Microhabitat selection for singing and other behaviour in great tits, Parus major: Some visual and acoustical considerations

Anim. Behav., 1980, 28, 468--475

MICROHABITAT SELECTION FOR SINGING AND OTHER BEHAVIOUR IN GREAT TITS, P A R U S MAJOR: SOME VISUAL AND ACOUSTICAL CONSIDERATIONS BY M A L C O L M L. H U N T E R JR.*

Edward Grey Institute, Department of Zoology, Oxford, OX1 3PS, Great Britain Abstract. Territorial male great tits (Parus major L.) usually forage on the ground, sing from perches with a mean height of 9.3 m, and perform most other activities between 2 and 4 m. Analyses of their habitat's visual and acoustical stratification indicate that great tits sing from high perches to make themselves hard to locate visually rather than to make tbeir songs carry farther. The birds' selection of inconspicuous singing spots is further substantiated by their striking preference for hawthorn trees, which have an extremely dense crown and produce leaves very early in the year. between 30 rain before dawn and noon; the balance occurred between noon and 30 min after sunset. In 1976 and 1977 all the observations were made within 3 h of sunrise. The study area was in Wytham Great Wood near Oxford, England, in a section that is characterized by an overstory of predominantly sycamore (Acer pseudoplatanus L.), oak (Quercus tobur L.), and ash (Fraxinus excelsior L.) and a sparse understory of mainly sycamore with a variety of less common species such as hawthorn (Crataegus spp.). In 1975 observations were confined to a 3.1-ha plot of this woodland that contained all or part of the territories of six individually colour-ringed male great tits. During 1976 and 1977 the study area consisted of a 30m-wide strip along both sides of a 2.3-km path that covered the wood in a rough figure of 8. I estimated that the path crossed about 20 great tit territories. The frequency distribution of different tree species was sampled at 10 randomly selected sites, half in the 1975 plot and half along the 1976-77 path. All the trees larger than 10 cm D.B.H. (diameter at breast height, 1.4 m) were counted in a 30-m-diameter circle; trees with a D.B.H. less than 10 cm but more than 2.5 cm were sampled in a 20-m-diameter circle. There was no significant difference in species composition between the 1975 and 1976-77 plots. In the 1975 study area the heights and approximate crown volumes of all large trees (D.B.H. > 30 cm) were measured; with very few exceptions, these trees formed the canopy. Observations were made while walking at a slow pace with frequent short steps along a regular route, but whenever I heard a great tit singing I went directly to it and resumed observations

Although there is a wealth of information on habitat selection by animals, only a few types of microhabitat selection have been investigated (Partridge 1978). The tendency of many animals to forage in a well-defined microhabitat is the best known of these (Gibb 1954; MacArthur 1958). The location of nests, roosts, and hibernacula has also been studied (Moore 1945; Collias 1964; Daan &Wichers 1968). These all represent relatively long-term investments in a specific site and are often characterized by the importance of avoiding predation and finding a benign microclimate. Short-term selection of microclimate is a very common method of behavioural thermoregulation, particularly among poikilotherms (Rand 1964; Grubb 1975). Although a few studies have described how an animal distributes its daily time budget among various types of behaviour, there seems to be no study that details how these different activities are distributed among microhabitats. This paper describes the perch height and tree species selected by male great tits (Parus major L.) while performing various activities during the prebreeding territorial period. I examine some visual and acoustical features of different forest strata, question the sufficiency of the conventional idea that birds sing from high perches to be heard farther away, and suggest that visual features also affect the choice of perches. Methods

Observations were made between 21 February and 27 April in 1975, 1976, and 1977 and are based on 343, 55 and 45 h of observation respectively. In 1975 about 85 % of the observation time and 93 % of the number of observations occurred *Present address: School of Forest Resources, University of Maine, Orono, Maine 04469, U.S.A. 468

HUNTER: MICROHABITAT SELECTION BY GREAT TITS from there. Observed behaviour was assigned to one of the following categories: Forage: Actively searching for food. Feed: Handling food; this behaviour was usually long enough to record only when i t involved breaking a nut. Flush: The bird flew up to a perch from the ground. Inactive: No overt behaviour except for small movements of the head, wings, and legs. An observation had to last at least 60 s or be preceded by a move of greater than 1 m in order to be rerecorded. Aggression: Display postures, vocalizations, and fighting in an interaction with another male or another species. Reproduction: All interactions with females; these were recorded too seldom for a suitable sample size and are not considered further. Preen: Manipulating feathers with beak or feet and/or ruttting feathers. Preening lasting less than 2 s was not counted. Call: Any vocalization that is not a song or is not made in an aggressive context. Sing: Loud vocalization of the male comprising a phrase of notes repeated rapidly at regular intervals. When I located a bird I recorded the first thing I saw it doing and then walked on. I f before I had completed my notes (ca. 20 s) it performed a different activity I recorded this as well. Consequently I sometimes recorded as many as four observations during one encounter with a bird, but I never recorded the same activity twice in the same encounter. This procedure assured the independence of observations of the same type of activity and avoided masking the interdependence between different activities. These constraints mean that the percentage of observations are a biased estimate of the birds' time budgets; they overestimate the importance of both frequently performed activities of short duration and conspicuous activities. Defining the transition from one activity to the next was easy except for Inactive, which had to be rather arbitrarily defined. For each observation I recorded the

469

species of tree, the height of the bird's perch, and the height of the tree to the nearest metre. Substantial heights were measured with a clinometer; heights less than about 6 m were estimated. All of the large trees (D,B.H. > 30 cm) on the 1975 pl0t were labelled and their heights known. The birds were not equally observable in all microhabitats. The consequences of this bias will be discussed in the appropriate section below. To better understand how visual communication might affect microhabitat selection I constructed a visual density profile of the study area by systematically 'estimating how much of a white board was obscured by branches at different heights. The method is only superficially simil a r to that used by MacArthur & MacArthur (1961) and thus warrants a detailed description here. The board was a 25 • 25-cm sheet of semirigid plastic (DARVIC) gridded into 25 5 • 5cm squares by black tape and attached to interlocking aluminium poles with an adjustable 90 ~ clamp. At 10 randomly chosen sites the board was placed at Board Heights of 0, 1.6, 3.2, 4.8, 6.4, 8.0, 9.6 and 11.2 m. At each height the board was viewed from four points, 10 m north, east, south and west of the pole. Sightings were made from Viewing Heights of 0, 2 and 4 m ; the 0 m sightings were made with my head resting sideways on the ground using my upper eye; a stepladder was used to make the 2 and 4 m sightings. I also made sightings from any climbable trees that were between 9 and 11 m from the poles; these Viewing Heights ranged from 0 to 12 m at 2-m intervals. By twisting the poles and adjusting the 90 ~ clamp, I changed the orientation o f the board in relation to both the direction of the viewing site and the difference between my height and the board height, in order to maintain a right angle between the line of sight and the plane of the board. In total 740 sightings were made. A sighting consisted of assigning each of the 25 squares to one of three categories: A. Open

B. Partial

Less than one-third of the square covered and with no twigs crossing the square through opposite sides. Less than two-thirds and more than one-third of the square covered and/or one or two twigs crossing the square.

470

ANIMAL

BEHAVIOUR,

C. Covered

Over two-thirds of the square covered and/or three or more twigs crossing the square. For each sighting an index was calculated as the number of category A squares plus half the number of B squares. Thus index values could range from 0 for a completely obscured board up to 25 for maximum visibility. Because small changes in the relative position of the viewer and board could easily affect the score, I fixed the position of my head before looking at the board and used only one eye. Tests were conducted only on calm days before leaf emergence. For each different Viewing Height all the index scores from the 10 sites and from the eight Board Heights were averaged to give an overall index of how good the view was from that height. Similarly, for each different Board Height all the scores for the 10 sites and seven Viewing Heights were averaged to indicate how visible the board was at that height. Using a different method, visual density measurements were made of the crown of two species, sycamore and hawthorn. Photographs were taken directly up through the crowns of four randomly selected trees of each species from points 1 m from their trunk and 1.5 m above the ground. The 35-mm negatives were enlarged by a factor of 12, and the density of branches was estimated five times on each print by randomly placing a 1 cm • 1 cm cross and counting the number of branches that crossed each of the four radial lines. I f the cross was located over a large branch, i.e. the width of the branch's image was greater than 1 cm, no score was read and another position was used. Thus the index is a measure of the density of twigs and small branches and not of the trunk and large branches. Other environmental measurements included the time of leaf emergence for different tree Table I. Mean Height (m) of Different Activities

Forage Flush Feed Inactive Aggression Call Preen Sing

X

SE

N

0.8 2.2a* 2.2a 3.5b 4.0bc 4.4c 6.0 9.3

0.1 0.1 0.3 0.2 0.2 0.2 0.7 0.3

(316) (133) (41) (173) (200) (373) (24) (334)

*Means were compared with a Newman-Keuls test for multiple comparison of ranked means: values followed by the same letter are not significantly different (P -0.05).

28,

2

species to the nearest week and the temperature at 15 cm, 2 m, 6 m and 12 m above the ground recorded with a thermograph. Results Height Selection

There was considerable variation in the mean heights at which various activities were performed (Table I, Fig. 1). It is obvious that the birds did the vast majority of their Foraging on the ground. When the distribution of Flush, Feed, Inactive, Aggression, Call and Preen are plotted together there is a strongly skewed distribution with most observations located just above the ground. The validity of pooling these data for this purpose was confirmed with a Z 2 test. However, a Newman-Keuls multiple comparison of the mean heights of these activities does show some significant variation. Much of this variation can be attributed to the association between different activities. Flushing and Feeding were almost always preceded by Foraging and hence occurred quite close to the ground. Most Preening observations occurred during breaks in song bouts, and Aggressive encounters often preceded or followed Singing. Singing occurred at distinctly greater heights, with a slight skew toward lower heights around a 9.3 m mean. The height distribution was almost certainly not the consequence of branch availability. Figure 2 shows that when perch height is expressed as a percentage of perch tree height, most activities were quite uniformly distributed, implying that the birds could easily move higher or lower. In contrast the Singing distribution was very skewed toward the tops o f selected trees. This is probably not due to a lack of branches lower in the tree; the birds obviously found many low branches on which to Call, Preen, etc. The large number of singing observations that occurred near the tops of perch trees may seem to imply that the maximum height at which birds sing was limited by tree height, but this is probably also untrue. Table II shows that in most tree species the singing perch height averaged less than three-fourths of the perch tree height; moreover the trees selected by the birds to sing in were not the tallest available. Large trees (D.B.H. 30 era) formed a,canopy that covered virtually the entire study area, and the average height of these large trees was significantly greater than the height of song perch trees of the same species (ttest, P < 0.01).

H U N T E R : M I C R O H A B I T A T SELECTION BY G R E A T TITS

471

24 OTHER

FORAGING

22

SINGING

BEHAVIOUR 20 18 16 14

~I (9

lO 8

61

,i

oi

,

i 20

i

, 40

,

~ 60

,

,I,

, 80

t

i 20

~

OF

.

40

i 20

i

t 40

OBSERVATIONS

Fig. 1. Frequency distribution of different activities at various heights from the ground (GR) upward at 2-m intervals. Other behaviour includes Feed, Flush, Call, Inactive, Preen and Aggression.

Perhaps the most obvious reason for birds singing from a relatively high perch is to be heard farther away. However, data from Marten & Marler's (1977) tests on sound propagation in temperate forests indicate that absorption of sound by the ground is likely to be very severe only for a bird singing on or near the ground. Singing at 10 m above the ground is unlikely to be more acoustically advantageous than singing at 2 m (Fig. 3). Predictions of a mathematical model (Roberts et al. 1979) based on sound interference patterns and atmospheric attenuation confirm the height effect described by Marten & Marler. One limitation of Marten & Marler's data is that they measured attenuation between a signaller and receiver at the same height. To avoid this limitation I have used our model to predict what acoustic advantage, if any, a great tit singing at 4 m (the average call height) would gain if it chose to sing at 10 m, assuming it wanted to be heard by an intruder at 4 m. The receiver height of 4 m was chosen because most aggressive encounters occurred at this height. Assume the bird sang a 90-dB song (see Brackenbury (1978, page 249); the absolute value does not affect the conclusion) with power at 3, 3.5, 3.75, 4, 4.25, 4,5 and 5 kHz (chosen to represent a great tit song) on a 0 C, 90 % R H day. Wind and vertical gradients of temperature are ignored; on my study area wind was usually negligible below the canopy in the morning; the temperature difference between 2 and 12 m was never more than 0.5 C and was

usually less than the sensitivity of my instrument (0.2 C) (see Henwood & Fabrick 1979). At 100 m distance the sound pressure level heard 4 m above the ground would be 40.9 dB if the song was sung at 4 m and 39.2 dB if sung at 10 m. For this set of assumptions it is slightly disadvantageous to sing from a high perch; for a wide range of assumptions--signaller-receiver distance and heights are the important ones--high perches offer no advantage. Figure 3 might be thought of as a vertical profile of the forest's 'acoustic impedance'; Fig. 4 depicts a vertical profile of the 'visual impedance' of the study area. Figure 4a gives an index of how good a view ! had from different heights of the OTHER 8EHAVIOUR

FORAGING lOO - '~1

SINGING

40

o~~ 20

o

20.

20

9o ~-/o

0/0 OF OBSERVATIONS

Fig. 2. Distribution of activities at different heights expressed as a percentage of the tree's height. Ground observations were not included. See Fig. 1 for list of other behaviour.

472

ANIMAL

BEHAVIOUR,

28,

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Table H. Heights (m) of Perches and Trees Used for Singing in Relation to Each Other and the Heights of Trees Forming the Forest Canopy Sycamore SE

Oak N

X

Ash

SE

N

R

SE

Hawthorn N

X

Canopy trees

19.1 • 0.3 (83)a*

17.5 • 0.4 (56)b

22.2 • 1.3 (12)

Song trees

13.0 • 0.4 (170)a

16.1 -E 0.5 (38)b

17.0 • 1.2 (24)b

Song perches

9.1 • 0.4 (170)a

11.1 :k 0.6 (38)b

12.5 • 1.1 (24)b

Song perches as ~ of song tree heights

70.0 zk 1.9 (170)a

70.2 • 4.0 (38)ab

74.2 zk 4.0 (24)ab

SE

Other species

N

X

(0)

SE

N

18.0 • 1.6 ( 5)ab

10.2 • 0.3 (56)

13.2 :k 1.2 (29)a

8.2 4- 0.3 (56)a

8.1 -4- 0.9 (29)a

80.7 • 2.0 (56)b

65.2 • 4.2 (29)a

*Variation among species was tested with a Newman-Keuls test. Reading horizontally, means followed by the same letter are not significantly different (P = 0.05).

10

ii

A

'BEING SEEN'

'SEEING'

+

+

E

--it

v

"1-

5

m1o

(.9

12

14

is

18 VISIBILITY

itl "1-

~o

~z

14

~6

18

INDEX

Fig. 4. A visual density profile of t h e forest. Large values of the visual index indicate an open view. Fig. 4A shows how well I could s e e a target board from seven Viewing Heights averaged over all eight Board Heights. Fig. 4B shows how well the board could b e s e e n at eight Board Heights averaged over all the Viewing Heights. Horizontal lines are • 1 SE.

1

0 -0

i -5

0

ATTENUATION INDEX

5

10

(dB/lOOm)

Fig. 3. An index of the effect of height on attenuation redrawn from Marten & Marler (1977). The data were obtained by broadcasting sounds from a loudspeaker at different heights and rerecording them at the same height 2.5 and 102.5 m away. Attenuation was calculated from the difference in signal power at these two distances. Attenuation averaged from nine sites and 25 frequencies is shown by a solid line. The dashed line is from data more directly applicable to the great tit situation, four leafless deciduous forests and 10 frequencies between 2 and 6 kHz.

board described in Methods. The high visual index values, indicating a good view, occurred at 2, 4 and 6 m and fell off above and below this open zone. Similarly Fig. 4b shows that the board was most visible when it was in this same zone

above the ground vegetation and below the canopy. The profiles have the same shape when only views with a line of sight close to horizontal (difference between height < 2 m) - are plotted. Both the height with the best view of the ground and the height most easily seen from the ground were quite high in the open zone, 6 m and 4.8 m respectively. Comparison of these data with the activity profiles indicates that most types of activities took place in a visually open zone, but the greater height of song perches would probably tend to place them in a visually dense stratum.

Tree Species Selection There is a considerable discrepancy between the frequency with which different tree species occurred in the study area and the extent to which they were used by great tits (Table III).

HUNTER: MICROHABITAT SELECTION BY GREAT TITS

I oO

<

./

c c~

"-

N/

< O

O

,.el

O

Z flS O O

.<

I

O

Za,

Iii

9

e~ o

uq

< O >o9

Hunter, Anita. Behav., 28, 2

HUNTER: MICROHABITAT SELECTION BY GREAT TITS The most prominent difference is that hawthorns were used about 16 times more than would be expected by their abundance; 'other species' were also over-utilized and sycamores were distinctly under-used. The heavy use of hawthorns was primarily due to their selection for singing sites and to a lesser extent for activities associated with singing, i.e. preening and aggression. The tree sample was based on trees with a D.B.H. greater than 10 cm because the great tits did not often use smaller trees. I also sampled trees with a D.B.H. between 2.5 and 10 cm. When they are included the percentage frequency distribution becomes: sycamore 83.3, oak 3.7, ash 1.6, hawthorn 0.5, and other species 10.9, and thus strengthens the discrepancy. Furthermore, hawthorns represent only about 0.6% of the total crown volume of trees available. Figure 5 (Plate I) shows the difference between sycamores and hawthorns that might explain their relative use by great tits: the density of twigs and branches. The visual density index I used gave a mean :t: SE score of 14.0 :t: 0.2 for hawthorn and 5.5 :k 0.3 for sycamore (t-test, P < 0.001), indicating that a great tit perched in a hawthorn would be far less visible than one in a sycamore. It is very unlikely that hawthorns are preferred simply because they provide more suitable-sized twigs on which to perch: both hawthorns and sycamores probably provide a superabundance of suitable twigs. Hawthorns were also the first tree species to produce leaves in the spring. Thus there was a , two- to three-week period when the extent to ' which hawthorns were visually denser than other species was considerably enhanced by their being

473

the only species in leaf. During this period the usage of hawthorns for singing was three to four times greater than that during the periods before the hawthorns had produced leaves and after other species had produced leaves as well (Table IV), This switch did not occur because great tits started foliage-gleaning in the hawthorns: only four times were great tits observed foraging in hawthorns, twice before leaf emergence and twice after. The birds were not equally observable to me in all microhabitats but the effect was not strong; moreover it tends to strengthen my interpretation of the results because I found a disproportionate use of visually dense strata. For example, Foraging, a relatively inconspicuous activity, was recorded as occurring almost exclusively on the ground, the most visually opaque stratum. Discussion

Foraging and singing are probably the two most important activities of territorial male great tits in February, March and April. This paper confirms their very strong tendency to forage on the ground during this season as previously reported (Hartley 1953; Gibb 1954; Lack 1971). One assumes that great tits forage on the ground because they find food most readily there. Relatively seldom were other types of behaviour performed on the ground. There are at least four possible reasons why great tits perched at 2 to 4 m during most activities. Avoiding ground predators, notably weasels (Mustela nivalis L.), is the most obvious. Being above the ground might also increase the chances of escaping from

Table HI. Frequency Distribution of Activities in Different Tree Species

Other Hawthorn species

Sycamore

Oak

Ash

Forage Flush Feed Rest Aggression Call Preen Sing

56.5 64.8 64.7 58.5 59.2 60.8 22.7 54.1

23.9 7.4 2.9 11.3 2.9 7.4 4.5 11.9

4.3 4.1 2.9 2.5 2.9 5.4 0.0 7.5

8.7 7.4 5.9 6.9 19.5 9.1 54.5 17.5

6.3 16.4 23.5 20.8 15.5 17.3 18.2 9.1

46 122 34 159 174 352 22 320

All behaviour Tree availability XZ

58.2 73.8 22.10I"

8,9 11.8 2.17

4.9 3.4 1.05

13.0 0.8 33.63

15.1 10.3 4.07

1229 263

***

*

***

NS

NS

N

tThe overall pattern of use was compared to the availability of different species with a Xa value derived from a 2 • 2 contingency table in the form [of Species A-Not Species A by Use-Availability (*, P < 0.05; ***, P < 0.005; NS, not significant)~

474

ANIMAL

BEHAVIOUR,

an avian predator because the number of potential escape routes is approximately doubled, i.e. a hemisphere of escape routes becomes a sphere. The better view from a perch would facilitate seeing predators and conspecifics. Finally, a bird is more conspicuous above the ground; although this might be advantageous for interacting with conspecifics it would probably increase the vulnerability to predators. The visibility factor would be modified by the tendency of great tits to select hawthorns for other activities besides singing. The tendency for birds to sing high above the ground has been recorded by Lemon & Herzog (1969), Lack (1971 ), Zimmerman (1971 ), Scherrer (1972), Wiens (1973), and Harrison (1977). Why do great tits sing from high perches? Since they spend most of their time on or near the ground, they are obviously choosing to sing high; it must cost some energy to fly up to these perches. The obvious answer, avoiding acoustic attenuation, is belied by (1) Marten & Marler's (1977) data, which show little reduction in attenuation above 2 m, and (2) our model (Roberts et al. 1979), which predicts that a male great tit singing at 10 m will not sound louder to a listener 4 m above the ground 100 m away than a male singing at 4 m. (Both Jilka & Leisler (1974) and Ficken & Ficken (1962) have sought to explain interspecific differences in the acoustic characteristics of songs in terms of the constraints imposed by different song perch heights, but both rely upon circumstantial evidence.) It seems likely that the mean song perch height is high because (a) the birds are singing above the

28,

2

2 to 6 m visually open zone, i.e. they are trying to be inconspicuous while singing, and (b) they cannot sing below this zone because of ground attenuation. This idea is strongly supported by their marked preference for hawthorns, especially when hawthorns are the only trees in leaf. The abundant branches of a hawthorn are almost certainly very good at camouflaging a great tit by disrupting its outline. It is not possible with the present data to use multivariate statistics to show which f a c t o r - height, branch density, or time of leaf emergence-contributes most to a singing male's inconspicuousness. However, it is easy to show that the effects are largely independent. First, great tits obviously do not sing on high perches as a consequence of choosing hawthorns, since their mean perch height in hawthorns is relatively low. Conversely, they cannot be choosing hawthorns in order to sing on high perches. It seems more likely that one of the reasons great tits prefer hawthorns is that they provide a safe perch at a relatively low height and thus decrease the height to which the birds have to fly. The choice of hawthorns is also not due solely to their early leaf emergence; selection of hawthorns is also very strong before and after the period when hawthorns are the only trees in leaf. Why be inconspicuous while singing? It is estimated that about 15 % of the adult great tits in Wytham Woods are captured by sparrowhawks (Accipiter nisus L.) during the breeding season (Geer personal communication). Singing is likely to increase a bird's conspicuousness and hence vulnerability, so it is not unlikely

Table IV. Usage of Different Tree Species Compiled for Three Categories of Behaviour during Three Periods Defined by Leaf Phenology

Percent frequency distribution Singing:

Before hawthorn leaves

Before sycamore leaves After sycamore and after hawthorn leaves leaves

N

Sycamore Hawthorn Oak, Ash, and other species

52.1 13.7 34.2

Ns *** NS

40.6 40.6 18.8

* *** NS

59.6 10.9 29.5

173 56 91

Foraging: Sycamore Hawthorn Oak, Ash, and other species

25.0 16.7 58.3

NS NS NS

43.7 12.5 43.8

* N$ NS

88.9 0.0 11.1

26 4 16

58.8 12.6 28.6

NS NS Ns

56.6 13.5 29.9

* * NS

65.9 7.6 26.5

516 100 247

Other behaviour:

Sycamore Hawthorn Oak, Ash, and other species

Differences between periods were compared for each category with a ~2 value derived from a 2 • 2 contingency table in the form of Species A-Not species A by Period X-Period Y (*P < 0.05; ***P < 0.005; NS, not significant).

HUNTER: MICROHABITAT SELECTION BY GREAT TITS t h a t they should choose perches that ameliorate this effect. The m a t t e d branches a n d t h o r n s o f a h a w t h o r n are also likely to shield a bird f r o m attack. However, if this was the sole reason for using h a w t h o r n s the leaf emergence effect w o u l d n o t exist. PuUiam & Mills (1977) have shown t h a t there are interspecific differences in the distances grassland sparrows forage away f r o m cover which m a y be d e t e r m i n e d b y differences in susceptibility to predation. A n o t h e r possible reason for r e m a i n i n g inconspicuous while singing involves the Beau Geste hypothesis for song repertoires (Krebs 1977; K r e b s et al. 1978). This hypothesis maintains t h a t individual great tits sing a n u m b e r o f different songs to deceive potential intruders into thinking that the a r e a is already s a t u r a t e d with territorial males. The facade is unlikely to w o r k if intruders can see t h a t just one bird is singing several songs. Thus i f the Beau Geste effect exists, great tits could be expected to hide while singing. M a n y bird species seem to choose particularly visible song perches on treetops o r fenceposts. However, this observation m a y be biased a n d atypical because people usually see singing birds in gardens a n d other open places where avian p r e d a t o r s are u n c o m m o n or easily detected from a distance. I n this situation the benefits o f visual c o m m u n i c a t i o n m a y outweigh the costs o f conspicuousness.

Acknowledgments R o s a m u n d A n n e t t , N i c k Davies, M a r t i n G a r n e t t , a n d A n d e r s Kiessling helped to measure the h a b i t a t parameters. N i c k Davies, Alex Kacelnik, J o h n Krebs, K e n M a r t e n , C h r i s t o p h e r Perrins, a n d R o n Pulliam c o m m e n t e d on the manuscript. J o h n R o b e r t s calculated the acoustic a t t e n u a t i o n results; M a x i n e H o r n e a n d Maggie N o r r i s t y p e d the manuscript. I was s u p p o r t e d b y a R h o d e s Scholarship.

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