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Publisher Summary The chapter discusses a review of sperm competition and behavioral adaptations to sperm competition in birds. Sperm competition is the competition between spermatozoa of different males to fertilize the eggs of a single female. The main emphasis in this chapter is on monogamy, extrapair copulations (EPCs), and associated behaviors, although sperm competition within other avian mating systems is also discussed. Four factors are known to influence the probability of extrapair paternity: (1) the timing and success of copulation— that is, sperm transfer by different males, (2) the relative numbers of copulations by different males, (3) the duration of sperm storage, and (4) sperm precedence. Extrapair paternity can be detected using genetic markers of two main types: those that are continuous variables (e.g., morphological characters) and those that are discontinuous variables (e.g., plumage color, enzyme polymorphisms). The chapter also discusses case studies on magpie ( pica pica ), common guillemot ( utria aalge ), swallow ( Hirundo rustica ), ringdove ( Barbary dove, Streptopelia risoria ), and indigo bunting ( Passerina cyanea ).

Most birds are monogamous, but recent studies have shown that extra-pair copulations (EPCs) occur frequently1,2 despite a range of paternity guards, including mate-guarding and frequent copulation1. Although EPCs are known to result in extra-pair paternity3–5, there are no previous quantitative estimates of the success of EPCs in fertilizing eggs. We present here estimates of the likelihood of success of extra-pair copulations in a monogamous passerine, the zebra finch Poephila guttata. We show that (1) EPCs occurring under semi-natural conditions in captivity result in extra-pair paternity, (2) sperm from the last male to mate has precedence over previous matings: a single EPC occurring last is disproportionately successful in fertilizing eggs, but EPCs followed by further pair copulations have a low probability of success. These results have important implications for sexual selection theory.

Individual differences in sexual behavior have received much attention by evolutionary biologists, but relatively little is known about the proximate causes of this variation. We studied the quantitative genetics of male and female sexual behavior of captive zebra finches and found surprisingly strong maternal effects (differing between individual mothers) on the aggressiveness and song rate of sons and on the daughters' mating preferences for these male traits. We also found that daughters differed in their choosiness during mate-choice experiments depending on whether they originated from eggs produced early or late within the laying sequence of a clutch. Because this effect of laying order occurred independently of hatching order in cross-fostered broods, it must have been caused by consistent within-mother variation in maternal effects transmitted through the egg. Our findings raise the question whether these maternal effects might represent strategic programming of offspring behavior in response to the environment experienced by mothers or whether they are merely nonadaptive byproducts of developmental processes.

Sperm competition, in which the ejaculates of multiple males compete to fertilize a female's ova, results in strong selection on sperm traits. Although sperm size and swimming velocity are known to independently affect fertilization success in certain species, exploring the relationship between sperm length, swimming velocity and fertilization success still remains a challenge. Here, we use the zebra finch ( Taeniopygia guttata ), where sperm size influences sperm swimming velocity, to determine the effect of sperm total length on fertilization success. Sperm competition experiments, in which pairs of males whose sperm differed only in length and swimming speed, revealed that males producing long sperm were more successful in terms of (i) the number of sperm reaching the ova and (ii) fertilizing those ova. Our results reveal that although sperm length is the main factor determining the outcome of sperm competition, complex interactions between male and female reproductive traits may also be important. The mechanisms underlying these interactions are poorly understood, but we suggest that differences in sperm storage and utilization by females may contribute to the outcome of sperm competition.

There is long-standing disagreement over the way in which seabird populations are regulated1. They may be regulated by density-dependent mortality during winter as a consequence of food shortage2–4, shortage of nest sites5–7, social factors8, or not regulated at all9. Ashmole10,11 argued that for tropical seabirds, density-dependent mortality of adults outside the breeding season is most unlikely because intraspecific competition for food close to the colony would result in prey depletion, causing adults to travel increasing distances for food as colony size increased.Breeding numbers could be regulated through a density-dependent reduction in reproductive output resulting from reduced rates of provisioning of chicks. This mechanism may not apply to seabird colonies in higher latitudes, however, because seasonal production might provide superabundant food supplies during the breeding season. Recent studies of the quantity of food consumed by breeding seabirds12–15 suggest that prey depletion also occurs in temperate regions and invites a critical test of Ashmole's hypothesis. We present here an analysis of interactions between spatial distribution and size of seabird breeding colonies and provide support for Ashmole's suggestion that seabird numbers may be limited by intraspecific competition for food around colonies during the breeding season.

Behavioral Mechanisms in Ecology. By Douglass H. Morse. Pp.383. ISBN 0-674-06460-7. (Harvard University Press: 1981.) $32.50, £17.50.