Egg Composition of American Coots in Relation to Habitat, Year, Laying Date, Clutch Size, and Supplemental Feeding

Abstract

Egg composition is generally assumed to influence the viability of hatchlings. We measured composition of 322 American Coot (Fulica americana) eggs to determine what factors influenced their variability. Eggs were obtained from 12 sample groups, which varied with respect to location (Delta Marsh or Minnedosa, Manitoba), year (3 yr at each location), and food supply (supplemental food was provided during 1 yr at Delta Marsh and 3 yr at Minnedosa). Egg size did not differ among sample groups, but measures of egg composition varied in both absolute and relative proportions. Egg composition differed between study sites, but only for shell and lean yolk masses. Year effects were pronounced. All variables except shell mass exhibited significant annual variation at one or both locations. Effects of supplemental feeding were generally insignificant, except in one year. In all feeding experiments, however, birds that received food supplements containing corn (high carbohydrate) produced eggs with greater absolute and proportional lipid content and greater energy density. Similar comparisons of birds receiving high-protein food supplements did not reveal any consistent trends in egg protein content. Among females, egg quality increased with clutch size and declined with laying date. Some of the among-group differences in egg composition appeared to be correlated responses to among-group variation in clutch size or laying date. There was no evidence of interwetland variation in egg composition. Although these results illustrate some potential plasticity in egg characteristics, we conclude that egg composition is a relatively inflexible component of the life-history strategy of American Coots.