Intraspecific Variability in Nestling Growth and Fledging Behavior of Cassin's Auklets at Triangle Island, British Columbia

Abstract

We compared nestling growth and fledging behavior of Cassin's Auklets (Ptychoramphus aleuticus) at Triangle Island, British Columbia to the predictions of a model that considers the timing of fledging to be an adaptive strategy. In the model, fledging mass and age depend on nestling growth rate, time remaining in the season, and the contrasting mortality costs and growth benefits experienced before and after fledging. As predicted, fast growing nestlings fledged heavier and younger than slow growing nestlings. Growth rates declined over the season and fledging behavior varied accordingly. When the seasonal variation in growth rates was statistically controlled, late nestlings did not fledge lighter and younger, in contrast to the model's predictions. Late in the season, nestlings reached a greater peak mass than expected based on their slower growth rates. Also, nestlings that grew more slowly due to a higher frequency of handling reached a higher peak mass than less frequently measured nestlings. We consider the possibility that parents adaptively modified their nestling's growth trajectory by altering provisioning behavior in response to nestling condition. The nesting habitat influenced fledging. Fast growing nestlings fledged at similar ages in both level and steep nest sites. In contrast, slow growing nestlings fledged at younger ages, but similar masses, in level sites compared to steep sites. We consider the possibility that the difference in parental predation risk between level and steep nest sites influenced provisioning decisions of parents, and consequently, fledging decisions of nestlings.