Communal feeding of nestlings in the Mexican jay (Aphelocoma ultramarina): Interflock comparisons

Anita. Behav., 1972, 20, 395-403

COMMUNAL FEEDING OF NESTLINGS IN THE MEXICAN JAY

(APHELOCOMA ULTRAMARINA): INTERFLOCK

COMPARISONS

BY J E R R A M L. BROWN

Department of Biology, University of Rochester, Rochester, New York 14627 Abstract. Observations were made on the individuals that fed the nestlings at six nests in four colourbanded flocks of the Mexican jay in Arizona. Communal feeding of the nestlings was found in one flock for the second successive year. Data collected over the entire nestling period show that helpers accounted for 46 to 68 per cent of the feedings at the five nests most intensively studied. At some nests certain individuals brought food to the nestlings more frequently than did the parents. Communal feeding of nestlings by the members of a flock of Mexican jays (Aphelocoma ultramarina) has been demonstrated by use of colourbanded birds in one flock during one breeding season (Brown 1970). The possibility remained, however, that this one occurrence might have been atypical, since pairs have been reported feeding young at some nests (Miller 1932; Brandt 1940; Gross 1949; Marshall 1957). The variability among nests in respect to the importance of helpers (in the sense of Skutch 1961) was still unknown. The object of the present study was to determine the generality and variability of communal feeding of nestlings in this species in the study area through additional observations on a larger number of nests. An attempt was made to gain as complete a picture as possible of the breeding activities of each member of a population of sixty-eight =j: two Mexican jays inhabiting an area of about 180 ha. Six of the nine successful nests in this area were selected for intensive observation, and the role of communal feeding of nestlings was estimated for each.

biologically significant respect different from the unselected ones. The conditions of observation for the six nests watched were as follows. Nest S I was 10 m f r o m the ground and was watched without a blind through a 20 • telescope from a distance of 43 m. Nest S 3 was 10-m high and was watched without a blind through 10 • 50 binoculars at 23 m. Nest A e was 6-m high and was watched without a blind from 46 m through a 20 • telescope. Nest A 3 was 5-m high and was watched from a blind through 7 • 35 binoculars at 13 m. The birds in the Station (S) and Acinom (A) flocks were relatively tame because of the year-round presence of people and because of feeders maintained most of the year. Nest Bry 1, belonging to the Bryce flock, was 2.5 m high and was watched f r o m a blind through a 20 • telescope at 29 m. Nest Brh 1, belonging to the Bruhlman flock, was 6-m high and was watched from a blind through 7 • 35 binoculars from 8 m. The telescope and binoculars were mounted on a tripod. The observer recorded which individuals brought food to the nest and the time of each feeding. The amount and kind of food brought could not be precisely determined. All watches on nestlings were for periods of at least 60 min; most were for about 2 or 3 hr. Nests A2 and A3 were watched for 3-hr periods, one per day between 08.30 and 12.00 hours. Other nests were observed at various times of day. In 1970, unlike 1969, two nests, A2 and A3, were observed simultaneously with the aid of Louis G. Herlin. This allowed direct comparisons between A2 and A3 of food delivery rates, uncomplicated by differences in time of day of observations. Potential observer bias at A2 and A 3 was minimized by alternating observers daily at each nest. In calculating the expected values for the chi-square tests applied to the data for each individual in Tables I and II the total number of feedings at both nests combined by the

Methods The methods used were similar to those of the preceding paper (Brown 1970). The observations on which this paper is based were made at the Southwestern Research Station in the Chiricahua Mountains of southeastern Arizona from 12 March to 17 June, 1970. The vegetation and bird population of a 14.6-ha plot of oak-juniper woodland within the study area and representative of the non-riparian part of the area were described in detail by Balda (1967, 1969). Nests were selected for observation on the basis of ease of observation without disturbing the birds and with preference for nests of flocks with the smallest number of unbanded jays. There was no reason to think the selected nests were in any 395

396

ANIMAL

BEHAVIOUR,

20,

2

Table I. Feeding Visits to Nests $1 and $3 by All Flock Members in 1970 I

I

I

I

~

SI Preincubation 1 Incubation Nestling 33 Per cent 22

---

3 16 10

5 3

1 7 15 10

10 7

17 11

~ --

8 5

--

24 16

9 6

2 1

5 25 16

1 5 59 38

1 3 2

-3 2

2 2 2

-4 3

1 1 1

1 1

8 5

1 6 4

42

59* 11

3

26*

13

3

14"

13

21

13 8

1 1

1 4 23 --

3 392 15 746 176 2587 100

1 2 2

-1 1

3 22 --

1 461 22 925 176 2583 100

12

2

45

352 5170

$3 Preincubation Incubation 2 -1 Nestling 1 24 14 Per cent 1 16 9 $I and $3 Nestling 34* 24* 30

Per cent is calculated from the total for identified visitors ($1: 176-23 = 153; S 3: 176-22 = 154). Year of hatching is given for each individual; 67-: means 1967 or earlier. S1Observation dates. Pre-incubation: 5 days from 26 March to 1 April. Incubation 7 days from 7 to 17 April; estimated incubation period from 3 to 20 J: 2 April. Nestling: 21 days from 21 April to 20 May, post-hatch days 3 to 30; young fledged 19-20 May. $3 Observation days. Pre-incubation: 5 days from 2 to 15 April. Incubation: 9 days from 16 April to 4 May; estimated incubation period from 16 April to 3 ! 1 May. Nestling: 21 days from 5 May to 2 June, post-hatch days 0 to 27; young fledged 1 to 3 June. *Statistically significant preference for either $1 or $3 (chi-square test; P < 0.01). individual was multiplied b y a fraction consisting o f the n u m b e r o f feedings b y all m e m b e r s o f the flock a t that nest d i v i d e d by the n u m b e r o f feedings by all m e m b e r s at b o t h nests combined. Results Co-Operative Feeding of Nestlings The c o - o p e r a t i v e m a n n e r o f delivery o f f o o d to nestlings was c o n f i r m e d at each o f the six nests t h a t were observed in the breeding season o f 1970. F o r five o f these nests the d a t a are r e a s o n a b l y c o m p r e h e n s i v e a n d are shown in Tables I to I I I a n d Figs 1 a n d 2. T h e tables s u m m a r i z e the total n u m b e r o f times each flock m e m b e r was observed delivering f o o d to nestlings (directly o r via the m o t h e r o n the nest). T h e d a t a f r o m a s a m p l i n g o f d a y s t h r o u g h o u t the nestling p e r i o d were p o o l e d for each laest. A t each o f the five nests in Tables I to I I I a n d Figs 1 a n d 2 at least nine different j a y s were seen

participating, a n d at the sixth nest five were observed. I n the S t a t i o n flock every m e m b e r o f the flock (fifteen) was observed to p a r t i c i p a t e in feeding at one o r b o t h nests ($1, $3). All fifteen were seen at S 3, b u t only twelve at S 1. I n the A c i n o m flock every m e m b e r o f the flock (thirteen) b u t one was seen feeding the y o u n g at one o r b o t h o f the successful nests (A2, A3). Eleven flock m e m b e r s were seen feeding nestlings at A2, a n d nine at A 3. T h e one n o n - p a r t i c i p a n t , M Y Y - Y X Y , was n o t seen associating with the flock d u r i n g the principal m o n t h s in which the y o u n g were being fed; it was t r a p p e d in the A c i n o m flock t e r r i t o r y d u r i n g the preceding winter (17 D e c e m b e r 1969) b u t was n o t seen a g a i n a n y w h e r e until 4 A u g u s t 1970, b y which time m o s t o f the feeding o f the y o u n g h a d ended. I n A u g u s t , however, M Y Y - Y X Y was a regular flock m e m b e r . It seems likely t h a t the

BROWN: C O M M U N A L FEEDING IN JAYS

397

Table II. Feeding Visits to N e s t s A2 and A a by All F l o c k M e m b e r s in 1970 o+

o+

G+

r-o

~-

~.

/-

~.

;-

~-

~-

~

2

~

--

1

~

1

4

110

Nestling

2

16

--

6

19

34

1

23

20

~

25

1

7

11

165

2139

12

22

1

15

13

~

16

1

5

~

100

13

~

65

15

~

1

265

5

~

25

6

~

0'4

100

33*

--

90

16

12

430

g

,

A2

Incubation Per cent

1

I0

--

4

A3 Nestling

75

3

3

48

--

41

--

1

Per cent

28

1

1

18

--

16

--

0"4

77*

19"

3

54*

19'

75

~

~

~

3116

A2 and A3 Nestling

1

24*

7*

Per cent is calculated from the total for identified visitors (A2: 1 6 5 - I1 = 154; A3: 2 6 5 - 1 = 264). Year of hatching is given for each individual: 67-: means 1967 or earlier. A2 Observation dates. Incubation: 2 days 23 to 28 April; estimated incubation period from 22 April to 9 or 10 May. Nestling: 12 days from 22 May to 6 June, post-hatch days 11 to 26; young fledged 4 to 6 June. A3 Observation dates. Nestling: 17 days from 21 May to 11 June, post-hatch days 5 to 26; young fledged 10 to 11 June. ~*Statistically significant preference for either A2 or A 3 (chi-square test; P < 0'01).

Table III. Feeding Visits to Nest B r y 1 by All F l o c k M e m b e r s in 1970

,.o

9

Bry 1 Nestling

8

31.

45

37

32

23

13

1

5

18

20

95

30

358

Per cent

2

9

14

11

10

7

4

0.3

2

5

6

29

--

100

4056

Year of hatching is given for each individual; 67- means 1967 or earlier. Observation dates: 14 days from 29 May to 16 June, post-hatch days 8 to 26; incubation from 4 to 21 May; young fledged 15 to 17 June. Per cent is calculated from the total for identified visitors (358 - 30 = 328). Unbanded jays could be identified sometimes by bill colour (W for white mark on bill, B for black bill) and sometimes by behaviour (female stayed long on the nest; male came sometimes while female on nest). It is assumed for the calculations in Figs 2 and 3 that only three unbanded jays were present, since in 1970 this seemed most likely. However, in 1971, at least four unbanded jays were present in this flock. N

398

ANIMAL

BEHAVIOUR,

20,

2

non-participation was due mainly to temporary absence from the flock. In the Bryce flock only one of the two active nests was watched. At least eleven members of the flock (thirteen) were seen feeding the nestlings in the nest Bry 1. The last nest studied, Brh 1 of the Bruhlman flock, was watched only 567 min over 5 days. During this time twenty-three feedings of the nestlings were seen; four were by the mother, MRY-YXR, and nineteen by at least four other jays (eleven by MRR-RXR, two by XOR-ROY, two by XOM-MOY, and four by unidentified jays). A sixth flock member, M M M - M X M , fed a fledgling that had just jumped to the ground under the nest but was n o t seen bringing food to the nest. Isolated pairs of Mexican jays rearing their own young exclusively and without helpers have never been found by me.

agreement with those for the Station flock in 1969, when most of the fourteen individuals showed a preference for one of the two nests (Fig. 1). The presumed father at these nests was identifiable with confidence only at A 3 (MBM-BXM), where he showed a strong preference for his own nest. At S 1 the father was probably XBY-YBY, but he exhibited no preference for feeding the young in S 1 or S 3. At S 3 the father was probably either XRO-ORO or XBY-YBY, but a preference for the young of $3 was shown only by XRO-ORO. The preferences of two of the five jays in the Station flock and three of the seven in the Acinom flock could not be explained through known or presumed parental relationship. There is no apparent explanation for their preferences, but such factors as nest location and unknown social relationships with the mother might be guessed to be important.

Nest Preference by Individuals No individual flock member was ever seen to feed nestlings or fledglings belonging to another flock. Within a flock most individuals delivered food to both nests. In the two flocks in which two active nests were watched it was possible to observe that certain individuals delivered food more frequently to one nest than the other. For such pairs of nests expected values for the number of food deliveries by each individual to a nest were calculated on the null hypothesis that the observed fraction of an individual's effort at each nest would be the same as the fraction of observations for all flock members combined at each nest. (In this way the expected values were corrected for unequal numbers of observation at each nest. If the numbers of observations of all flock members combined at both nests were equal, then the expected numbers of observed food deliveries by an individual at each nest would be equal.) Individuals whose relative contributions at each of the two nests differ at the 0.01 level from their expected values have been marked with an asterisk in Tables I and II and Figs 1 and 2. Restricting the analysis to those individuals who brought food to at least one nest at least seven times, one finds that five of twelve qualified individuals exhibited a preference for a particular nest. At all four nests the bird that incubated the eggs and brooded the young (the mother) confined her attentions nearly exclusively to her own nest (XMY-YMY at $1, X R R - R R Y at $3, MBR-RXB at A2, MOM-OXO at A3). These results are in rough

The Station Flock in 1969 and 1970 For the Station flock it was possible to compare the behaviour of most individuals in 1969 and 1970. Of the fourteen members of the 1969 flock, twelve were stilI present in 1970, and these were joined in 1970 by the yearlings raised by the flock in 1969. One of the 1969 members disappeared from the study area (XYR-RYY), and one (XOY-YOY) became a regular member of the adjacent Roth flock and was not seen again in the territory of the Station flock. The overall impression conveyed by Fig. 1 is that the performance of individual flock members in 1970 differed only slightly from that in 1969. The same two females owned nests, and the proportion of the feedings by each individual in the flock changed rather little. Some individuals (XBOOBO, XRY-YRY, XOO-OOY) showed a preference for a particular nest in one year but not the other, but these changes were small. Two of the older birds of the flock (XYM-MYY, XBMMBM) reduced their percentage contributions. Three of the five younger flock members (class of 1968) increased their contributions.

Delivery Rates Overall rates of delivery of food to the nestlings for the entire nestling period for each nest are shown in Table IV. The size and type of food brought to the nest changed as the nestlings grew. Young nestlings were fed mainly small, soft arthropods or pieces of them; older nestlings were fed larger, coarser arthropods in addition. However, since it was impossible to estimate

BROWN: COMMUNAL FEEDING IN JAYS

399

PERCENTOFFEEDINGSOF NESTLINGSOF XMY-YMY 1969 XRR-RRY XMY-YMY1970 XRR-RRY 30 20 lo o lo,2,o,3,o,4o#o

~o 30 20 . io. . o . ,lo,2o 3o,4,o .

XMY-YMY

*~

xRo oRo

XBY-YBY XBO-OBO XYM-MYY XBM-MBM ................

* l ~

J~ _ C_t_A~A #_ Lt67_- . . . . . . .

~!~.

CLASSr 190,

xRu YRY

XSR- RBR XYO- OYY XO0 - OOY XYY-YYY XOY-YOY

~ I _j~___

~

..........

CHANGEDFLOCKS DISAF~EARED

XYR-RYY "ffE~'C'NGO--TIxRR-'R'R~_ _ _ "C~A'SS'~-Ib6-9. . . . . OXV-VMR XMO-OMY NESTUNGOFIXMY-YMY RX-MO N ~

E ...........

30 20 m o 1o 2o 3o 4o 50 6o ~o'2'o' ~b' o' io' Co' 3'0'4'0 PER CENT N:198

N:218

PERCENT N:153

N:154

Fig. 1. Relative contribution of each flock member to the feeding of the young in each of two nests in the Station flock in 1969 and 1970 (1969 data from Brown 1970). An asterisk designates a statistically significant (P<0.01) preference for delivering food to one of the nests. Year of hatching is indicated where known; jays in class of 1967- were hatched in 1967 or earlier and are of uncertain age. Sample size is indicated at the bottom for each nest. amounts of food brought on a visit, all visits were counted equally, regardless of quality or quantity of food brought per visit. Overall delivery rates ranged from 2.5 to 5.3 feedings per hour. They appeared to increase with later nestings; the broods that fledged earlier had lower rates, those fledging late had the highest rates. The Spearman rank correlation coefficient between feedings per hour and the date of fledging from Table IV, was significant at P < 0.01. This oorrelation might be due to the seasonal warming trend, which could be expected to increase the availability of arthropod prey to the jays. However, because the number of nests is small probably little importance should be attached to this correlation unless it is found again in subsequent years. There was no correla-

tion at the 0.01 level between overall delivery rate and number of nestlings, number of adults, total number of jays in the fock, calculated nestling period, number of helpers, or per cent of deliveries by helpers (Table IV). Delivery rates varied greatly from hour to hour and day to day. Daily variation was studied in the two Acinom nests that were watched by two observers simultaneously at the same time of day (08.30 to 12.00 hours). Contrary to expectations, there were no consistent trends. An inverse relationship between maxim u m temperature and delivery rate was noted in the combined data for A2 and A3 (P < 0.05, sign test). The one exception occurred when after a cold rainy day (27 May) the rates went up for both nests (28 May). Because of the large num-

400

ANIMAL

BEHAVIOUR,

20,

2

PER CENT OF FEEDINGS OF NESTLINGS ,~r: A2 1970 A3 Bry i 1970 20 i0 MBR- RXB MOM- OXO UNBANDED MBM-BXM MMY-MXR MRB-BXR MBY-YXB MMR-YXM MYB-BXY MYY-YXY

10 20 30

20 10 0

"LE of AI

* *

~

A

CLASS of

AB SENT 1967- 1969 V

u

MRM-MXY MRM-MXY

UNBANDED B UNBANDED B 0~ RXY-YYR MXR-RYR MXM-MMR MXY-YBR YXR-MBR ~) YXM-RMR UNBANDED W BXM-MRR RXM-MYR

i 1969 2O t0 0

1O 20 30

PER CENT N:i54

N:264

20 10 0

PERCENT N:328

Fig. 2. Relative contribution of each flock member to the feeding of young in three nests of the Acinom and Bryce flocks. See legend for Fig. 1. The unbanded jays in the Bryce flock were recognized by bill colour and behaviour, when this was possible.

The Role of Helpers

NESTS t969-1970

OWNER

a" 'MATE' OLDER HELPERS YEARLING HELPERS L/

Fig. 3. Summary of roles of helpers and owners at all nests studied extensively in 1969 and 1970. The numbers of helpers of each age class at each nest are shown by numerals placed appropriately. Nests Nt and N2 were in the Station flock in 1969 (Brown 1970). ber o f uncontrolled variables that affect delivery rates a more detailed quantitative analysis o f them has not been attempted.

The contributions o f the parents and helpers are summarized in Table IV and Fig. 3 for the five nests that were watched extensively. A t all six nests watched in 1970 plus two in 1969, the female that incubated the eggs performed a moderate percentage (10 to 28 per cent) o f the feedings o f her young. In the two flocks in which two nests were watched the females fed only their own nestlings or almost so. The contributions o f the male parents are more difficult to establish because male parentage was in no case k n o w n with complete certainty. In only two cases were the male parents k n o w n with a high degree o f probability. These were M B M - B X M at A3 and an unbanded, black-billed jay at Bry 1. At S 1 the most likely candidate was X B Y - Y B Y and at $3, X R O - O R O (or perhaps XBY-YBY). The male parent was completely u n k n o w n at A2, but in order to estimate conservatively the role o f helpers it was assumed that the most frequent feeder at A2 ( M R B - B X R ) was the male parent. When parentage is allocated as above, the

BROWN: COMMUNAL FEEDING IN JAYS

401

Table IV. Comparison of Nests with Respect to Feeding Rates and Role of Helpers

Nest

Feedings per hr

s~

4.1

$3

4"1

A2

4.6

A3

5"1

Bry 1

5"3

Brh 1

2.5

Young left nest 19-20 May 1-2 June 4-6 June 10-11 June 15-17 June 19 4- 1 May

No. nestlings

No. adults in the flock

No. yearlings in the flock

No. helpers at the nest

Per cent Per cent Per cent feedings feedings feedings female male helpers* parent* parent* 9

4-3

12

3

10

22

11

67

4

12

3

13

16

38

46

5

12

1

9

10

22

68

4

12

1

7

28

18

53

5

9

3

9

22

19

59

2

8zb

1+

3

--

--

m

*Percentages of total feedings by identified jays. contributions by helpers are as shown in Table IV and Fig. 3. It is evident that the percentage contribution by helpers is not determined simply by their number; the nest with the largest number of helpers had the lowest percentage contribution by them, and the nest with the highest contribution had the second smallest number of helpers. The table also illustrates that two nests within the same flock area may differ considerably both in the number of helpers and in the percentage contributions of helpers. Figure 3 illustrates the variability among nests in the percentage contributions of owners and helpers in the 1969 and 1970 seasons. In all cases the contribution of helpers is about 50 per cent or greater of the total number of observed deliveries of food to the nestlings. The data for N~ and N 2 are for the Station flock in 1969 (from Brown 1970). A comparison (Fig. 1 and Fig. 3) between 1969 and 1970 for the nests belonging to female X M Y - Y M Y (N1, S1) and to female X R R - R R Y (N2, $2) tentatively suggests that the relative contributions of a female owner and her mate tend to remain the same in successive years; for example, the contribution of XMYY M Y was greater than that of her presumed mate (XBY-YBY or XBO-OBO) in both 1969 and 1970, while the contribution of X R R - R R Y was less than that of her presumed mate (XROORO) in both years. Feeding of the Female before Hatching

Three nests ($1, 53, A2) were watched before hatching for a total of 2634 rain, of which 853min were before incubation began and

1781 min during incubation (Tables 1 and II). The overall rate at which deliveries of food to the female were made before hatching (1.03 feedings per hour) was considerably lower than after hatching (4.1 to 4.6 feedings per hour at the same nests, Table IV). I had hoped that pre-hatch observations would reveal which male was paired with each nestowning female. Unfortunately, this method of identifying the male parent proved to be unreliable, since more than one jay fed the female at every nest at which prolonged observation was made. The female owner was observed to be fed on her nest before hatching by nine individuals at $3, four at S 1, and three at A 2. At none of these three nests did the presumed male mate perform a clearly predominant share of the feedings, although at $3 the two main contenders seemed to have an edge over other individuals in the flock. At a very late neat observed by Hardy (1961) near the Southwestern Research Station in 1956 the female was not seen to be fed by other jays at all until the day of hatching. One breeding female, X M Y - Y M Y , fed another breeding female, X R R - R R Y , both at S 3 and at S1. This unexpected observation can be partly explained with a knowledge of the history of the nests of these females. Rather incomplete evidence suggests that S 1 was built and initially owned in part by female X R R - R R Y ; she was fed on S 1 by female X M Y - Y M Y . Several days later X M Y - Y M Y abandoned her own nest ($2) for unknown reasons, and took over St from X R R - R R Y , who then built $3. The feeding of

402

ANIMAL

BEHAVIOUR,

one female by another on the latter's nest was also seen in an earlier study (Brown 1963).

Discussion The present report establishes that the communal pattern of breeding reported earlier in one flock on the basis of one season of study (Brown 1970) also occurs in neighbouring flocks and persists in the same flock in successive years. The literature gives little support of this point. Various authors with much field experience with Mexican jays have reported pairs feeding nestlings or fledged juveniles, and some of these have reported this pattern to be common or the rule (Miller 1932; Brandt 1940; Gross 1949; Marshall 1957). Occasional observations of the feedings of nestlings or fledglings by three or more birds have also been reported (Brandt 1951; Wagner 1955; Marshall 1957; Hardy 1961). Since no previous author was able to recognize more than a few jays in a flock individually (because colour banding was either not extensive or not used at all) it would have been essentially impossible for them to be sure that only two jays were tending a given nest. In most cases only when three or more jays were present simultaneously would it be possible to establish that more than two jays were involved. The present study has shown that even when the entire flock participates in feeding the young in a particular nest the presence of more than two feeders simultaneously on a nest is unusual. Under these conditions it is hardly surprising that earlier naturalists failed to verify the typical and important role of helpers in feeding the young. The present data strongly suggest that communal feeding of the young is the typical pattern. In no case have I found isolated pairs caring only for their own young. Unfortunately, it is difficult without colour banding to establish the number of jays that tend a given nest; consequently, reliable evidence is not available on this point for most of the range of the species. Nevertheless, throughout its range the species is commonly seen in flocks of about the same size as those in south-eastern Arizona. This mildly supports the concept of communal feeding of nestlings as the norm, but detailed observations on individually identifiable birds in other parts of the range would be desirable. There was no evidence of sub-territories; no areas within any of the flock territories were defended against other members of the" same flock. Although in each flock there were some individuals that brought food only or mainly to

20,

2

one nest, these same individuals could be seen close to the other nest about as often as anywhere else in the territory. Partiality for feeding the young in a particular nest is, of course, not evidence of a sub-territory. Although I plan to treat the evolutionary problem posed by the existence of communal feeding of the young in the Mexican jay in a separate paper, a few points deserve preliminary mention. A correlation between amount of communal aid and fledging success might have been expected, but it is difficult to demonstrate in the present small sample of nests and flocks. Further work on this point is in progress. Since the theory of Hamilton (1964) for the genetical evolution of social behaviour predicts that selection for altruistic traits should occur mainly among close relatives, it is interesting to examine how closely the members of a flock of Mexican jays are related. Because of the delay in achievement of reproductive maturity in the Mexican jay, it has not been possible to observe jays breeding that were banded as nestlings; consequently, direct observation of inbreeding was not possible in 1970. An indication of a high degree of inbreeding within a flock is given by the history of the Station flock from 1969 to 1970 as shown in Fig. 1. There seemed to be a strong tendency for jays of all age classes to remain in the flock from 1969 to 1970. Of the seven jays that fledged in 1969, two died in June 1969, two disappeared in the summer of 1969, and three remained with the flock through their first year to the summer of 1970. Of the seven jays banded as yearlings in 1969, five remained with the flock through 1970, one moved to an adjacent flock, and one disappeared. Of the seven older jays present in 1969 all were present in 1970. No jays from outside the Station flock joined it between 1969 and 1970. If this pattern of jays remaining with the natal flock and excluding outsiders is typical of the species, it must lead to a high degree of inbreeding within a flock and consequently to close genetic relationship among flock members. It thus appears likely that Hamilton's prediction will be fulfilled in this species. Studies on this point are continuing.

Acknowledgments This study was supported by United States Public Health Service Grant M H 16345-02 from the National Institute of Mental Health. I thank Louis G. Herlin for his invaluable help in watching nests A2 and A3. Dana Slaymaker

BROWN: COMMUNAL FEEDING IN JAYS helped greatly by finding s o m e o f the nests. M r s Esther Brown c o n t r i b u t e d observations a t nest Bry 1 a n d helped in n u m e r o u s ways. I would like to t h a n k the A m e r i c a n M u s e u m o f N a t u r a l History a n d M r Vincent R o t h , D i r e c t o r o f the Southwestern Research Station for their cooperation. This study w o u l d n o t have been possible without the generous permission o f the Pugsleys, B r u h l m a n s a n d Bryces to w o r k on their land. REFERENCES

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Hardy, J. W. (1961). Studies in behavior and phylogeny of certain New World jays (Garrulinae). Univ. Kans. Scient. Bull., 42, 13-149. Marshall, J. Y. ,Jr (1957). Birds of the pine-oak woodland in southern Arizona and adjacent Mexico. Pacific Coast Avifauna, No. 32, 1-125. Miller, A. H. (1932). The summer distribution of certain birds in central and northern Arizona. Condor, 34, 96-99. Skutch, A. F. (1961). Helpers among birds. Condor, 63, 198-226. Wagner, H. O. (1955). Bruthelfer unter den V6geln. Veroff. Oberseemus. Bremen. Reihe A, 2, 327-330. (Received 4 May 1971 ; revised 17 July 1971 ; MS. number: A1163)