Trumpeter Swan Incubation in Areas of Differing Food Quality

Abstract

Intraspecific comparisons along geographic gradients are useful in evaluating the adaptive sig- nificance of avian incubation behavior. Incubation constancy in northern-breeding Anserinae is correlated with breeding success, and trumpeter swan (Cygnus buccinator) populations vary in growth and other population characteristics. Thus, we compared incubation constancy and other behaviors of female trumpeter swans nesting in 2 areas suspected to differ in food quality and availability: the Copper River Delta, Alaska (1988-89), and the Tristate Region of Idaho, Montana, and Wyoming (1991). Alaskan females fed on abundant emergent macrophytes, whereas Tristate females fed on submerged macrophytes that may require more search and handling time. Alaskan swans took shorter (P = 0.001) incubation recesses and maintained higher (P = 0.017) incubation constancy than Tristate birds. Our data are consistent with the hypothesis that the Tristate Region is lower quality swan habitat than that in Alaska because it appears to lack food resources necessary for female swans to maintain a high incubation constancy in an area with a short and harsh breeding season.