Adult prey choice affects chick growth and reproductive success in pigeon guillemots

ABSTRACT.-Pigeon Guillemots (Cepphus columba) are diving seabirds that forage near shore and feed their chicks demersal and schooling fishes. During nine years between 1979 and 1997, we studied chick diet, chick growth rate, and reproductive success of Pigeon Guillemots at Prince William Sound, Alaska, to determine factors limiting breeding populations. We found evidence for prey specialization among breeding pairs and detected differences in reproductive success between specialists and generalists. Pairs that specialized on particular prey types when foraging for their chicks fledged more chicks than those that generalized, apparently because they delivered larger individual prey items. Reproductive performance also varied among guillemot pairs as a function of the proportion of high-lipid schooling fishes fed to the chicks. Pairs that delivered primarily high-lipid fishes (Pacific sand lance [Ammodytes hexapterus] and Pacific herring [Clupea pallasii]) had higher overall reproductive success than pairs that delivered primarily low-lipid demersal fishes (e.g. sculpins, blennies, stichaeids, and pholidids) and gadids. The proportion of high-lipid fishes in the diet was positively related to chick growth, suggesting that piscivorous seabird chicks benefit from eating species with high-energy densities during development. The diet of Pigeon Guillemot chicks showed high annual variation from 1979 to 1997, presumably because of fluctuations in abundance of Pacific sand lance, a high-lipid schooling fish. Regression analyses suggest that the percent occurrence of high-lipid fishes in the diet affected chick growth rate at the population level. We conclude that Pigeon Guillemots benefit by specializing when selecting prey for their chicks, and that high-lipid schooling fishes enhance chick growth and reproductive success. Received 30 September 1998, accepted 5 May 1999.

WITHIN POPULATIONS of generalist predators, some individuals demonstrate high degrees of prey specialization (Werner and Sherry 1987, West 1988, Wendeln et al. 1994). Differences in patterns of prey choice among individuals within populations are of interest from an ecological standpoint because they represent alternate strategies to the general life-history challenge of maximizing lifetime reproductive success. Yet, relatively few studies have compared the reproductive performance of a population in which the adults specialize on different prey types (but see Trillmich 1978, Trivelpiece et al. 1980, Pierotti and Annett 1991, Spear 1993). Much more common are studies that relate intercolony differences in diet to reproductive performance (e.g. Harris and Hislop 1978, Monaghan et al. 1989, Hamer et al. 1991).

Guillemots (Cepphus) are semicolonial seabirds that eat a wide range of prey types (Bradstreet and Brown 1985, Ewins 1993). Some individuals are highly specialized, however, and prey selection may differ markedly among birds within the same breeding colony (Drent 1965, Slater and Slater 1972, Cairns 1981, Kuletz; 1983, Emms and Verbeek 1991). Thus, guillemot colonies present valuable opportunities for studies of foraging ecology in relation to chick growth and reproductive success.

Guillemots often forage solitarily, or in small groups, and they primarily select nearshore demersal fishes (sculpins, blennies, stichaeids, and pholidids) for their chicks (Drent 1965, Cairns 1987a, Ewins 1993). These prey tend to be dispersed but may be predictable in time and space (Rosenthal 1979, Cairns 1987a). In contrast, most other piscivorous alcids (e.g. murres [Uria] and puffins [Fratercula]) feed in foraging flocks on dense aggregations of pelagic schooling fishes (e.g. Pacific sand lance [Ammodytes hexapterus], capelin [Mallotus villosus], Pacific herring [Clupea pallasii], and gadids; Piatt 1990, Hatch and Sanger 1992). Given that many pelagic schooling fishes have higher lipid content (gadids are an exception), and consequently higher energy density than demersal fishes (Montevecchi et al. 1984, Hislop et al. 1991, Anthony and Roby 1997, Van Pelt et al. 1997), it is perhaps surprising that guillemots do not prey on schooling fishes more extensively. High-lipid schooling fishes sometimes are available to guillemots, as instances of individual birds specializing on them demonstrate (Slater and Slater 1972, Cairns 1981, Kuletz 1983). Only rarely, however, have guillemots been reported to exploit schooling fishes to a large degree (Kuletz 1983).

To better understand the foraging ecology of guillemots, we studied chick diet, chick growth, and reproductive success of Pigeon Guillemots (Cepphus columba). We tested two main hypotheses, the first being that adults that are highly specialized when selecting prey items for their chicks have higher reproductive success than adults that are less specialized. This might be expected if specialization increases foraging efficiency by reducing prey handling time or enabling adults to select larger or more nutrient-rich prey (Futuyma and Moreno 1988). The second hypothesis is that reproductive success varies as a function of the percent of high-lipid prey items in the chick diet. Adults that select high-lipid prey for their chicks may be expected to have higher reproductive success than those that select low-lipid prey for a number of reasons. Field and laboratory studies of seabird nestling growth suggest that chicks fed high-lipid prey grow faster than chicks fed low-lipid prey because lipids are energy-rich (Harris and Hislop 1978, Massias and Becker 1990, Roby 1991). Because lipids tend to replace water and not protein, highlipid prey fishes typically are not lacking in other nutrients (Harris and Hislop 1978). A further benefit of high-lipid prey is that they generally yield higher assimilation efficiencies by seabirds than do low-lipid prey (Massias and Becker 1990, Brekke and Gabrielsen 1994).