Causes and Consequences of Egg Loss in Rockhopper Penguins, Eudyptes chrysocome

Much debate has surrounded the fate and function of the first-laid egg of crested penguins (Eudyptes spp.) because that egg is much smaller than the second, frequently disappears during incubation, and seldom survives to produce a fledgling. We studied the causes and timing of first-egg mortality in two populations of rockhopper penguins (E. chrysocome), assessed the frequency with which first eggs replace second eggs (i.e. provide effective insurance against second-egg failure), and compared the rates of egg retention in different microhabitats. Observed egg losses were due mainly to skua (Catharacta skua) predation, but also stemmed from aggressive behavior among conspecifics and movements by a newly-hatched sibling. Rates of first egg loss were similar between geographic locations, but varied markedly among microhabitat types; significantly more nests in a vegetated area retained both offspring to hatching. The insurance value of first eggs was realized in less than half the nests from which second eggs were lost. We suggest that nesting habitat is important in determining the cost effectiveness of retaining a surplus offspring in crested penguins.