Food Supply and Egg Mass Variation in the European Coot

Female European Coots (Fulica atra) typically lay clutches of seven to eight, in which egg mass increases through the laying sequence. The proportions of yolk and albunmen do not change with egg mass, and hatchling mass is proportional to egg mass. Hatching is asynchronous, and laterhatced chicks often starve or die from exposure. These breeding characteristics are proposed to be related to the exploitation of a food supply showing both long— and short—term unpredictability in abundance. The higher mass of the later chicks within a brood may allow them to survive long enough to exploit any sudden increases in food abundance at little parental cost. Experimental manipulation of food supply during the prelaying period shows that this systematic variation in mass within a clutch may be due to a facultative response in females; individuals fed upon an extremely constant daily food supply laid less variable clutches than females exposed to the natural food supply. The function and mechanism of this response are discussed.