The adaptive significance of nutrient reserves to breeding American Coots: A reassessment

Female American Coots (Fulica americana) lay large clutches of nutrient-rich eggs, and both sexes assist with incubation and brood-rearing. We studied the role of stored fat, protein, and ash reserves in meeting the nutritional demands of reproduction by analyzing carcass composition of 138 male and 181 female coots collected in southwestern Manitoba during 1987-1991. For females, mean reserve levels did not change between prelaying, laying, incubation, early brood-rearing, and late brood-rearing; however, fat reserves of males doubled between early and late brood-rearing (21.1 vs. 45.2 g, respectively). Females with larger fat reserves began nesting earlier, but paradoxically, they did not utilize nutrient reserves for egg laying, and 1991 females even accumulated fat reserves during laying. Both sexes accumulated fat reserves during incubation and brood-rearing. Coots that had access to supplemented food during laying, incubation, and late brood-rearing had larger fat and protein reserves, but supplemental food did not otherwise after patterns of nutrient-reserve usage. Nutrient-reserve dynamics of American Coots differed markedly from sympatric populations of breeding ducks, which utilize fat reserves extensively during laying and incubation. We believe that two principal factors contribute to this difference: (1) the nutrient demands of egg formation in coots are about one-half those of comparable-sized ducks, and (2) biparental care in coots allows males and females to share the costs of incubation and brood-rearing, whereas in ducks these costs are incurred entirely by females. We conclude that reproduction in American Coots is not constrained by the availability of endogenous nutrients.