Nest-site safety predicts the relative investment made in first and replacement eggs by two long-lived seabirds

Comparative studies of birds suggest that differences in nest-site characte ristics, such as between open-nesting and cavity-nesting species, might ult imately underlie much interspecific variation in clutch size and renesting capacity. The "renesting hypothesis" holds that in species that typically e njoy high breeding success because they nest in safe sites, individuals max imize fitness by laying large first clutches early in the season while with holding few resources for renesting attempts. We tested whether differences in nesting habits would be associated with differences in egg size and ren esting capacity in razorbills (Alca torda) and Brunnich's guillemots (Uria lomvia), long-lived marine birds of the tribe Alcini (family Alcidae) that lay single-egg clutches. Razorbills nest in a dispersed fashion, often buil ding nests of small stones and other matter in enclosed crevices where thei r eggs and chicks are safe from avian predators. Brunnich's guillemots nest very densely on exposed cliff ledges and do not build nests. Due to these differences in nesting habits, razorbills experience higher hatching succes s than Brunnich's guillemots. As predicted by the renesting hypothesis, raz orbills laid larger eggs than guillemots, after accounting for differences in adult body mass. Associated with this larger investment in first eggs, r azorbills were less likely than guillemots to replace lost eggs, and they t ook longer to do so. As these results were obtained by experimentally remov ing eggs front early-laying females, they presumably assessed the tendencie s of more capable birds in both species. Data from other colonies and years were consistent with these results. We conclude that differences in nestin g habits have led to evolutionary divergence in the investment made in firs t and replacement eggs by these two closely related species.