The costs and benefits of egg destruction by conspecifics in colonial cliff swallows

-We studied egg destruction by conspecifics in colonial Cliff Swallows (Hirundo pyrrhonota) in southwestern Nebraska. Between 8.5% and 19.6% of all nests lost at least 1 egg to an intruding conspecific. Egg destruction occurred when nests were left momentarily unattended, often during colony alarm responses. Birds that destroyed eggs maintained nests of their own, and usually nested within 75 cm of their victims. Egg destruction was not related to attempts to usurp nests. Over a third of perpetrators of egg destruction lost eggs from their own nests to conspecifics. Egg destruction occurred more often in nests initiated early in the nesting season and in nests with large clutch sizes. Egg destruction usually occurred during a victim's egg-laying or early during incubation and declined in frequency as incubation proceeded. Breeding in a colony's peak period of nesting did not afford an advantage to potential victims by diluting their chances of being victimized. Cliff Swallows seldom destroyed all of the eggs in a neighbor's clutch, usually destroying only 1 egg at a time even though other eggs were present. There was little direct evidence that egg destruction was associated with parasitic egglaying by Cliff Swallows, but nests with egg destruction were over 3 times more likely than nests in general to have an egg physically transferred into them. The costs of egg destruction to victims were obvious, but the benefits to destroyers of eggs were not. Egg destruction is possibly a prelude to physical transfer of eggs, reducing host clutch sizes and ultimately within-brood competition among host and parasitic nestlings. Males may also benefit by destroying a female's eggs during laying, thereby causing her to continue laying and remain sexually receptive to forced extrapair copulations. Incidence of egg destruction by conspecifics increased with Cliff Swallow colony size and thus, for potential victims, represents a definite cost of coloniality. Received 22 January 1988, accepted 26 June 1988. A POTENTIAL cost of colonial breeding is an increased number and proximity of neighbors that may interfere with one's attempt to reproduce. A dramatic form of reproductive interference is the destruction of eggs and chicks by conspecifics within the colony (Wittenberger and Hunt 1985). Potential destruction of eggs or chicks represents an enormous cost of coloniality. This cost is incurred routinely by some colonially nesting birds, especially skuas, gannets, and large gulls (Wittenberger and Hunt 1985, references therein). Conspecific interference centers on prefledged chicks, perhaps often to prevent giving care to unrelated offspring. Chicks can be attacked, killed, and eaten. Cannibalism is an important component of this form of reproductive interference (Wittenberger and Hunt 1985). A steady supply of available chicks presumably represents a major benefit of nesting in a colony for a cannibalistic individual. Less attention has been paid to egg destruction and its effects in colonial birds. Lesser Blackbacked Gulls (Larus fuscus; Harris 1964, Brown 1967, Davis and Dunn 1976), Herring Gulls (L. argentatus; Paynter 1949, Harris 1964, Brown 1967), Arctic Terns (Sterna paradisaea; Pettingill 1939), Carrion Crows (Corvus corone; Yom-Tov 1974, 1975), and Marsh Wrens (Cistothorus palustris; Picman and Belles-Isles 1987) are reported to steal, puncture, or eat eggs from nests of neighboring conspecifics. Egg destruction in Cattle Egrets (Bubulcus ibis) occurs when nesting materials are stolen and during fights (Siegfried 1972). In communally nesting Ostriches (Struthio camelus), Groove-billed Anis (Crotophaga sulcirostris), and Acorn Woodpeckers (Melanerpes formicivorus), eggs in a clutch are removed and destroyed by later-laying females, a manifestation of reproductive competition among females (Vehrencamp 1977, Bertram 1979, Mumme et al. 1983). Egg destruction can have major effects on the fitness of both destroyer and victim (see Wittenberger and Hunt 1985). We must understand fully both egg destruction and chick-killing, because their costs and benefits must be measured if we are to construct a coherent general theory for the evolution of avian coloniality. 737 The Auk 105: 737-748. October 1988 This content downloaded from 157.55.39.103 on Mon, 25 Apr 2016 07:00:08 UTC All use subject to http://about.jstor.org/terms 738 BROWN AND BROWN [Auk, Vol. 105 We discovered that colonial Cliff Swallows (Hirundo pyrrhonota) in southwestern Nebraska often destroy the eggs of conspecifics. Egg destruction varied with colony size and may affect the evolution of coloniality. Although the costs to a victim are clear, the benefits to an egg destroyer are not. We report on egg destruction in Cliff Swallows and examine the potential costs and benefits to both victims and perpetrators. Further, we investigated whether egg destruction was associated with alternate reproductive tactics such as intraspecific brood parasitism (Brown 1984, Brown and Brown 1988). Because Cliff Swallow nestlings are highly altricial and are very immobile prior to fledging, chick-killing of the kind described in colonial seabirds does not occur in this species. We therefore restrict our focus exclusively to consideration of egg destruction.