Egg Size, Eggshell Porosity, and Incubation Period in the Marine Bird Family Alcidae

A .—Although the ultimate factors that infl uence the duration of avian incubation periods are well known, we know much less about the proximate mechanisms by which birds adjust incubation period in response to selection. We tested the hypothesis that an adjustment in eggshell porosity is one such proximate mechanism (i.e., that avian species with higher ratios of incubation period to egg size lay eggs with less porous shells). Eggshell porosity aff ects the rate of gaseous exchange between the developing embryo and the external environment; thus, to the extent that embryonic metabolism is diff usion-limited, eggshell porosity could directly determine incubation period. To test that hypothesis, we collected eggs from seven species of Alcidae, a family of marine birds that exhibits an unusual degree of interspecifi c variation in incubation period, and measured egg mass and eggshell porosity (determined by the number and size of pores and the thickness of the shell). Incubation periods were obtained from the literature. Egg mass and eggshell porosity combined explained 87% of the variation in incubation period among the seven species, which included at least one member of each of the six main alcid lineages. As predicted, eggshell porosity and incubation period were negatively related, a er controlling for egg mass. Our results are consistent with the hypothesis that evolutionary changes in avian incubation period may be a ributed, at least in part, to adjustments in eggshell porosity. Received 21 December 2004, accepted 14 March 2006.