Colonial Breeding in the Barn Swallow (Hirundo Rustica) and Its Adaptive Significance

COLONIAL BREEDING IN THE BARN SWALLOW (HIRUNDO RUSTICA) AND ITS ADAPTIVE SIGNIFICANCE

BARBARA DENNISTON SNAPP

Social patterns in the swallow family (Hirun- whether active interactions among the breed- dinidae) involve varying degrees of gre- ing pairs allow the development of a Barn gariousness during the breeding season. Some Swallow colony, or whether such aggregations species, such as the Bank Swallow (Riparia are the result of a passive congregation at a TipaTia) and the Cliff Swallow (Petrochelidon nesting site. The presence or absence of such pyrrhonota), breed in dense colonies of up interactions may be inferred from an exami- to several hundred pairs. Other species, like nation of the variation in within-colony breed- the Tree Swallow (ITidopTocne bicoloT) and ing synchrony and occurrence of second- the Rough-winged Swallow (Stelgidopteryx brooding, as well as by direct observations of Tuficollis), are solitary breeders. A third pat- the social behavior involved in predator de- tern of social behavior is that of breeding in fense, foraging, and nest site selection. Last, small colonies of variable size. The Barn Swal- I attempted to relate the observations for Barn low (HiTundo mcstica) is such a species, found Swallows to those for other swallow species in nesting alone and in aggregations of 2 to 40 order to explain the rise of coloniality in the or more pairs ( Bent 1942). family as a whole. This diversity of social breeding habits is the result of adaptation to a number of en- METHODS viromnental pressures ( Crook 1965). These pressures may involve habitat and food type, I conducted the present investigation in Tompkins County, New York during the breeding seasons of dispersion of food supplies, and availability 1970 and 1971. The study included 34 colonies (table and nature of nest sites. The latter is particu- l), most of them in barns or sheds. In all but one larly important in elucidating the contrasting case the barns were abandoned. No persistent local types of protection offered against predation, insect populations could arise due to the presence of domestic animals, and all foraging had to take place including both inaccessibility and crypticity of away from the nest sites themselves. nest sites. The colonies ranged in number from 1 to 30 nests, Mayr and Bond (1943) pointed out that the and I arbitrarily distinguished 5 size categories: a) development of coloniality in the swallow single: one pair, b) small: 2-5 pairs, c) medium: family paralleled the development of the abil- 69 pairs, d) large: lo-13 pairs, e) x-large: more than 13 pairs, in this study 1730 pairs. ity to build nests. Burrow-excavators of the I visited all colonies 2 or 3 times per week, from genus RipaTia and mud-nest builders of the mid-May to late August. I recorded date of hatch, genus PetTochelidon often breed in large colo- clutch size, brood size, and number of fledglings for nies. On the other hand, the genera that utilize both first and second broods. I weighed nestlings with a 30-gram Pesola ( Ruti-Zurich, Switzerland) naturally-occurring cavities for nest sites, e.g., spring balance (& 0.25 g) 3 times per week, from IridopTocne and Stelgidopteyx, breed in a the first or second day after hatching in 1970 and dispersed fashion, perhaps because of the er- from the 8th or 9th day in 1971 (day of hatching is ratic distribution of such cavities. defined here as day 1). As a rule, I did not weigh nestlings after the 16th day, since they were quite Why has coloniality developed in nest-build- restless and often fledged prematurely. ing genera? At least 4 factors, singly or in Variances within and among colony sizes with re- combination, could have contributed to the spect to all data collected usually were not homo- evolution of colonial breeding habits: reduc- geneous and did not allow the use of parametric sta- tion in loss to predators, increased foraging tistics. Therefore, I used nonparametric tests for most analyses. The Kruskal-Wallis test provided a method efficiency, increased reproductive success for comparing colony size means where ranking was through social stimultation, and increased utili- possible. The Spearman rank correlation ( rs) test indi- zation of available nest sites. In order to un- cated the degree of correlation between two co- derstand the phenomenon of coloniality in varying variables. In cases where ranking was im- practical, I used binomial and multinomial chi-square swallows and in Barn Swallows in particular, statistics. I undertook three analyses. First, I sought to determine how each of the 4 factors men- RESULTS tioned above varies with Barn Swallow colony size, utilizing data concerned with growth SEASONAL REPRODUCTIVE OUTPUT and survival of young and responses to preda- Some measure of the relative fitness of pairs tors and food supply. Second, I investigated breeding in colonies of different sizes is needed

14711 The Condor 783471480, 1976 472 BARBARA DENNISTON SNAPP

TABLE 1. Distribution of Barn Swallow colony the potential number of young that can be sizes studied in Tompkins County, New York, in produced in a season, such as variation in 1970 and 1971. clutch size, occurrence of second-brooding and

Me- X- replacement-brooding, and date of hatch. Sin& Small dium Large Large‘ Total

1970 6 3 3 3 1 16 SOURCES OF ’ MORTALITY 1971 Whole-clutch mortality. The major source of Undisturbed 12 6 4 3 3 28 Experimental” 1 2 3 mortality in both years consisted of whole- 1970 & 1971b 4 3 2 3 1 13 clutch loss of eggs and young due to suspected predation and abandonment (Table 2). This Total 18 9 7 7 6 47 mortality represents a loss of 12.5% (218/ ~1Data from experimental colonies are considered only in 1750) of the eggs laid over the two seasons, analysis of timing of reproduction. “Number of colonies included in the study during both 17.2% (318/1850 eggs) if losses due to cats seasons. and humans are included. These figures cor- respond to an average loss of 0.6 and 0.8 to assess the benefits or disadvantages of young per pair, respectively. group nesting for the Barn Swallow. Seasonal The magnitude of losses due to suspected reproductive output of an average pair in predation and abandonment was not affected each colony is an appropriate measure of rela- in any consistent or significant manner by vari- tive fitness and is expressed as the number of ation in colony size and ranged from 9.5% to young fledged per breeding pair per year, 15.6% of the eggs laid (0.4 to 0.7 young per the number of pairs being equal to the num- pair). Whole-clutch mortality in colonies of ber that initiated a first brood. Reproductive all sizes was equally distributed between loss output did not vary consistently or signifi- of eggs (5.6%, 98/1750) and loss of young cantly with colony size over the 2 years of the (6.9%, 120/1750). The size of the colony also study ( r, = 0.226). The reproductive output had no effect on the ratio of abandoned to averaged 5.6 young (r = 2.7-8.0 young) cor- plundered nests. A total of 146 eggs and young responding to a seasonal reproductive success (7.7%) in 33/376 nests were destroyed by un- of 78.5% (r = 57-94%; 100% x number of known agents, probably birds or rodents. The young fledged/number of eggs laid) and was remaining 76 eggs and young (4.1%) perished similar in both years. after abandonment of 17nests.’ Abandonment The lack of consistent differences in repro- accompanied or closely followed predation in ductive output of different colony sizes sug- the same colony in lo/17 cases, suggesting that gests that Barn Swallows derive no benefits or the destructive agents also caused the aban- disadvantages from nesting in colonies. How- donment. ever, the failure to discern dramatic differ- Many colonial birds will mob potential pred- ences in reproductive output does not preclude ators, and the Barn Swallow is said to be an the existence of more subtle and perhaps con- aggressive mobber in comparison to other co- flicting effects of group nesting. In examining lonial swallows (Bent 1942, Lind 1964). I the data, I will consider two sources of varia- tion in reproductive output. One deals with have observed mobbing by Barn Swallows on the relative importance of different sources of several occasions. Two of the attacks involved mortality, e.g., predation, failure to hatch, lack prevention of Starling (Sturnus vulgaris) of food, and probability of post-fledging sur- predation, and the swallows successfully drove vival. The second concerns factors influencing off the Starlings. Three other occasions in-

TABLE 2. Barn Swallow mortality attributable to failure to hatch, whole-clutch loss, and partial-clutch loss for all colony sizes.

Total Colony Nests Whole-clutch Failure to Pago”-;luFh size (eggs 1 mortality5 hatch Eggs Young

Single 27( 127) 9.5%oh‘ 13.4% 2.4% 19.7% - 5.5% Small 47( 218) 13.3% 6.4% 1.8% 11.0% 10.6% Medium 79( 366) 15.6% 2.5% 3.3% 9.8% 11.5% Large 107(498) 13.3% 3.8% 3.6% 9.8% 10.9% X-large 116(541) 10.0% 5.9% 4.1% 12.4% 7.6% Total 376( 1750) 12.5% 5.2% 3.4% 11.5% 9.5%

RNests destroyed by cats and humans not included. 1,Percentages based on the total number of eggs laid in each colony size. COLONIAL BREEDING IN BARN SWALLOWS 473

volved mammals (2 cats and a woodchuck), unable to measure post-fledging mortality per and the mobbing did not appear to disturb se, but I did collect data concerning the gen- the mammals greatly. It is unlikely that mob- eral health of older nestlings that might be of bing is more effective in large Barn Swallow use in estimating the probability of post-fledg- colonies than in small ones, although a con- ing survival. clusion on this matter must await further in- Differences in nestling growth curves among vestigation. On the occasions that I observed the colony sizes were slight. The curves mobbing, the only pairs that responded to the reached peak weights of 20-23 g on the 13th presence of the potential predator were those or 14th day after hatching. Subsequent to this possessing nests in the immediate vicinity of period, the weights began to taper off, as has the danger. I observed no colony-wide re- been reported by other investigators (e.g., sponse other than an increase in alarm calls. Stoner 1935, de Braey 1946, Nitecki 1964, Kuz- Failure to hutch. Failure of part of a clutch niak 1967, Ricklefs 1967, 1968a, 1968b, George to hatch ranked second in importance as a and Al-Rawy 1970). Variation in the growth source of mortality, accounting for the loss of curves did not begin to appear until after the 5.2% (91/1750) of the eggs laid (table 2). 9th day in 1970, by which time the major por- The proportion of eggs failing to hatch did tion of the increase in size and weight had al- not vary consistently or significantly with col- ready occurred. Therefore, it seems that fac- ony size in either year and ranged from 0.0% tors of food supply and adult foraging ef- to 16.3% of the eggs laid. I did not attempt ficiency may not be important until the latter to determine the cause of the failure to hatch, half of the nestling period. Since young swal- so I do not know to what extent infertility, lows must be able to forage on the wing at malformed embryos, or parental neglect con- the time of fledging, deviations in the later tributed to the failure. portions of the growth curve could be signifi- Partial-clutch mortality. The third compo- cant in terms of post-fledging survival. nent of mortality accounted for the loss of In order to investigate the significance of 3.4% (59/1750) of the eggs laid and included these deviations, I used the weights of nest- death due to starvation and accident (see ta- lings on the 15th day as an indication of the ble 2). The incidence of death to 1 or 2 mem- general health of the young that would be bers of a clutch was not linked significantly fledging. This day occurs after peak weight with colony size and varied from 0.0% to 8.5% has been attained and only a day or two be- of the eggs laid in a colony, usually contribut- fore the young will fledge prematurely if dis- ing less to overall mortality than either preda- turbed. Variation among the colony sizes in tion or failure to hatch. 15-day nestling weight was significant in only It was difficult to determine the causes of 1 of 4 tests (the first brood of 1970: P < 0.005 partial-clutch mortality of young. In some for the Kruskal-Wallis test), and even this instances, the young died in the nest; in others, variation was not consistent with respect to they disappeared without a trace or were colony size. Mean weights tended to be higher found dead on the ground. The young that for solitary nesters and in medium-sized colo- were found dead were never marked in any nies (22.3 g). The lowest mean weight in 1970 way that suggested predation as a cause of was recorded in the x-large colony size (20.3 death, and I suspect that those young that dis- g). Standard deviation in mean 15-day weight, appeared were taken only after falling from one indication of the degree of difficulty in the nest. In 76% of the cases (36/47), the obtaining sufficient food for all nestlings young that perished had been retarded in ( Ricklefs 196813)) was not related significantly weight and growth, and therefore starvation to colony size at any time during the two year investigation. appears to have been an important factor in One variation in nestling weight with brood their deaths. size is pertinent to this discussion of colonial nesting. In general, mean 19day weight NESTLING WEIGHT AND GROWTH varied significantly with brood size only in the The ultimate reproductive success of a pair first brood of 1970, when weight was inversely of birds is represented by the number of off- related to brood size (P < 0.005, F = 9.33 for spring that reach reproductive maturity. The l/70 df; see Snapp 1973). However, this pat- nestling stage is only the first period of ex- tern was not found in all colony sizes. Figure posure to selective pressures, and post-fledging 1 shows that mean 15-day nestling weight de- mortality may be an extremely important part creased with increasing brood size only in of natural selection of breeding habits. I was large and x-large colonies (P < 0.005 for both 474 BARBARA DENNISTON SNAPP

25 70 1 24 I - 60 23

22 50 21 I- ;; 20 z 40 P w 3t IQ T Q It: (3 I8

I- 20 I 24 1 0 w 23 IO 3 22 21 L 20 I 2 3 4 5 >5 IQ FORAGING GROUP SIZE I6 FIGURE 2. Percent of foraging events encountered for different foraging group sizes of Barn Swallows 23456 2 3 4 5 6 observed around 4 colonies with more than 8 pairs in 1971. Horizontal lines represent mean values and BROOD SIZE vertical lines indicate the range of values observed.

FIGURE 1. Relationship between mean X-day nest- ling weight and Barn Swallow colony size for dif- ferent brood sizes. Each point represents one nest values fluctuated around mean rates of 29.1 mean. Data are from the first brood of 1970 in sin- visits per nest per hour and 6.9 visits per nest- gles and small colonies (A), medium colonies ( B ), ling per hour (ranges, 21.0-37.6 and 5.1-10.6 large colonies ( C ), and x-large colonies ( D ). respectively ) . A number of factors could have contributed colony sizes, F = 21.4 and 12.6 respectively). to the large variability observed in the feeding This observation implies that adults might rates and might have masked any differences have had more difficulty finding food for nest- attributable to colony size. It was impossible lings in large colonies than in small ones. How- to insure that foraging conditions were identi- ever, the variation did not appear at any other cal at a11colonies or in all observation periods. time during the study, so its validity requires In an attempt to control some of the vari- substantiation. ability, I limited observations to the hours of 0700 to 1100, refrained from collecting data if FORAGING AND FEEDING RATE any rain fell during observation hours, and re- The frequency with which adults visit the stricted analysis of data to nests containing nest to bring food to nestlings is a crude mea- young between the ages of 8 and 13 days. A sure of the availability of food in the immedi- fourth factor affecting the feeding rate- ate area. If feeding rates in any particular brood size-was probabIy not an important breeding location are lower than the average source of variation in the colony size rates. for the species, this could indicate local scar- Although feeding rate varied significantly with city of food and/or the necessity to forage at brood size (Snapp 1973)) brood size itself did greater distances than normal. One factor that not vary with colony size. could affect local availability of food for the Although colony size appeared to have no Barn Swallow, an aerial insectivore, is the size effect on feeding rates, it is possible that adults of the colony that is exploiting food supplies derived some benefit from foraging in groups in the immediate vicinity of the breeding site. in the larger colonies. In 1971, I observed No such effect was evident in 1970 when I ob- adults in the field at 4 colonies of 8 or more served feeding rates at 11 colonies. The vari- pairs in an attempt to assess the role of social ability in all colony sizes was great, and there facilitation in feeding and location of food was considerable overlap in feeding rates supplies. Observations on foraging patterns among the colony sizes. Mean rates did not over a total of 75 hr support the conclusion vary significantly among the colony sizes, that Barn Swallows forage as individuals and either on a per-nest or per-nestling basis. The do not take advantage of the possibility of COLONIAL BREEDING IN BARN SWALLOWS 475

group foraging and social facilitation offered Within-colony synchrony. Synchrony of by larger colonies. Most of the feeding ap- hatching dates within a colony was unrelated peared to take place within 400 m of the col- to colony size, 16 days being the mean dura- ony, and little clumping of feeders occurred. tion of hatching over both broods of both ’ ’ Figure 2 indicates that the majority of feeding years. First-brood hatching took place over adults foraged singly or in groups of 2 (360, ’ a period of more than 16 days in 9/10 colonies 476 cases). in 1970 and in 11/16 colonies in 1971 (r, = -0.20 and f0.33, respectively, for colony size TIMING OF REPRODUCTION analysis of standard deviations of hatching The data presented above indicate that losses date). Within-colony synchrony was greater due to actual mortality and to potential post- for the second brood in both years, only 2/B fledging mortality did not vary consistently of the 1970 colonies and 3/15 of the 1971 colo- nies taking more than 16 days to hatch out all with colony size. Next, it is important to ex- amine whether any active interaction among nests ( r, = -0.32 and -0.27, respectively). The the breeding birds might have contributed to increase in synchrony from the first brood to variation in the reproductive output among the second is attributable to the lack of second the colony sizes. If social interactions occurred broods among late breeders. among breeding Barn Swallows, differences in Colonies of all sizes were more synchronized date of hatch, synchrony, and/or second- in 1971 than in 1970, possibly the result of a brooding should be related to changes in the long, cold spring in 1971 when mean tempera- size of the colony. tures in April and May were 2.8”C lower than Mean date of hatch. Clutch size showed a those for the comparable period in 1970. This non-significant tendency to increase in the increase in synchrony in 1971 coupled with early stages of the first and second broods and the similarity of the mean hatching dates in to decrease thereafter ( Snapp 1973). Compar- the two years suggest that the entire breeding ison of the breeding biology of late breeders season was not delayed by the cold weather. to that of other Barn Swallow pairs indicated Rather, it appears that only early breeders that pairs which hatched young more than 5 were delayed, resulting in a clustering of days after the mean first-brood hatching date hatching dates close to the mean for each possessed clutches that averaged 0.8 eggs less colony. Therefore, it seems that only external than those of earlier breeders in 1970 and 0.45 conditions such as weather and not active in- eggs less in 1971 (P < 0.025 in 1970; non- teraction among breeding pairs can act to in- significant in 1971 for multinomial chi-square crease the synchrony of a colony. test). In addition to laying a small first clutch, Since the duration of first-brood hatching late breeders also produced fewer second within most colonies was less than that of the broods. Only 18% (10/55) of those late breed- population as a whole (-5.5 days average in ers that should have been able to start a second 1970; -12.2 days in 1971), it is possible that brood actually did so, compared to 82% (144/ within-colony synchrony, irrespective of col- 176) of the earlier breeders. ony size, could have had an impact on re- Because of the effect of date of first brood productive output in a colony. Such an impact hatching on clutch size and second-brooding, might be expected if social stimulation played the number of young fledged from a colony a role in determining the timing of breeding could be influenced by the mean hatching within a colony (Darling 1938). The critical date of that colony and by the percentage of timing parameter is the within-colony syn- late breeders present. However, variation in chrony of the first brood, and this parameter mean hatching date among the colony sizes bore no relationship to seasonal reproductive was significant only in the first brood of 1970 output ( rs = -0.08). Nor were there signifi- when medium-sized colonies nested later than cant correlations in either first or second brood other colonies (P < 0.025 for Kruskal-Wallis when each was considered separately ( rs = test). The pooled mean dates were similar in -0.07 and +0.15). both years: 13 June 1970 ( k11.0 days) and 14 Occurrence of second broods. I examined June 1971 (k8.8 days) for first broods, 23 July second-brood occurrence in two ways: actual 1970 ( * 7.9 days) and 24 July 1971 ( *4.5 number present and potential number avail- days) for second broods. There was likewise able. There was no significant correlation no consistent relationship between colony size among colony sizes with respect to the pro- and percentage of late breeders in a colony portion of pairs actually having a second (mean = 26% or 79/301 nests). brood (r, = -0.16). An average of 49% ( 147, ’ 476 BARBARA DENNISTON SNAPP

301) of all first brood pairs and 57% (147/ swallows, although Barn Swallows were able 259) of the successful first brood pairs initi- to drive Starlings away from their nests when ated second broods. the latter attempted to enter. By contrast, a Three timing factors operate together to de- solitary natural-cavity nesting species, the termine the potentiality of second-brooding: Tree Swallow, is reported to be very aggres- date of hatch of the first brood, date of fall sive in defending its nest against nest-site migration (late August), and an inter-brood competitors from other species (Kuerzi 1941, interval of approximately 45 days (R = 35-54 Chapman 1955). days). The combination of the latter two fac- The Barn Swallow is not alone among co- tors allows the determination of a date by lonial swallows with respect to the ineffective- which first-brood hatching would have to oc- ness of predator defense. In fact, the Bank cur if a second brood were to be raised. This Swallow, Cliff Swallow, and House Martin date is 23 June. An average of 77% of the (Delichon urbica) are reported to be even Barn Swallow pairs successfully completing less aggressive and less effective in mobbing a first brood met the date restrictions and predators and nest-site competitors (Windsor therefore were theoretically able to raise a and Emlen 1975, J. T. Emlen 1952, 1954, Lind second brood. However, only 65% (147/225) 1962, 1964, respectively; but see also Hoog- of all pairs with time to raise a second brood land and Sherman 1976, on Bank Swallow actually attempted to do so. Neither percent- mobbing behavior). One reason for the in- age varied significantly with colony size. effectiveness of mobbing in colonial swallows The lack of a consistent correlation between may be the lack of crypticity of the nest site. colony size and first-brood timing factors, Horn (1968, 1970) found that mobbing by inter-brood intervals, or percentage of late Brewers’ Blackbirds (Euphagus cyanocepha- breeders suggests that the occurrence of sec- Zus) was effective only against flying birds, ond-brooding is not regulated by social inter- and not against mammals, snakes, or perching action among breeding pairs and is probably birds; crypticity of the nest site was the main unaffected by the density of breeding pairs. deterrent to predation by the latter. Likewise, Similar observations hold true for the occur- Kruuk (1964) found that the mobbing be- rence of replacement broods laid after de- havior of Black-headed Gulls (Lams ridibun- struction of previous clutches. Although aban- dus) served primarily to distract potential doned nests and destroyed second-brood predators and thereby enhanced the effective- clutches were never replaced, 68% (25/37) of ness of nest crypticity and diminished the abil- the nests destroyed in the first brood were re- ity of the predator to locate eggs or young. A placed in the two seasons. The rate of re- similar distraction effect might occur as a re- placement, interval between destruction and sult of the aggressive activity of the solitary- replacement, and the success of replacement nesting Tree Swallow. However, there is no broods were all unrelated to colony size, ir- evidence to indicate that predator defense is respective of the nature of the destructive enhanced by increases in breeding density agent. among colonial swallows, in contrast to the situation in Black-headed Gull colonies DISCUSSION ( Kruuk 1964). Hoogland and Sherman (1976) provided some data to indicate a po- Because of the scarcity of literature on many tential enhancement of predator defense in species of swallows, the following discussion Bank Swallow colonies but have yet to demon- will deal for the most part with northern strate an actual decrease in predation with in- temperate zone swallows and with species creasing colony size. from other bird families. The observations of Barn Swallow mobbing behavior lead one to the conclusion that ag- EFFECT OF PREDATION gregation has not conferred any benefit from The impact of predation can be reduced by the standpoint of decreasing mortality due to judicious placement of nests in inaccessible or predation. On the other hand, colonial breed- cryptic locations and/or by behavioral activi- ing does not appear to have been detrimental ties on the part of breeding pairs, such as mob- with respect to predation loss. Although the bing, increased alertness, and greater effective- large number of birds increased the conspicu- ness of alarm systems. ousness of the nesting site, the level of preda- Mobbing activity in the Barn Swallow is not tion in Barn Swallow colonies was not greater as highly developed as that observed for other than that suffered by solitary nesting pairs of swallow species, notably the solitary-nesting the same species. COLONIAL BREEDING IN BARN SWALLOWS 477

EFFECT OF FOOD SUPPLY not in smaller colonies, indicating that adults might have had some difficulty finding suf- In his examination of the breeding of Brewers’ ficient food. Blackbirds, Horn (1968) suggested that uni- The absence of a consistent effect of colony form dispersal of food would result in a simi- size on weight in Barn Swallows does not rule lar dispersal of nests, while temporal and spa- out the possibility that these birds are food- tial uncertainty of food supply would result limited in some situations. Considering the in a clumping of nests. Brown (1964) had general impression of the instability of insect previously related territoriality with defend- populations and the extreme influence of food ability of a uniformly dispersed food supply. supply on success in other aerial insectivores Aerial insect populations are often spatially (Common Swift [Apus upus], Lack and Lack and temporally variable (e.g., MacLeod and 1951; House Martin, Bryant 1975), it would Donnelly 1957,1958, Johnson 1969) and hence be surprising if food supply did not exert a se- not defendable. Since these animals provide lective pressure on Barn Swallows at some almost all of the swallow diet ( Beal 1918)) one point during some breeding seasons (see would not expect to find swallows defending Snapp 1973). feeding territories and therefore colonial breeding would not be prohibited by this type EFFECT OF SOCIAL STIMULATION of aggression. Horns’ hypothesis of nest distribution does Darling (1938) hypothesized that the size of not imply or require the existence of social a colony of breeding birds has a direct influ- facilitation in foraging. However, if a colony ence on the development and synchronization becomes very large, it is reasonable to expect of the reproductive condition of the individual that local food supplies might become de- members of the colony, He suggested that pleted to an extent that would compel the larger colonies should be both more synchro- birds to forage at greater distances and make nized and more successful. “Social stimulation” social facilitation invaluable in reducing the is a term that encompasses a wide variety of time spent searching for food (see also Ward possible colony-size effects, including the en- and Zahavi 1973, Krebs 1974). Several co- hancement of breeding success through in- lonial swallow species have been observed crease of sexual motivation and parental care feeding in large flocks during the breeding as well as through increase in breeding syn- season (Bank Swallows, Stoner 1936, S. T. chrony. Emlen and Demong 1975; Cliff Swallows, J. At this point, it appears that social stimula- T. Emlen 1952, 1954; House Martins, Knopfli tion plays a small role, if any at all, in the 1971; Purple Martins [Progne suhis], Johnston establishment of Barn Swallow colonies. Syn- and Hardy 1962). The absence of social forag- chrony of breeding was not strong, and it was ing patterns in Ram Swallow colonies may not correlated with colony size. Each pair ap- indicate that the colonies have not reached peared to operate independently of others with the size at which foraging distance becomes respect to the initiation of breeding and the so great that nestling growth suffers unless timing of second broods. Although I did not social feeding enhances foraging efficiency. observe pre-copulatory behavior in detail, Barn Swallow colonies are smaller than those breeding behavior did not appear to be con- of the Bank Swallow, Cliff Swallow, and tagious as in Brewers’ Blackbirds (Horn 1968) House Martin. and some other swallow species (e.g., Cliff In fact, most of the evidence presented in Swallows, J. T. Emlen 1952, 1954; Bank Swal- the section on nestling weight and growth in- lows, Petersen 1955, S. T. Emlen and Demong dicates that food supply is not always a criti- 1975; Purple Martin, Allen and Nice 1952). cal factor in determining Barn Swallow colony Possibly the mere presence of birds at a size. For example, nestling death due to sus- colony resulted in the recruitment of addi- pected starvation was very low in all colony tional pairs by drawing attention to the exist- sizes (1.8% to 4.1% of eggs laid), and was less ence of suitable breeding sites. However, important as a mortality source than preda- there was no indication that the recruited in- tion or failure to hatch. Nestling weight varied dividuals responded physiologically to the significantly among the colony sizes only in established pairs, or that recruitment occurred the first brood of 1970, but in a manner that differentially with respect to the degree of was unrelated to the density of nesting pairs. similarity in reproductive condition between On the other hand, mean weights of 15-day- the established pairs and the newcomers. Any old nestlings varied inversely with brood size effect of recruitment on synchrony appeared in large and x-large colonies at this time, and to operate independently of Barn Swallow col- 478 BARBARA DENNISTON SNAPP ony size. In addition to this lack of correla- Barn Swallows defend the approaches to the tion between colony size and synchrony, there nest as well as the area immediately surround- was no influence of synchrony or colony size ing the nest site itself (Davis 1937, pers. on breeding successin the Barn Swallow. observ. ) . The absence of data for many colonial spe- These nest-site characteristics, in combina- cies of swallows makes it difficult to compare tion with the defense of an area around the the role of social stimulation in the Barn Swal- nest, result in dispersal of nests within a build- low with its role in the family as a whole. ing, and Barn Swallows are seldom found nest- Strong synchrony of activities has been re- ing less than 3 m apart. Although nests on ported for the Cliff Swallow (J. T. Emlen, different beams may be closer, they almost al- 1952, 1954) and Bank Swallow (Stoner 1936, ways have different approaches. Small build- Petersen, 1955, S. T. Emlen and Demong, ings, therefore, have only 1 to 3 pairs nesting 1975). However, the literature is by no means in them, as do larger barns that are tightly in agreement on this point. Myres ’ (1957) shuttered and do not provide the wide en- data indicate no effect of colony size on syn- trances attractive to Barn Swallows. In Scan- chrony of egg laying within Cliff Swallow dinavia the Barn Swallow is not known as a colonies, but his study involved a different colonial breeder ( Curry-Lindahl 1961, Lind colony each year, and he did not investigate 1964, von Haartmann 1969), although groups breeding success at all. A similar lack of cor- of 2 to 3 pairs are not uncommon. The well- relation between colony size and breeding syn- kept nature of the barns and sheds may con- chrony was found by Lind ( 1964) in his study tribute to the solitary nature of the breeding of the House Martin. S. T. Emlen and De- habits there (pers. observ. ). In the United mong (1975) reported that breeding success States the large number of abandoned barns is significantly related to the degree of syn- and the wide-open nature of most cow sheds chrony within Bank Swallow colonies, but has greatly increased the number of nest sites there is no relationship between colony size available in any one place. A similar situation and synchrony. occurs in Germany (pers. observ. ), where the Barn Swallow is also known to nest in large EFFECT OF NEST-SITE AVAILABILITY groups (von Vietinghoff-Riesch 1955). Availability of nest sites may be a critical fac- Nest-site limitation occurs in other species tor influencing colony formation for many bird of swallows as well. The high degree of ag- groups, notably offshore seabirds ( Lack 1967)) gressiveness displayed by at least 4 species of natural-cavity nesters (Tree Swallow, Kuerzi herons (Lack 1968), nomadic blackbirds (Or- ians 1961)) and Savannah weaverbirds (Crook 1941, Chapman 1955; Rough-winged Swallow, 1962, 1964). The Barn Swallow also has nest- Lunk 1962; Purple Martin, Allen and Nice ing requirements that may result in a scarcity 1952, Johnston and Hardy 1962; and Blue Swallow, of suitable nest sites. Hirundo atrocaerulea, Snell 1969) Barn Swallows prefer to nest inside struc- may be connected with the need to defend a tures such as barns, sheds, and the undersides limited resource, namely a suitable site for of bridges and culverts. Before the develop- nesting. All of these species, except the Blue ment of human settlements, they probably Swallow, readily use artificial nest sites, sug- used shallow caves and crevices in cliffs and gesting that their breeding density may be limited by the availability of natural cavities. are occasionally reported to do so today (Bent 1942, Murray 1962). Since Barn Swallows do The Blue Swallow builds its mud nests in pot- not inhabit the darker recesses of the areas holes in the shallow valleys of Rhodesia they use for colonies (Davis 1937, pers. and vigorously defends these limited nest sites observ. ), most natural caves do not provide (Snell1969). much room for nesting. This situation con- In conclusion, the Barn Swallow seems to trasts markedly with the situation for other have evolved mechanisms that permit pairs to colonial swallows. Numerous nest sites are nest near one another but at well-spaced in- available on the exposed vertical surfaces oc- tervals, a condition that allows groups of 2 or cupied by Cliff Swallows, Bank Swallows, and 3 pairs to nest together in most natural breed- House Martins, as well as in the inner recesses ing sites. It appears that the larger colonies of large caves inhabited by Cave Swallows now are a passive extension of the natural oc- ( Petrochelidon fulva). Therefore, the actual currence of small nesting groups, and that the number of suitable nest sites available to Barn nature of the buildings available has had a Swallows may be few, and it may be that the profound influence on the development of co- shortage of nest sites explains the fact that lonial breeding in the Barn Swallow. COLONIAL BREEDING IN BARN SWALLOWS 479

SOCIALITY IN THE SWALLOW FAMILY SUMMARY Evidence suggests that nest-site availability Four factors could account for the presence may have played an important role in the evo- of coloniality in the Barn Swallow: predation, lution of both social patterns and nest-building starvation, social stimulation, nest-site avail- behavior in swallows (Mayr and Bond 1943). ability. Neither the number of pairs that col- Geographical distribution of swallow species lected in a colony nor the degree of within- is such that an area contains at most one spe- colony synchrony affected reproductive SUC- cies in each of the following nesting cate- cess by consistently increasing or decreasing gories : burrow-excavator, exposed-surface losses due to predation or food shortage. Col- mud-nester, recessed-surface mud-nester, tree ony size also did not influence the degree of cavity-nester, substrate cavity-nester (Snapp synchrony, occurrence of second broods or 1973). However, the presence of a high de- replacement of destroyed clutches, as might gree of synchrony within the colonies of cer- h ave been expected if social stimulation were tain swallow species indicates that nest-site important. However, the number of pairs in availability alone is not sufficient to account a colony rose in parallel with the increase in for the high degree of sociality observed in the the size of the building and/or the number breeding season. It is perhaps significant that of entrances to the building. Therefore, the those species which are most synchronized are implication is that Barn Swallow colonies rep- also found in the largest colonies (Bank and resent passive aggregations of breeding birds Cliff swallows). and do not actively recruit additional pairs. Evidence that synchrony in colonial swal- The lack of close synchrony and the absence lows is a response to predation pressure is of colony-size effects on reproductive success scanty, and behavioral defense mechanisms indicate that coloniality in the Barn Swallow have not developed to a great degree. Ob- has not reached the level of breeding density servations of the foraging behavior of some where pressures other than nest-site avail- species indicate that difficulty in locating food ability begin to exert an influence on the de- sources may play a role in producing within- velopment of social breeding patterns. colony synchrony. The fact that there is less synchrony between colonies than within colo- ACKNOWLEDGEMENTS nies in these species (J. T. Emlen 1952, 1954, I am indebted to S. T. Emlen for generous advice and Myres 1957, on Cliff Swallow; Stoner and encouragement throughout the course of my in- 1935, Petersen 1955, S. T. Emlen and Demong vestigation. I also appreciate his comments and those 1975, Hoogland and Sherman 1976, on OF P. Colgan during preparation of the paper. K. Hall and P. Colgan provided valuable assistance concerning Bank Swallow; Snapp 1973, on Barn Swallow) statistical techniques. This research was supported by suggests that there may not be a severe tempo- an NSF Graduate Fellowship. ral concentration of food abundance that would impose breeding synchrony on the LITERATURE CITED birds. ALLEN, R. W., AND M. M. NICE. 1952. A study of In addition, House Martins and Barn Swal- the breeding biology of the Purple Martin (Prognesubis). Am. Midl. Nat. 47: 606-645. lows both produce two broods each year, indi- BEAL, F. E. L. 1918. Food habits of the swallows, cating that synchrony is not solely the result a family of valuable native birds. U.S. Dept. of an intense concentration of food in the early Agric. Bull. 619. part of the summer. Although all swallow spe- BENT, A. C. 1942. Life histories of North Ameri- cies feed on the same insect orders (Beal can flycatchers, larks, swallows, and their al- lies. U.S. Natl. Mus. Bull. 179. 1918), highly synchronized and colonial spe- DE BRAEY, L. 1946. Aupres du nid de lhirondelle’ de cies do not have two broods, whereas those cheminee Nirundo rusticu rustica Linne. Gerfaut nesting in smaller less synchronized colonies 36: 133-194. do. There is no reason, from timing consider- BROWN, J. L. 1964. The evolution of diversity in avian territorial systems. Wilson Bull. 76: 160- ations, why Cliff Swallows and Bank Swallows 169. could not produce a second brood throughout BRYANT, D. M. 1975. Breeding biology of the most of their ranges. It is possible that food House Martin Delichon urbica in relation to aerial supplies in the latter half of the summer are insect abundance. Ibis 117:180-216. low enough to prohibit successful nesting in CHAPMAN, L. B. 1955. Studies of a Tree Swallow colony. Bird-Banding 26 :45-70. large aggregations. Changes in insect quantity CHOOK, J. H. 1962. The adaptive significance of or quality may also explain why some Barn pair formation types in weaverbirds. Symp. Swallows with time to raise a second brood Zool. Sot. London. 8:57-70. CROOK, J. H. 1964. The evolution of social organi- do not always attempt to do so (see also Bry- zation and communication in the weaverbirds ant 1975 on the House Martin). (Ploceinae) Behaviour Suppl. IO. 480 BARBARA DENNISTON SNAPP

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Accepted for publication Delichon u. urbicu (L. ), zu ihren Feinden. Ann. 4 December 1975.