Feeding Behavior, Size Asymmetry, and Food Distribution in Magellanic Penguin (Spheniscus magellanicus) Chicks

Abstract

How food is distributed among siblings has long been of theoretical importance because the interests of siblings differ (Trivers 1974). The manner in which food is apportioned among siblings appears to be important in the determination of chick mortality patterns. One of the factors that could be affecting this food distribution is the size relationship between siblings. Penguins vary in their degree of hatching asynchrony, egg-size dimorphism, and fledging success (Lamey 1990). In Spheniscus penguins, hatching asynchrony creates size asymmetries that can lead to brood reduction (Lamey 1990, Boersma 1991, Boersma and Stokes 1995). Magellanic Penguins (Spheniscus magellanicus) at Punta Tombo, Argentina, show high variability in breeding success among years, mainly as a result of changes in food availability (Boersma et al. 1990). During years with low food availability, most pairs raise no offspring or only one as a result of the starvation of one or both chicks (Boersma and Stokes 1995). When one sibling dies, the surviving chick receives all the food brought to the nest by the parents, resulting in an increased growth rate (Boersma 1991). We describe the feeding behavior of Magellanic Penguin chicks, quantify the way food brought to the nest is apportioned among siblings, and analyze how this distribution relates to chick size asymmetries. Study area and methods.-The study was conducted in the Punta Tombo Provincial Reserve, Chubut, Argentina (44002S, 65?11'W), during January 1990 and 1991. Punta Tombo is the site of the largest continental colony of Magellanic Penguins (Boswall and McIver 1975), with 225,000 breeding pairs (Boersma et al. 1990). Magellanic Penguins have a seasonal breeding schedule in which they begin returning to the colony in late August and early September, and lay two eggs in October that are laid three to four days apart (Boersma et al. 1990). Eggs are always nearly equal in size with an average difference of 5 g between the larger and smaller eggs (Boersma and Stokes 1995). Chicks hatch in November approximately two days apart, and fledge in late January and February. Both adults defend the nest site, incubate