Motivationally controlled stimulus preferences in chicks of the black-headed gull (Larus ridibundus L.)

Anim . Behav., 1969, 11, 252-270

MOTIVATIONALLY CONTROLLED STIMULUS PREFERENCES IN CHICKS OF THE BLACK-HEADED GULL (LARUS RIDIBUNDUS L,) BY

MONICA IMPEKOVEN*

Department of Zoology, Oxford University

Initial Interactions between Parent and Chick Begging-feeding. The movement of a recently hatched black-headed gull chick in the nest-cup may stimulate the parent to get up and to look down into the nest . When the parent is standing with lowered head and its beak is pointing down, the chick may beg for the first time by repeatedly pecking at the adult's bill-tip . This behaviour can induce the parent to regurgitate semidigested food which, when presented between the lower ends of the mandibles, may be taken and swallowed by the chick (Weidmann 1956 ; Beer 1966) . Thus, usually the first sight that the chick gets of one of its parents is the red underside of the vertically held beak . At nest relief the brooding parent sometimes gets up or leaves before its mate (often alighting 1 to 2 m away) has reached the nest. On this occasion, the chick for the first time experiences the complete sight of one of its parents . It may respond to the approaching bird by turning towards it or even by climbing up the nest-rim in the appropriate direction (I have seen this as early as 12 hr posthatching). Faced either with the profile or underside of the newly arrived parent's beak, the chick may again beg for food . From the second day onwards a chick shows an ever increasing tendency to follow a departing parent or to approach a parent standing some distance away from the nest . The chick often approaches the parent from the side and takes the food either from between the adult's bill-tips or from the ground. Snuggling-brooding . On the first day after hatching a chick is brooded most of the time . Sheltered by the parent from above, the chick usually turns round after feeding and snuggles under the parent's breast or between its wing and body. Older chicks that had been fed away from the nest may return to the nest by themselves or following the parent. They either try to snuggle immediately or they lean against the parent from the front or side . Crouching and hiding . In response to some ground predators (humans, foxes) adults fly up in great numbers and utter characteristic alarm calls (Kruuk 1964) . Chicks less than 24 hr after

An animal's sense organs limit the range of stimuli to which it can respond but there is usually a further restriction on stimuli to which it actually responds . This stimulus filtering (Marler 1961 ; Marler & Hamilton 1966 ; Broadbent 1958) is particularly striking when one finds that selectiveness favours different stimuli at different times . Such changes in stimulus selectivity were found to occur in visual stimulus situations between one hue and another (Ilse 1941 ; Crane 1955), or between hues and darkness (Tinbergen et al. 1942) . Grayling butterflies of both sexes when searching for food approach preferentially yellow and blue flower models, but when sexually active the males approach dark female models, irrespective of their hue . In all of these examples the switches in preference are claimed to be brought about by changes in the animal's motivation . Alternately, they could result from differences in the stimulus objects presented in the two situations, irrespective of internal state ; for example, Curtius (1954) and Hailman (1967) showed that colour preferences can differ with changing background . In order to resolve this, one has to study stimulus preferences of an animal in different states to a constant stimulus situation or object, so that it must respond selectively to certain stimuli, while other stimuli, which may be effective in a different context, are simultaneously impingeing on it . In the laboratory it has been possible to train animals to change their preferences in a constant situation according to their motivational states (e .g . Bolles & Petrinovitch 1954 ; Bailey & Porter 1955) . In a field study Baerends (1959) showed that the herring gull responds selectively to the speckling of its eggs and ignores their shape when rolling them in for incubation, but to their shape and wholeness when searching for them as food. The present experiments were carried out to investigate stimulus preferences in some responses occurring naturally in the chicks of the black-headed gull and to see in what way they were affected by motivation . *Present address : Institute of Animal Behavior, Rutgers University, Newark, New Jersey 07102 .

252



IMPEKOVEN : STIMULUS PREFERENCES IN BLACK-HEADED GULL CHICKS

hatching usually remain motionless in the nestcup . From the second day on, they may leave the nest and seek cover in the nearby vegetation . When the disturbance has subsided, the adults return to their nests. The chicks emerge from their hiding places and approach their parent, and usually snuggle or lean against the parent . Although the chicks' responses to their parents seem to be orientated to a great extent by visual stimuli, calls certainly play an important part as well, though they will not be considered in the present study . The Problem On the basis of qualitative field observations, it would seem that pecking followed by swallowing of food, if provided, is likely to occur in chicks that have not been fed for some time ; and snuggling followed by resting under the parent is likely to occur in chicks that have been chilled for some time . Different motivational states labelled `hungry' and `cold' respectively must underlie these responses . It will be seen later, however, that the label `cold' is misleading in that the preferences of chilled chicks are not primarily controlled by temperature . The question arises whether chicks that approach their parent from a distance respond to the same or to different aspects of it depending on whether they are `hungry' or `cold' . The effective stimuli eliciting the pecking response in the chicks of this species had been studied earlier by Weidmann & Weidmann (1959) and Weidmann (1961) . Nothing comparable has been done with respect to snuggling .

The present experiments were designed to discover whether `hungry' chicks would show the same stimulus preferences when approaching objects as when pecking and whether `hungry' and `cold' chicks would show the same or different preferences when approaching objects, followed by pecking or snuggling respectively . Chicks which run to cover in response to the adults' alarm calls were labelled `alarmed .' As a side line experiments were designed to discover what stimulus preferences `alarmed' gull chicks would show . Methods During the breeding season 1965 to 1 .967 approximately 1300 gull chicks were tested under daylight conditions in a wooden hut : near a large black-headed gull colony in Ravenglass, Cumberland, England . Subjects Pipped eggs collected from the gullery were hatched in darkened incubators (Glevtim Superior). After hatching, the chicks were .individually numbered with small labels stuck on their backs and kept for most of the time in darkness up to 24 to 36 hr post-hatching, separated from one another in small cardboard boxes (7 x 8 x 7 cm). Without food the young of this species do not survive well beyond 12 to 18 hr after hatching. The experimental animals were fed singly, in a standard light grey box (painted with French grey Robbialiac, No . 435 of known reflectance, Fig . la) from tweezers, every 3 to 4 hr (with an 8-hr gap overnight), in total

so

eal.

~~\\o 2 -

French LL

253

ey

0\\e 1~

~,40

Grey c Medium grey

f

r

;

Dark grey Black

0 400m B LU

500

700

660m1 YELLOW

GREEN

a

RED

b Fig . 1 . Reflectance of greys and colours used in the experiments, showing the amount of light reflected at each wavelength as a percentage of the light reflected by a white manganese oxide surface . (a) Paints for backgrounds, (b) Standard Ostwald colour-papers,

Grey k

25 4

ANIMAL BEHAVIOUR, 17, 2

three to four times before the first experiment . The food consisted of a mixture of boiled egg (in 1965) or egg-white alone (in 1966, 1967) with dog-meal or brown bread and added vitamins and calcium. After the end of the experiments the chicks were replaced in wild birds' nests . Apparatus The experiments were carried out in an arena (Fig. 2) painted in French grey as was the feeding box (Fig . la). Light coming in through a large

pretations of any preferences with respect to their adaptiveness to the natural situation . In some pecking tests (repeating Weidmann's 1956 experiments with modified standard background and choice rather than successive presentation) a single profile was presented at the time (series 1 to 3, see below) . It was coloured light grey like the background and two model beaks, differing in one of the parameters described above, were fastened to the head in a vertical position, one beside the other (Fig . 4a). In other

a b Fig . 2. Testing arenas and models seen from above . From the experience gained in the first season 1965 (test series 1) the apparatus was modified for experiments in 1966 and 1967 (test series 2 to 5) : the sharp corners and edges which had attracted the chicks to peck or to snuggle were rounded . The two sections of the end wall from which the models protruded were put at an angle of 120 degrees to each other so that all parts of the models appeared at roughly equal distance for a chick placed in the centre . (a) 1965 (test series 1) . (b) 1966, 1967 (test series 2 to 5). window facing north fell directly on to the end wall of the apparatus from which the stimulus objects protruded . The stimulus objects for approach tests of `hungry' and `cold' chicks (see procedure) were flat life-sized profile models of an adult gull (Figs 2a and b, 3 a and b,) presented two at a time and differing in overall colour, brightness, contrast or `pattern' . The models were made gull-shaped on the assumption that their form or size might contain additional characteristics that were effective in enhancing approach, even in chicks that had not had any previous experience with real parents . Whether this assumption is correct or not remains to be investigated. In addition it was thought that the use of gull-shaped models might facilitate inter-

pecking tests two anterior gull profile models were presented (series 4 to 5, see below, Fig. 4b) . Similarly shaped models had been used earlier by Weidmann & Weidmann (1959) and Franck (1966). In the first season, 1965, `alarmed' chicks were tested with small rectangular boxes (7 x 6 x 7 cm, Fig . 5) lined with different colours, into which they would creep. In the following season, gull-shaped models were presented to them, so that any difference in preference from `hungry' and `cold' chicks could definitely be attributed to motivational factors rather than to differences in the stimulus objects presented . The models were covered (or lined in the case of boxes) with one of the following pigment colours from the Ostwald series (earlier



IMPEKOVEN : STIMULUS PREFERENCES IN BLACK-HEADED GULL CHICKS

255

b a Fig. 3 . Gull models for approach tests of `hungry' and 'cold' chicks in 1966 and 1967 (test series 2 to 5 ; for 1965, test series 1, see Fig . 2a) and 'alarmed' chicks in 1966 (for 1965 see Fig . 5) . (a) with a 'hungry' chick pecking . (b) with a 'cold' chick snuggling .

a b Fig. 4 . Models for pecking tests . (a) test series 1 and 2 (b) test series 3 to 5 .

Fig. 5. Test-situation for alarm responses . Rectangular cardboard boxes (test series 1) . used by Ilse 1937 ; Weidmann 1961 ; Schaefer & Hess 1959) ; red (7 pa), yellow (2 pa), blue (13 pa), green (21 pa), white, light grey, dark grey and black (grey a, c, k, p) (see Fig . lb) . Backgrounds of various shades of grey were obtained by mixing French grey Robbialac paint with different amounts of black paint (Fig . la) .

The following series of stimuli were used with 'hungry', and 'cold' chicks : Series 1 : red, yellow, blue, green, black and white paired equally often in all 15 combinations (Table I, Figs 6 and 7) . Later, testing was continued with only eight combinations (red/blue, red/black, blue/black, blue/yellow, green/black, green/yellow, blue/green, yellow/black, Table 1 b) . Series 2 : red, yellow and green were compared with light grey, dark grey and black (Fig . 8) . Series 3 : white, light grey, dark grey and black were compared with one another in all possible combinations (Table III) . Series 4 : white and black were compared with one another on a light grey, a medium grey, a dark grey and a black background (Fig. 9) . Series 5 : differently patterned models were compared with one another and with uniformly coloured models (Fig . 11) . For the approach tests of 'hungry' and 'cold' chicks, the dummies were moved from side to



256

ANIMAL

BEHAVIOUR, 17, 2

side over a distance of ca . 5 cm at a standard speed, at 50 to 60 to-and-fro swings per min in test series 1 to 3, and at 40 to-and-fro swings per min in series 4 and 5 . Hailman (1967) found that the latter was the optimal speed for eliciting pecking behaviour in laughing-gull chicks . The movement was in the first case produced by a pulley system operated through a foot pedal by the experimenter, and timed with a stop clock and in the second case driven by the motor of an electrical record player. For pecking tests the models were moved at the same speed as in the approach tests but over a reduced distance of about 2 .5 cm . `Alarmed' chicks were only used in series 1 and 2 : for these chicks the models were kept still . Assessment of Stimuli A rank order of subjective brightness of the colours and greys used in the tests was computed from the spectral sensitivity of the light-adapted pigeon after Blough (1957) and the reflectance curves of the Ostwald papers (Fig. ib). Blough's data for the pigeon were chosen for the reason that they cover a wider range of the visual spectrum than data obtained by other authors and for other birds, and also because they stem from behavioural rather than physiological measurements . An investigation of the spectral sensitivity of gulls by G . Thompson (in preparation) will show to what extent they coincide with the data from the pigeon . Measurements of pupil contractions in two 6-months-old blackheaded gulls towards narrow-band interference filters showed that the maximum of their sensitivity is like that of the pigeon at 560 run . Black was assumed to be the darkest and white the brightest stimulus . Together with the calculations for the other stimuli the following rank order was obtained starting with the darkest and ending with the brightest stimulus : (black), blue, dark grey, green, red, light grey, yellow (white). This order turned out to coincide with the ranking of stimuli based on my own visual impressions . The brightness contrast of a white and a black model on a light (French) grey, a medium grey, a dark grey and a black painted background (Fig. la) was assessed by five human subjects . According to them black contrasts more from a French grey background than white . On all the darker backgrounds the contrast of white predominates over that of black . Procedure Chicks were tested between 24 and 36 hr after

hatching. Before the age of 24 hr they usually did not approach objects at all . Groups of chicks received differential treatment before the experiment. Chicks labelled `hungry and warm' or in short `hungry' were not fed for 3 to 4 hr and were kept warm before testing (in the standard grey feeding box under a gas-brooder at 26 to 30°C) . Chicks labelled `cold and satiated' or in short `cold' were fed and subsequenty chilled in the feeding box by being exposed for 1 to 2 min to temperatures of about 10 to 17°C (measured at 15 cm above the floor of the box), either outside the hut or under the open window, according to outside temperatures . Some preferences of chicks under these treatments were compared with those of `hungry but cold' and with those of `satiated but warm' chicks . Birds labelled `alarmed' were fed and kept warm but during the experiment alarm-calls were played back to them through a tape recorder. Thus this group of chicks differed from `hungry' and `cold' chicks in that the motivational state was given not by pre-treatment alone but by introducing an acoustic stimulus just after a chick had been placed into the testing arena . The alarm calls were the response of a chorus of adult gulls in the colony to the human intruder recording them. The chicks responded to these taped calls by seeking cover just as they do to real calls . Some preferences of these chicks were compared with those of `alarmed but hungry' chicks . The birds that were kept warm before the experiment were tested at about 21 to 24°C, those that were chilled at a temperature of approx . 17 to 19°C as measured near the starting point about 20 cm above the floor . All experimental subjects were light-adapted in the light grey feeding box for approx . 3 min (including feeding and chilling time), before they were presented with the choice stimuli . For some pecking tests (series 1) the chicks were not lightadapted . For approach tests `hungry' or `cold' chicks were placed singly in the centre of the arena, 40 to 45 cm from the two models (Fig . 2) . No further precautions were necessary, since (apart from very few exceptions) the animals did not approach the models immediately . Gull chicks that did not approach within the first 5 min (test series 1) or 3 min (test series 2 to 5) were subsequently placed at a distance of 25 cm from the models . The tests were concluded when a chick had reached a model and pecked at it (when food-deprived) or crawled under or behind



IMPEKOVEN : STIMULUS PREFERENCES IN BLACK-HEADED GULL CHICKS it (when chilled) (see Fig. 3 a and b) . Choices of `hungry' chicks that subsequently snuggled, `cold' chicks that pecked and chicks that did not peck or snuggle or showed both responses in immediate succession were excluded from the final scores . Such chicks are considered in a special section : factors contributing to a difference in stimulus preference . Choices of `hungry' chicks that showed intention movements of snuggling and choices by `cold' chicks showing intention movements of pecking were included (test series 1, 2 and 5) . No chicks were discarded in series 3 and 4 ('uniformly grey models' and `white and black models on backgrounds in different shades of grey'), irrespective of their overt response . `Alarmed' chicks were placed at approx . 25 cm from rectangular boxes or gull models . The test was concluded when the chick had clearly approached one model, even if it had not attempted to reach it, which was frequently the case . Chicks that pecked at the model were excluded from the score . In cases where the object was reached, no sharp distinction could be made between snuggling and crouching, but when the chick stopped half-way it often crouched in a typical fashion pressing its body against the ground . Unlike `hungry' or `cold' chicks, which usually walked in a more or less upright position, `alarmed' chicks often advanced with curious pedalling movements, pressing their bellies against the ground. In pecking tests `hungry' chicks were placed directly in front of the model beaks and left there for a period of I min from the first peck . The experimenter could observe the chicks' behaviour in the arena through a mirror hanging in an oblique position above the apparatus, without attracting the chicks' attention . Since the supply of chicks was limited any single chick had to be used for more than one test situation . Thus, starting early in the morning, all chicks were first tested `hungry' (after 1I to 2 hr of food deprivation) in a pecking situation . No pecking tests were carried out after approach tests later in the day since it was found that the chicks did not stay with the pecking model for a I min testing period . Approximately 1 hr after the last pecking test the subjects were divided into two groups, one kept `hungry and warm', the other `satiated and cold' and tested in an approach situation . All chicks were given `alarm' tests either after the end or between two `hungry' or `cold' approach tests . In the pecking and alarm situation each chick

257

was presented with one or two subsequent choice tests, or pairs of choice tests respectively (see below), separated by I hr. In the `hungry' and `cold' approach situation three (series 1, part 1, Table I and Fig. 6) or two choice tests or pairs of choice tests respectively were given (all other tests), separated from one another by 2 to 3 hr. In series 1 any pair of stimuli was presented to a chick in only one position and the left/right positions were switched with subsequent chicks . In all other series the stimuli were presented twice to a chick with left/right position switched . Therefore in series 1 each choice of a particular colour was made by a different chick so that the number of choices equalled the number of chicks making a choice . In all other test series a chick contributed to two choices in any particular colour combination . In subsequent choice tests or pairs of choice tests of any one test situation different colours (greys, backgrounds or patterns) were presented . For instance a chick starting off with red/blue was later tested with for example green/black . A chick starting with light-grey/ white was later tested with dark-grey/black . However in series 4 a chick tested with white/ black on a medium background was later tested with the same stimuli but on a different background . With subsequent chicks the sequence of stimulus pairs was alternated . If for example one chick started with red/blue followed by green/black, a different chick would start with e .g. green/black followed by red/blue . With three consecutive stimulus pairs presented to chicks (series 1) the sequence of them was alternated according to a Latin square arrangement . Any single chick was presented with the same colour choices (greys, backgrounds or patterns respectively) in the pecking, subsequent `hungry' or `cold' approach situation and `alarm' situation. For instance a chick first tested with red/ blue in pecking was later also tested with red/blue in `hungry' or `cold' and `alarm' approach . In the approach tests of `hungry' and `cold' chicks records were taken of the stimulus chosen, the first response to the model, the latency of approach, and the calls . In the pecking tests, the number of pecks towards each stimulus was counted over I min period, starting with the first recorded peck . The stimulus to which the majority of pecks was directed was then scored as one choice and the actual number of pecks were not used. In the `alarm' approach tests, the stimulus

of



258

ANIMAL BEHAVIOUR, 17, 2

initial approach and the overt behaviour, i .e . crouching (pecking or snuggling) were recorded . Results Uniformly Coloured Models 'Hungry' chicks . In tests where red, blue, yellow, green, black and white were compared with one another in all possible combinations red and blue were preferred to all other colours, yellow to green and black, and white scored hardly at all (Fig . 6) . The pecking preferences for these colours (using the number of pecks) have also been ranked according to Dawkins' choice threshold model (Dawkins & Impekoven 1969). Figure 6 shows the broad similarity for the

Were these preferences due to genuine chromatic responses or merely to the brightness of the stimuli presented? From the present colour comparisons it must be concluded that the preferences shown cannot be due to as simple a brightness preference as either for the brightest or the darkest stimulus, since on the one hand yellow, which is lighter than for example red, received less response ; on the other hand, red, which is lighter than blue, was approached more (according to the order of subjective brightness, Fig . 6) . Thus, if the preferences were due to brightness alone, it would have to be to some intermediate shade . For the pecking response of black-headed

HUNGRY

COLD

Approach

Pecking IIIilllllllllllllllllllllllllllllllllllllilll

/O////%//////%/G//%/

%000/0///////////

IIIIIIIIIIIIIIillllllllllllllllllillll

Approach

Order of subjective brightness

IIIIIIIIillllllllllllllllllllllllllllll

Black

FORMEMANNI/J//

Blue Illllllllilllli Green I Red Yellow

0 0

40 SO

0 0

11 20

40

0

White I 20

40

Ho . of chicks choosing

Fig. 6. Preference order for six different colours in pecking and approach tests of `hungry' chicks and approach tests of `cold' chicks, together with the subjective brightness order of the stimuli . The effect of any particular colour is assessed as the number of chicks choosing it, irrespective of what that colour was paired with . E .g. the score for red represents the number of chicks choosing that colour, when paired with blue, yellow, green, black or white. Each colour was paired equally often with each other colour . (Ca. 100 chicks tested).

preference order of colours for `hungry' chicks when approaching and when pecking . No statistically significant differences appeared in any of the 15 individual colour pairs (Table I a and b) with the exception of red/blue (P< 0 .001, chi-squared 2-tailed), blue being preferred when pecking and red when approaching (followed by pecking) . Naive birds tested for pecking before their first feeding experience showed a relative red/blue preference which differed somewhat though not significantly from that of experienced chicks, suggesting that the initial preferences may have been modified by experience (Table I) . It is conceivable that the high blue preference in pecking was subsequently acquired in the feeding box which turned out to reflect a certain amount of blue (Fig . la) .

gull chicks a comparison of the preferred colours red and blue with a finely graded series of greys led to the conclusions that these colours were discriminated by means of hue (Weidmann, unpublished) . Hailman (1967) demonstrated colour vision in the laughing-gull chick by comparing narrowband interference colours of assumed equal subjective brightness to the chicks (on the basis of Blough's, 1957, spectral sensitivity curve for the pigeon) . Since the preferences in the approach response of `hungry' chicks were found to be similar to those of pecking, we may assume that hue discrimination is also involved in approach . This is furthermore supported by experiments (test series 2, Fig. 8) showing that red and, to a lesser



IMPEKOVEN : STIMULUS PREFERENCES IN BLACK-HEADED GULL CHICKS

259

Table L Relative Preferences of `Hungry' Chicks Pecking at and Approaching and of `Cold' Chicks Approaching Pairs of Different Colours (a) Shows Details of Data Used in Fig . 6 . (The Values Refer to Numbers of Chicks) Treatment : Behaviour :

`HungryI Pecking

'Cold' Approach

Approach

Stimuli lighter/darker

P-values

P-values

red/blue naive experienced

11/8 6/14

8/2

yellow/red

10/10

4/6

red/green green/blue yellow/blue

18/2 0/20 4/16

<0 . 001

P-values 2/8

<0 . 01

0/10 7/3

<0 .001

10/0 2/8

<0 .05

2/8

2/8

7/3

4/6

<0 .001

8/2

7/3

10/0 8/2 6/4

yellow/green red/black 1

14/6 18/2

blue/black 1 yellow/black 1

20/0 14/6

<0 .001

green/black 1

15/5

<0 . 05

<0 .01

2/8

<0 .01

4/6 2/8 3/7

white/red 2

0/20

<0 .001

0/10

<0 .01

1/9

<0.05

white/blue 2

0/20

0/10

white/yellow 2

0/20 2/18

<0 .001 <0 .001

<0 .01 <0. 05

0/10 1/9

<0.01 <0.05

<0 . 05

2/8

<0 . 05

2/8

white/green 2 white/black

0/20

<0 . 001

1/9 1/9

<0 . 001

1/9

Statistical analysis : Binomial 2-tailed, Wilcoxon matched pairs, signed ranks 2-tailed P<0 .05 for 'hungry -'cold' approach. (b) Shows Eight Colour Pairs with which Twice as Many Chicks had been Tested Treatment : Behaviour :

'Hungry' Pecking

'Cold' Approach

P-values

Stimuli

Approach P-values

P-values

lighter/darker red/blue yellow/blue green/blue yellow/green red/black

15/24 11/29

17/3 5/15

<0 .01

<0 . 01

<0 . 05

11/9 5/15

2/38

<0 . 001

3/17

<0 .01

6/14

29/11

<0 .001

14/6

blue/black

35/3 36/4

<0 .001 <0.001

16/4 14/6

yellow/black

28/12

<0 . 05

12/8

green/black

24/16

9/11

<0 . 05

6/14 <0 .05

12/8 10/10 5/15

<0 . 05

7/13

Statistical analysis : Binomial 2-tailed, Wilcoxon matched pairs ; signed ranks 2-tailed F<0 . 05 for 'hungry'-`cold' approach .

extent, yellow were preferred to three different greys . While these results suggest that colours highly preferred to greys, like red, must owe their effectiveness to true chromatic properties, they do of course not prove that colours like green,

not preferred to greys, cannot be discriminated by their hue (Fig . 8) . `Cold' chicks. In test series 1 where red, blue, green, black and white were compared with one another their order of preferences differed from



2 60

ANIMAL BEHAVIOUR, 17, 2

that of `hungry' chicks (Fig . 6) : Black, the darkest stimulus, had risen in preference whilst yellow, the lightest stimulus but one, had considerably fallen . The proportion of black and yellow choices differed significantly between the two treatments (Table II) . The change in the rank order of preferences seemed to approach the order of presumed subjective brightness for the colours tested ; however, not to the extent that black, the darkest stimulus, became top scorer. Black was not preferred to red and blue, colours most responded to by 'hungry' chicks . According to a more detailed analysis of the 15 individual colour pairs tested the change in preference showed a consistent tendency ; compared with 'hungry' chicks the scores of 'cold'

apparent with respect to their relative preferences for green. `Alarmed' chicks . In experiments where rectangular boxes in red, blue, yellow, green, black and white were compared with one another (series 1), the ranking of the colours corresponded with the subjective brightness order starting with the darkest stimulus black and ending with the lightest, white (Fig . 7) . In tests where gull-profile models in red, yellow and green were compared with different greys and black (series 2) 'alarmed' chicks scored higher for the darker greys and black than 'hungry' or 'cold' chicks . According to the Wilcoxon test the difference between 'hungry' and `alarmed' chicks is not significant (P<0 . 1, 1tailed) . A difference in 'cold' chicks was only

Table II. Proportion of Black or Yellow Choices Respectively of 'Hungry' and 'Cold' Chicks in Tests of any Colour Other Paired with Black or Yellow Respectively Approach 'Hungry'

`Cold'

No. of chicks approaching black

30

48

No . of chicks not approaching black

60

42

No. of chicks approaching yellow

44

25

No. of chicks not approaching yellow

36

55

Colours paired with black

Colours paired with yellow

Statistical analysis (chi-squared 2-tailed) : Black choices 'hungry'-'cold' Yellow choices 'hungry'-'cold'

P<0'05 P<0 . 01

Values from Table I (a) and (b) combined .

chicks were shifted in favour of the darker stimulus of a pair (Table la and b) . The shift was particularly striking in combinations containing black 1) and did not occur in any combinations containing white 2 ) . But white, the lightest stimulus, was lowest in preference even for 'hungry' chicks and thus could not fall any lower. In experiments where red, yellow and green were compared with different greys and black (series 2, Fig . 8), 'cold' chicks again scored higher for the darker greys and in particular for black. According to the Wilcoxon matched-pair signed ranks test (as for series 1, Table I) the difference between 'hungry' and 'cold' chicks was not significant (P<0 . 1, 1-tailed) . The difference between 'hungry' and 'cold' chicks was least

Number of chicks choosing 0

25

Order of subjective 50 brightness Black

Idill'IlfilGlllililillllllllllllilllliili:llllilllllliai'illll6

Blue Green

%/////////////////%//////////

1111110111111

LJ

Red

%//////i

Yellow White

L

I

Fig . 7 . Preference order for six different colours in approach tests of 'alarmed' chicks together with the subjective brightness order of the stimuli . The effects of the different colours are assessed as in Fig . 7. (Ca, 1OQ chicks tested),



IMPEKOVEN : STIMULUS PREFERENCES 1N BLACK-HEADED GULL CHICKS No. of choices 0 15 30 0 1 1 I

Fig. 8. Relative preferences of `hungry', 'cold' and 'alarmed' chicks approaching red, yellow and green when compared with light grey, dark grey and black. (Symbols for colours as in Fig. 6 and 7 . Light grey, fine black stipling on white background ; dark grey, fine white stippling on black background) . According to the subjective brightness order the stimuli were ranked in the following way starting with the darkest and ending with the brightest stimulus : black, dark grey, green, red, light grey, yellow. (Ca. 132 chicks tested) . The values indicate the number of choices, each chick being exposed twice to the same stimuli with reversed position .

15

161

30 0

F 1 1

15

30

1 1 1

/////////////// 0///d//////O/// COLD Is

I

a

L-

//G//////.

ALARMED

%///////0 I

I

0

15

-1

1

30 0

I

1

I.

15 30 0

15

30

Statistical analysis to Fig . 8 . P-values

Stimuli

'Hungry'

Difference 'Hungry''cold'

light-grey/red

<0 .001

dark-grey/red

<0 .001

ns

black/red

<0 . 001

<0 . 001

light-grey/yellow

<0 . 05

dark-grey/yellow

<0 .001

light-grey/green

Difference 'Cold''alarmed'

<0 .001

<0 .001

Difference 'Alarmed''hungry'

<0 .01 <0 .05 <0 . 001 <0 . 001

<0.001

<0 . 001

<0 .001

<0 .001

<0 . 001

<0 .001

<0.001

<0 . 001

<0'001

<0.05

dark-grey/green black/green

'Alarmed'

<0 .001

<0.001

<0 . 05

black/yellow

'Cold'

<0.05 <0 .05

Statistical tests : Binomial and chi-square 1-tailed, since direction of differences was predicted from previous results .

noticeable with respect to yellow, the brightest of the three colours tested in this experiment . Uniformly Grey Models (Test-series 3) In these experiments a white, a light grey, a dark grey and a black model were compared with one another in all possible combinations on a light grey background . `Hungry' chicks. 'Hungry' chicks did not differ significantly in their preferences whether pecking at model-beaks or approaching entire model gull-profiles . However, there is an indication that when pecking, chicks differentiated more between different greys than when approaching, particularly between light-grey/white and dark-grey/black (Table III) . Whether this difference is due to chance, or indicates a real difference in preference between pecking and

approach cannot be decided from the small amount of data. `Cold' chicks . In an approach situation 'hungry' and `cold' chicks showed the same order of preference, passing from black (at the top) through dark grey to white and ending with light grey . There is however an indication that 'cold' chicks differentiated more between different greys than 'hungry' chicks (Table III) showing in particular a higher preference for the darkest stimulus, black . These differences may be due to chance or hint at a real difference between the two treatments . White and Black Models on Backgrounds in Different Shades of Grey (Test Series 4) Experiments were designed to see whether and to what extent the low preference for white

262

ANIMAL BEHAVIOUR, 1'f, 2

Table III. Relative Preferences of `Hungry' Chicks when Pecking (HP) and Approaching (HA) and of 'Cold' Chicks Approaching (CA) Pair Combinations of Different Greys Stimuli

HP

P

HA

P

Difference HA - CA P

CA

P

white/light grey

6/14

ns

9/11

13/7

white/dark grey

6/14

ns

7/13

8/12

white/black

0/20

<0 . 005

4/16

<0 .05

2/18

<0.005

light grey/black

4/16

<0 . 005

3/17

<0 .005

1/19

<0 .005

dark grey/black

6/14

ns

11/9

2/18

<0.005

light grey/dark grey

4/16

<0 . 005

4/16

1/19

<0.005

<0 .01 <0 . 05

Statistical tests : Binomial 2-tailed . Chi-squared 2-tailed .

was due to its low contrast with the light grey background on which it had been presented, and to what extent the high preferences for black (shown by `cold' chicks) was due to its absolute darkness or high contrast with the light grey background . `Hungry' chicks . The scores of pecking and approach preferences were found roughly to coincide. A black model was preferred on a light grey background (as shown earlier and represented again on Fig. 9) and a white model on a black background, a result that ties in with Weidmann's (1961) findings on the black-headed gull and Hailman's (1967) findings for the laughing gull . On a medium grey background on which a white model still contrasted more than a black one both models received equal response . `Cold' chicks. `Cold' chicks preferred the white model on a black background as did `hungry' chicks . On a dark grey background, on which white seemed to contrast more than black `cold' chicks unlike `hungry' ones, gave equal preference to a white and a black model. On the medium grey background they preferred the black one. Thus the overall score for black was higher (Table IV) . Factors Contributing to Shift in Stimulus Preference

In order to see to what extent the differences in preference between `hungry' and warm' and `satiated and cold' chicks were due to fooddeprivation and/or chilling, chicks treated in this way were compared with 'hungry but cold' and with `satiated but warm' chicks . With respect to the relative preferences between a red

and a black model `hungry and cold' birds differed from `satiated and cold' birds but hardly from `hungry and warm' ones (Fig . 10) . The approach scores of 'satiated and warm' chicks were very similar to the ones of `satiated and cold' chicks . Chicks which were `alarmed and hungry' showed relative red/black preferences intermediate between `alarmed' (satiated) and 'hungry' (not alarmed) chicks . In some of the earlier tests (series 1, 2 and 5) only the choices of pecking individuals in the case of `hungry' chicks and only the choices of snuggling individuals in the case of `cold' chicks had been included in the final scores . In Fig. 10 choices of six `hungry' and of two `cold' chicks were included which showed in at least one (out of two) test not the `required' behaviour . The relative scores for red/black scarcely differ from the earlier result (compare Fig . 10 with Fig. 8) . From `satiated and warm' and from `hungry and cold' birds the choices of all individuals were included irrespective of whether they pecked or snuggled . Whilst `hungry and warm' chicks had shown predominantly pecking behaviour and `cold and satiated' ones snuggling behaviour, `hungry and cold' and `satiated and warm' chicks both snuggled and pecked. The occurrence and strength of pecking although primarily dependent on the state of `hunger' varied also to some extent with the temperature : `Satiated and warm' chicks pecked more than `satiated and cold chicks' . Snuggling movements seemed to be both induced by satiation and low temperature ; `satiated and warm' as well as `hungry and cold'



IMPLKOVEN : STIMULUS PRP+F'MENC1.S IN $LACK-HI A1) D GULL CHICKS °1°(P) ornumber(A) of choices 0 50120

Backgrounds. 100140 N=18

,P H `A CA Fig. 9. Relative preferences of `hungry' chicks when pecking (HP) and approaching (HA) and of 'cold' chicks approaching (CA) a black and a white model presented on backgrounds of different greys . (Ca. 80 chicks tested). The values indicate the number of choices as in Fig. 8) .

263

light grey N=42

P

: .,

A

medium grey

CA N=40 1

P H`A CA

arkgrey N=36

P 'A

black

CA 0

50120

100140

Statistical analysis to Fig . 9. P-values Background HP Light grey

Difference HP - HA

<0'001

HA

Difference HA - CA

<0 .02

CA <0 . 001

<0 . 05

Medium grey Dark grey

<0 . 001

<0-001

Black

<0.001

<0 . 001

ns <0 .001

Statistical tests : Binomial 2-tailed . Chi-squared 1-tailed, since direction of difference was predicted from previous tests.

chicks showed snuggling movements (Table V) . Furthermore, it was of interest to find that the scores for the stimuli red/black did not entirely correspond with the overt behaviour shown ; although 'hungry and cold' chicks snuggled much more than 'hungry and warm' chicks their relative preferences for red/black did not differ significantly from those and although 'satiated Table IV. Total Number of Approaches of `Hungry' and `Cold' Chicks to a Wbite and a Black Model Presented on Backgrounds of Different Greys (See Fig. 9) 'Hungry' 'Cold' Total number of white-choices

88

64

Total number of black-choices

72

96

Statistical analysis : differences between 'hungry'-'cold' P<0 . 01 (chi-squared, 1-tailed) .

and warm' birds pecked more than 'satiated and cold' ones their scores coincided . `Patterned' Models and Adaptedness (Series 5) Previous experiments designed to see what colour, brightness and contrast preferences chicks under different motivational states would show yielded results which we should like to understand in terms of adaptedness to the natural situation . When the aim of a study is to investigate the mutual adaptedness of certain species characteristics and the preferences in the animal responding to them, the best way is to start from a near natural situation and assess the effectiveness of the feature in question by varying it, leaving everything else as natural and constant as possible . For instance, in order to see how the redness of the beak contributes to the parent's attractiveness to a chick one would compare a model of the parent's head with a red



ANIMAL BEHAVIOUR, 17, 2

26 4 °!o choices 0

•

50

beak but in their overall colouration . The results suggest that e .g . a strong red-preference as shown by `hungry' chicks approaching may be adaptive with respect to the parent's red beak . However tests with more naturalistically coloured models, in which only the beak colouration would be varied, have not been carried out . In one experiment (Fig . 11, 1) it was shown that a white model with a red beak only was less effective than an entirely red dummy . This difference is only significant for `cold' chicks not for 'hungry' ones . With respect to the adult's body colouration earlier tests showed that although white and light grey were least preferred stimuli, their effectiveness was enhanced when seen against a darker background . In further tests a white model with a red beak and head was found to be as effective as an entirely red model and a white model with a black beak and head as effective as an entirely black model for 'hungry' chicks

100

•

C Al

N

•

-

52

+ +

40 40

•

- +

5

44 40

+

6 + - + 20 Fig . 10 . Relative preferences of (1) 'hungry + warm' (2) 'hungry + cold' (3) 'satiated + cold' (4) 'satiated + warm' (5) 'alarmed + warm' and (6) 'hungry + alarmed' chicks approaching a red and a black model . The values refer to the percent of choices made, each chick being tested twice to the same stimuli with reversed position . Statistical analysis to Fig . 10. P-values Difference between 1 . 'Hungry + warm' and 3) 'satiated + cold' <0 . 001 1 . 'Hungry + warm' and 4) 'satiated + warm' <0 . 001 2 . 'Hungry + cold' and 3) 'satiated + cold' <0 . 05 Statistical test : chi-squared 2-tailed .

Table V. Behaviour of Differently Pretreated Chicks Towards the Model on Approaching it, in Choice Tests with a Red and a Black Model (See Fig . 10). Treatment

Behaviour

'Satiated +warm' 'Hungry+cold' 'Hungry + warm' 'Satiated+cold' difference difference difference difference between between between between treatments treatments treatments 'satiated+ P P P cold' and 'satiated + warm' P

Pecking

29

<0 .01

64

92

Peck-intention

27

<0.01

5

2

Snuggling

73

59

Snuggle-intention

14

0

Total no . of tests

41

42

<0.001

11-5

11 . 5 52

<0 .001

<0.001

7

<0 .05

2

<0 . 001

98 0 44

Statistical test : chi-squared, 2-tailed . The values indicate the percentage out of the total number of tests, in which the particular behaviour was shown .

beak with models with differently coloured beaks. With respect to pecking this has been done for the black-headed gull and related species (Weidmann & Weidmann 1959 ; Hailman 1967 ; Tinbergen & Perdeck 1950) and the preferences found have at least in part confirmed that the chicks' selectivity was adapted to the parent's beak or vice versa, although the high blue preference could not be explained in this way (Hailman 1968) . With respect to approach tests the models presented to the chicks in the present study did not only differ in the colour of their

approaching it. This suggests that the body colouration in these situations did not affect their choice . In 'cold' chicks, however, the entirely red or black model respectively received more response implying that here the body colouration was of importance (Fig . 11, 2 and 3) . This conclusion was supported by a further test showing that 'satiated' chicks preferred a model with a black body to one with a black head (4) . Such findings are not easy to interpret in functional terms . Time did not allow for the performance of further tests which may have



IMPEKOVEPt : STIMULUS PHMMENCES IN BLACK-1#EA15ED GULL CI#ICkS 0 50 100

0/0 choices

N-46 I N=35 I N=32

WAWSNAITAVAVAWM

Nz34 N=34

CA H

N=38 N=36

P A

N=30

CA

N=37

P

N=30

A

N=32

CA

N=33

P H A

N=30

4)

N=31 'C

N=32 N=33 N=30 N=34 N=30 N=32 N=30 0

50

100

Fig. 11 . Relative preferences of `hungry' chicks when pecking (HP) and approaching (HA) and `cold' chicks when approaching (CA) differently 'patterned' models. (Ca. 115 chicks tested) . Left side of diagram refers to left model but models were given in both reversed positions . Values refer to to percentage of choices made (as in Fig. 10) . Symbols for colours as before . Statistical analysis to Fig. 11 . P-values Test no. HP

Difference HP - HA

1

HA

Difference HA - CA

ns

CA <0.05

2

ns

<0.05

3

ns

<0 .01

4

<0 .001

5

<0 .001

6 7

ns

<0 . 001

<0. 001

ns

<0 . 001

<0 .05

<0 . 05

Statistical tests : Binomial 1-tailed for pecking tests, since direction of difference was predicted from previous tests and from literature (see text). Binomial 2-tailed for 'hungry' and 'cold' approach tests (HA + CA) . Chi-square 2-tailed for difference between treatments .

263



266

ANIMAL $$IIAVIOUk, 17, 2

thrown more light on this problem : e .g. how a white model shaded darker on the lower side in the way a solid body of a gull looks in natural lighting would compare with an entirely white or an entirely black model . Thus at this point it has to remain suggestive whether e .g . a black body preference would direct the approach of `satiated' chicks into the dark shadow of the parent's belly. Weidmann and Hailman found earlier that the pecking response towards a red beak is not affected by the colouration of the model's head, leading to the conclusion that the facial mask of the parent was of no importance to a chick pecking at its beak . Present results confirm these findings (Fig . 11, 6 and 7) . However, for both `hungry' and `cold' chicks approaching, a red or black head added to a red beak on a white model increased its effectiveness considerably (5, 6). Also a white model with a black head was superior to an entirely white model (7) . These results suggest that the facial mask of the parent is of significance to a chick approaching it from a distance . Discussion In the experiments presented here `hungry' chicks showed similar colour and grey preferences when pecking at model beaks and when approaching entire model gulls, with the exception of red and blue, blue being preferred when pecking and red when approaching (Table I, Fig . 6) . Related studies on pecking preferences of gulls supported by my own experiments suggest that the preferred colours were in both a pecking and an approach situation decided by their hue and not merely by their subjective brightness. The relative preferences for red and blue of `naive' chicks coincide with Weidmann's (1956) and Franck's (1966) findings, who had tested unfed chicks about 6 to 10 hr after hatching . Hailman (1967) obtained a bimodal preference curve with peaks in blue and red in a related American species, the laughing gull (Larus atricilla L .), irrespective of whether testing with narrow-band interference filters of equal quantum intensity or pigment colours . Strong red and blue responses were also recorded for the pecking behaviour of other Laridae (Quine & Cullen 1964) and for red only in the herring gull (Tinbergen & Perdeck 1950 ; Kear 1964) . No comparable results are available so far on approach preferences of other Laridae . Closest to the present investigation comes Schaefer & Hess's (1959) study on the following response

in domestic chicks, which show also strong red and blue preferences . However, these authors obtained their results not from initial choices but from the strength of retention of `imprinted' stimuli . Other investigators using different methods found a high preference for red, but blue was less responded to than other colours (reviewed in Bateson 1966) . None of the studies specify the motivational state in which the chicks were tested, although Schaefer & Hess indicated that the `goal' was shelter and protection rather than food . The colour preferences of `cold' chicks differed from the ones of `hungry' birds in that black had risen in preference whilst yellow had considerably fallen (Table II and Fig. 6) . Darker colours (according to a subjective brightness rank order, _Fig . 6) also received higher scores (Table I) . `Alarmed' chicks differed both from `hungry' and `cold' chicks in that they scored still higher for darker colours and greys (Fig . 7 and 8) . One important point to consider when comparing the results of `alarmed' chicks with those of `hungry' and `cold' chicks is that the difference between the preference order of coloured stimuli (series 1, Figs 6 and 7) may have arisen from the differences in stimulus objects presented rather than motivational factors . Some data suggest that the differences between the stimulus objects did affect the relative colour preferences (Table VI) . `Alarmed' chicks differed in their relative proportions of red and black choices depending on whether they were presented with gull-models or boxes . Also `cold' chicks were found to choose black predominantly when tested with boxes (difference gull profilesboxes P<0 . 001) . `Hungry' chicks initially did not approach boxes at all but after being tested first in the state of `alarm' or `chilled' they subsequently did, preferring red to black as in the case of gull profiles (differences 'hungry'-'cold' P<0 . 001, 'hungry'-'alarmed' P<0 .01) . These pilot tests are briefly mentioned here to point out possible interactions of external (visual) stimuli and internal state in controlling colour preferences . The differences may in some way be correlated with differences in illumination, the colours inside the boxes appearing slightly darker than on a flat gull model, but surely not dark enough to prevent the colours from being recognized . The results definitely show that colour preferences differed according to the motivational state and raised the question about the nature of this



IMPEKOVEN : STIMULUS PREFERENCES IN BLACK-HEADED GULL CHICKS

267

Table VI . Comparison of Red/Black Preferences with Respect to Two Different Stimulus Objects : 2-dimensional Gull Models and 3-dimensional Rectangular Boxes (Figs 3 and 5) Treatment Stimulus objects

'alarmed'

'cold'

boxes gull models 1966

boxes

'hungry' gull models 1965 1966

boxes

gull models 1965 1966

red

12

15

0

12

14

16

15

28

black

28

15

16

8

16

6

4

2

The values indicate the number of choices, each chick being tested twice to the same stimuli with reversed position .

change . It could be that for 'cold' chicks and even more so for 'alarmed' chicks the brightness dimension has become more important than for 'hungry' chicks with the effect that they decided their preferred stimuli to a greater extent on the basis of their darkness. In other words, chicks under different motivational states would shift their stimulus selectivity along different physical dimensions : hue and brightness-darkness . Such a possibility would tie in well with the results of other studies indicating that animals attend to different physical dimensions in different behavioural contexts (Tinbergen et al. 1942 ; Baerends 1959, reviewed in Manning 1967) . Alternately the relative importance of hue and brightness could be the same but brightness preferences could differ with different motivational states . Thirdly both brightness and hue preferences might be different . An answer to this could be provided with the aid of interference and neutral density filters . The first possibility would be supported if brightness changes of hues affected the choices of 'cold' and 'alarmed' chicks more than those of 'hungry' ones . The second possibility would be supported if changes of brightness were equally effective for differently pretreated chicks but the preferred brightness associated with each hue differed . The slight differences between 'hungry' and 'cold' chicks with respect to greys-unless due to chance-could be interpreted in favour of the first idea according to which 'hungry' chicks would discriminate less between different greys because they were responding more to chromatic properties of stimuli presented to them ; whilst 'cold' chicks would differentiate more between different shades of grey and respond in particular to black because the brightness-darkness dimension was more important to them . It was surprising to find that for both 'hungry' and 'cold' chicks white and light grey, predominant colours of the parent gull should be

lowest in the preference hierarchy . This may have been due to lack of brightness contrast with the light grey background . The results showing that the darker the background the more a white model is preferred imply that the light plumage of the adult gull may be effective through its contrast with the often darker background, against which the chick sees it. The results with differently shaded backgrounds also show that in the case where a white and a black model were presented on a black background the contrast between the object and the background was more important to 'cold' chicks than the absolute shade of the object. However, irrespective of contrast 'cold' chicks directed a higher number of responses to black, suggesting that at least to some extent their preferences for this stimulus were due to its absolute shade . In what way were the preferences shown by 'hungry and warm' and by 'satiated and cold' chicks controlled by their motivational state? In this study a particular behavioural response had been taken as a criterion for a particular underlying state, i.e. pecking for 'hunger' and snuggling for 'cold', but it was found later (Table V) that even 'satiated' chicks pecked slightly more when warm than when 'cold' and snuggled even if not chilled . Hand-raised gull chicks peck frequently as early as 6 hr after hatching, and at an age when they do not swallow the food presented to them thus do not appear 'hungry' . This may be different from wild birds which could occasionally be seen fed by their parents whilst still wet (Weidmann 1956). Such 6-hr-old chicks showed already high red and blue preferences when pecking (Weidmann 1961) . Hailman (1967) compared natural colour preferences of laughing-gull chicks at different ages more systematically and found no striking differences between recently hatched and older, presumably more 'hungry' birds . Similarly Weidmann (unpublished) testing in black-headed

268

ANIMAL BEHAVIOUR, 17, 2

gull chicks natural preferences after different lengths of deprivation towards a red and a grey rod and discs of the same two colours found no change. It appears then that pecking preferences are little or not affected by food-deprivation. Stimulus generalization as obtained for pigeons (Thomas & King 1959) did not occur even for conditioned preferences (Hailman 1967) . Marler (1961) and Weidmann (1956) pointed out that some of the stimulus characters eliciting pecking behaviour in gull chicks may owe their effectiveness to more general properties of the sensory perceptual pathways . Thus red may be effective in various behavioural contexts . Duecker (1963) speculated from the similarity of feeding preferences and plumage colouration of various song-birds, that similar preferences may occur in different behavioural situations . Tinbergen (pers. comm .) speculates that red preferred in pecking may become effective again in territorial disputes of adult gulls and malefemale encounters. Yet colour preferences in eggrolling and egg-shell removal have been tested in adult black-headed and laughing gulls and were found to differ from pecking preferences of chicks in the same species (Tinbergen et al. 1962 ; Hailman 1966 ; pers . comm.) . These differences may be due to other differences in the stimulus objects presented (e .g . `shape') and to the preferences in egg-rolling and egg-shell removal being modified by experience . An analysis of the factors leading to the differences in preference between `hungry' and `cold' chicks showed that they were due to fooddeprivation rather than chilling . Accordingly the label `cold' is not very appropriate and should (in first approximation) be replaced by `satiated' . The argument that the chilling may not have lasted long enough to be effective is invalidated by the fact that chicks chilled for longer did not approach at all . According to personal observation and Kleitman's review (1963) the activity of very young animals is affected both by the amount and hours of availability of food . `Satiated' animals tend to go to sleep . Concluding from this the state of `satiated' chicks might most adequately be described as `sleepy' or drowsy'. Gwinner (1966) showed that in an experiment song-birds selected the darker of two rooms for sleeping and the hypothesized darkness preferences in `satiated' gull-chicks may appear somewhat more plausible in the light of these results . In conclusion the difference in stimulus select-

ivity may not so much be specific for the particular underlying states of 'hunger-satiation' but may be the result of a difference in activity, i .e. alertness-drowsiness, although food-deprivation may in turn control the animal's selectivity by affecting its state of alertness . This hypothesis might be tested by measuring other behavioural responses of `hungry' and `satiated' chicks like differences in mobility, rate of calling, strength of pecks or responsiveness to strong stimulation . Although this was not systematically recorded, chicks tested in an alarm situation immediately after satiation responded very poorly to the calls, but their responsiveness was high when tested about 15 min later . With respect to `alarmed' chicks it is suggested that in this situation where acoustic stimuli of a high intensity are suddenly impingeing on the animal a mechanism might be operating that would lead to a stimulus reduction or `cut-off' (Chance 1962) . In this context a study on cats may be relevant showing that potentials evoked by acoustic stimuli were larger when the eyes were simultaneously exposed to light (Gerard et al. 1936 ; in Hinde 1966) . Summary The main aim of this study was to investigate the effect of motivation on the stimulus selectivity of the black-headed gull (Larus ridibundus L.) towards a constant stimulus situation . Soon after hatching the young of this species respond to the parent bird in either of two ways ; when `hungry' they beg for food by pecking at its billtip, when satiated but `cold' they seek warmth and shelter by snuggling beneath the parent . From the second day after hatching both activities can be preceded by approach from a distance. At the same time the chicks start responding to the adults' alarm calls by running into cover and crouching there . The present experiments were carried out to see (1) whether `hungry' chicks when approaching objects would show similar or different stimulus preferences than when pecking ; (2) whether `satiated cold' chicks would show similar or different preferences than `hungry' chicks when approaching the same stimulus objects .. In addition stimulus preferences when running into cover as response to alarm-calls were studied. Hand-raised chicks (24 to 36 hr posthatching) were therefore tested (a) several hours since being fed, i .e . `hungry', (b) just after feeding, i .e . `satiated but chilled' and (c) in a situation where alarm-calls were played to them from a tape recorder, i .e. `alarmed' .

IMPEKOVEN : STIMULUS PREFERENCES IN BLACK-HEADED GULL CHICKS

For approach `hungry' and `cold' chicks were tested in standard apparatus with flat cardboard models resembling an adult gull in profile presented two at a time which differed in either of the following dimensions : colour, brightness, background relations or pattern . `Hungry' chicks, upon reaching a model, would usually peck at it, `cold' chicks would snuggle beneath it . In the alarm situation the objects presented were either small rectangular cardboard boxes or gull models and the chicks approached them and then crouched. A subjective brightness order of the stimuli was assessed by computation from the spectral sensitivity curve of the pigeon and the reflectances of the colour papers used . With uniformly coloured models, `hungry' chicks showed similar colour preferences whether pecking at model beaks or approaching entire model gulls . It was concluded that in both situations the colours were discriminated by hue and not merely by their brightness . The preferences of `satiated cold' and `alarmed' chicks differed from those of `hungry' chicks . Thus the results clearly show a shift in preference with changing motivational state . With uniformly grey models, `hungry' chicks showed similar grey preferences whether pecking at model beaks or approaching entire model gulls. `Hungry' and `cold' chicks showed the same preference order when approaching models . With white and black models on backgrounds in different shades of grey, `hungry' chicks showed roughly the same scores of pecking and approach preferences . `Cold' chicks chose the black model more often than did `hungry' chicks suggesting that they responded less to the contrast between stimulus-object and background than `hungry' chicks and to a greater extent to the absolute shade of the object . Tests designed to discover whether food or temperature contributed most to the change in preferences between `hungry warm' and 'satiated cold' chicks showed that the change in preference was more affected by the degree of satiation than chilling . The possible adaptiveness of the preferences was discussed on the basis of some previously described tests and some other tests specially designed to find out about the mutual adaptiveness between the chicks' stimulus selectivity and the characteristics of the parent bird . It is suggested that the change in stimulus selectivity found was possibly not so much response or motivation specific but due to a

269

general change of activity . Acknowledgments I would like to thank Professor J . W . S . Pringle, FRS, for his permission to work in the Department of Zoology, Oxford, Professor N . Tinbergen, FRS, for his encouragement and advice, Drs J. M . Cullen, J. P . Hailman and R. D . Martin for criticizing the manuscript, Professor B .Tschanz, Drs H .J. Croze, M. NortonGriffiths and Miss D. Werner for valuable discussions and practical help in the field . I further want to express my gratitude to Sir William Pennington-Ramsden, Bart . and the Cumberland County Council for permission to work on the Drigg Peninsula near Ravenglass, Cumberland. Financial support was received from the Science Research Council . REFERENCES Baerends, G. P. (1959). The ethological analysis of incubation behaviour. Ibis, 101, 357-368 . Bailey, C . J . & Porter, C . W . (1955) . Relevant cues in drive discrimination in cats . J. comp . physiol. Psycho!., 48, 180-182. Bateson, P . P. G . (1966) . The characteristics and context of imprinting. Biol. Rev., 41, 177-220 . Beer, C . G . (1966) . Incubation and nest-building behaviour of black-headed gulls . V : The posthatching period. Behaviour, 26, 189-214 . Blough, D. S . (1957). Spectral sensitivity in the pigeon. J. Opt . Soc . Amer., 47, 827-833 . Bolles, R. & Petrinovitch, L . A. (1954). A technique for obtaining rapid "drive" discrimination in the rat . J. comp . physiol. Psycho!., 47, 378-380. Broadbent, D. E . (1958) . Perception and Communication . London : Pergamon Press . Chance, M . R . A . (1962) . An interpretation of some agonistic postures ; the role of "cut-off" acts and postures . Symp . zool . Soc. Lond., 8, 71-89 . Crane, J. (1955) . Imaginal behavior of a Trinidad butterfly Heliconius erato hydara Hewitson, with special reference to the social use of colour . Zoologica, N.Y., 40, 167-196 . Curtius, A . (1954) . Ueber angeborene Verhaltensweisen bei Voegeln, insbesondere bei Huehnerkuecken . Z. Tierpsychol ., 11, 94-109. Dawkins, C. R . & Impekoven, M . (1969) . The peck/nopeck decision-maker in the black-headed gull-chick . Anim . Behav., 17, 243-251 . Duecker, G . (1963) . Spontane Bevorzugung arteigener Farben bei Voegeln . Z. Tierpsychol., 20, 43-65 . Franck, D. (1966). Moeglichkeiten zur vergleichenden Analyse ausloesender and richtender Reize mit Hilfe des Attrappenversuchs, ein Vergleich der Successiv- and Simultan-methode . Behaviour, 27, 150-159 . Goethe, F . (1955). Beobachtungen an Aufzuchten von jungen Silbermoewen (Larus argentatus a . B.). Z. Tierpsychol., 12,402-433. Gwinner, E. (1966) . Tagesperiodische Schwankungen der Vorzugshelligkeit bei Voegeln . Z. vergl. Physiol., 52, 370-379 . Hailman, J . P . (1966) . Four color preferences of the laughing gull (Larus atricilla). Am. Zool., 6, 288,



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