Intraspecific Fledging Mass Variation in the Alcidae, with Special Reference to the Seasonal Fledging Mass Decline

Abstract

Species in the avian family Alcidae show enormous inter- and intraspecific variation in age and mass at nest departure (fledging). We develop a dynamic programming model of the nest departure decision that incorporates the differential growth benefits and mortality costs of the pre- and postfledging habitats (nest and ocean). The model's most basic prediction is the existence of a negatively inclined fledging boundary in mass-time space: nestlings fledge as their growth trajectory hits the boundary. The consequence is that faster-growing nestlings fledge younger and heavier than slower growers and that later-hatched nestlings fledge younger and lighter. A review of the comparative and experimental data in the literature largely supports these predictions. We use the model to show how a widespread pattern reported in the literature-the seasonal fledging mass decline-arises because of the interaction of growth and hatching date variation with the fledging strategy modeled here. The seasonal decline occurs in the absence of any of the causes most commonly suggested in the literature, particularly the seasonal decline in food availability. A detailed case study of the rhinoceros auklet (Cerorhinca monocerata) strongly supports the mechanism outlined here.