EFFECTS OF SUPPLEMENTAL FOOD ON EGG-PRODUCTION IN AMERICAN COOTS

Previous researchers have suggested that egg production in American Coots (Fulica americana) is limited by food availability for the laving female. I studied patterns of variation in laying date, clutch size, egg size, and laying rate of American Coots from 1985-1991 in southwestern Manitoba. Annual variation was evident for all four of these traits. Clutch size declined seasonally during most years, but the rate of decline differed markedly among years. These temporal patterns of variation in laying performance provided a template for assessing the relative importance of food availability. I provided coots with supplemental food during four breeding seasons (1987-1989 and 1991) to determine whether variation in laying performance could be attributed to food availability for the laying female. Supplementally fed coots nested one to two days earlier and laid about one more egg than did controls, but this represented only 1 and 3% of the observed variation in laying date and clutch size, respectively. In contrast, annual effects accounted for 12% of the variation in laying date, and annual plus seasonal effects accounted for 38% of the variation in clutch size. Supplemental food did not affect egg size or laying rate. Feeding effects were independent of year and date, suggesting that most natural variation in laying performance was independent of food availability. I conclude that reproduction in American Coots was weakly limited by food availability for the laying female. Alternatively, supplemental food may have caused female coots to modify their laying date and clutch size in anticipation of future food abundance during the brood-rearing period, but this hypothesis will be difficult to test.