The effect of age on timing of breeding and reproductive success in the Thick-Billed Murre

Ecology, July, 1996 by Leah N. de Forest, Anthony J. Gaston

INTRODUCTION

In many long-lived seabirds breeding in seasonal environments, timing of breeding is closely related to reproductive success. Individuals laying early in the season are more successful than late breeders, with clutch size, egg volume, probability of relaying if eggs are lost, hatching success, fledging masses, and fledging success declining with date of laying in a wide range of species (Wandering Albatross Diomedea exulans, Croxall et al. 1992; Northern Fulmar Fulmarus glacialis, Ollason and Dunnet 1978; Antarctic Fulmar Fulmarus glacialoides, Weimerskirch 1990; Shag Phalacrocorax aristotelis, Amundsen and Stokland 1990; Herring Gull Larus argentatus, Parsons 1975; Black-legged Kittiwake Rissa tridactyla, Coulson and White 1958, 1961; Razorbill Alca torda, Lloyd 1979; Common Murre Uria aalge, Birkhead 1977, Hedgren 1980, Birkhead and Nettleship 1987, Wanless and Harris 1988, Hatchwell 1991; Thick-billed Murre Uria lomvia, Birkhead and Nettleship 1981, 1982, Gaston and Nettleship 1981; Cassin's Auklet Ptychoramphus aleuticus, Ainley et al. 1990; Atlantic Puffin Fratercula arctica, Harris 1980). In addition, young, inexperienced breeders often differ from older breeders in laying later in the season and in various measures of reproductive performance (Coulson and White 1958, Davis 1975, Potts et al. 1980, Ryder 1980, Pugesek 1981, 1983, Nisbet et al. 1984, Nelson 1988, Reid 1988, Saether 1990, Pyle et al. 1991).

Because of the correlation between age and date of laying, the direct effects of age on reproductive success are difficult to distinguish from time-of-year effects. Other confounding variables, such as seasonal and annual changes in prey availability and weather, may also contribute to seasonal changes in reproductive parameters (Hedgren and Linnman 1979, Coulson and Thomas 1985, Sydeman et al. 1991).

Several hypotheses have been suggested to explain observed declines in reproductive success with date of laying in colonially breeding seabirds: (1) the risk of predation on eggs or chicks is greater for late birds, which are more isolated and exposed after earlier chicks have departed ("late predation hypothesis," Birkhead 1977); (2) the risk of predation is highest for pairs laying out of synchrony with their local breeding group ("synchrony hypothesis," Hatchwell 1991); (3) a seasonal decline in prey availability and/or quality causes later chicks to have reduced fledging success or lower fledging masses ("food availability hypothesis," Hedgren 1979, Hedgren and Linnman 1979, Gaston and Nettleship 1981, Birkhead and Nettleship 1982); and (4) young and/or inexperienced birds that breed later in the season have lower reproductive success than older birds ("age/experience hypothesis," Hedgren 1980).

The direct effects of age and those mediated by timing of laying can only be distinguished through experimental manipulation. To examine this problem, we removed the eggs of early breeders to induce laying of a replacement egg [approximately equal to]14 d later. By forcing early-laying birds to breed late in the season, we were able to detect the effect of date alone by comparing the performance of the experimental birds to that of a control sample that laid their first eggs at the same date. We also compared different components of reproductive success for experimental birds with those for young breeders to demonstrate the effects of age/experience in a situation where older birds had laid later than the young birds.

Several studies have found chick growth rate to be associated with food supply (Boersma 1978, Gaston et al. 1983, Brown and Nettleship 1984, Ricklefs et al. 1984, Barrett et al. 1987, van Heezik 1990). To detect changes in prey availability as the season progressed, we measured early control chicks and chicks from experimental replacement eggs to assess changes in growth rates early and later in the season.

MATERIALS AND METHODS

The study was carried out at Coats Island, Northwest Territories, Canada, on a colony of [approximately equal to]30 000 breeding pairs of Thick-billed Murres (Gaston et al. 1993a). Thick-billed Murre chicks there have been banded with year class color bands each year since 1984. In addition, some adult breeders have been banded each year, using a single color to denote "banded as a breeder," as well as the year class colon All birds were also banded with numbered metal bands.

Reproductive success

We recorded reproductive success for marked and unmarked birds on five study plots comprising, in aggregate, 415 and 361 pairs of Thick-billed Murres in 1990 and 1991, respectively. The plots were situated in the part of the colony where banding was carried out and to which most banded chicks return (Noble et al. 1991). Our measures of reproductive success in relation to age were confined to birds breeding on our study plots. We rejected the possibility of using birds of known age on other sites in case such selective observations should constitute a biased sample of potential breeding sites. In 1990 and 1991, 57 and 72 birds banded as chicks bred on the study plots, at ages from 3 to 7 yr.