Variation in laying date, clutch size, egg size, and egg composition of yellow-headed blackbirds (Xanthocephalus xanthocephalus): a supplemental feeding experiment

I studied the effects of food availability, habitat quality, and timing of breeding on egg production in yellow-headed blackbirds (Xanthocephalus xanthocephalus). Food availability was experimentally manipulated by providing females on six wetlands with supplemental food; six additional wetlands served as unsupplemented controls. Mean nest initiation dates varied by up to 6 d among wetlands, and supplementally fed blackbirds initiated nests 2 d earlier than controls, on average (although this latter difference was not quite significant; P = 0.07). Clutch size declined with laying date, but was unaffected by wetland location, food supplementation, or interactions between these two factors and laying date. Although egg size did not vary among wetlands or in relation to supplemental feeding, egg composition varied with both of these factors. All egg components except wet and dry shell and dry albumen varied among wetlands, whereas total water, wet yolk, and lean yolk were the only components that varied with food supplementation. Large blackbird eggs contained proportionately more water and albumen, but proportionately less yolk and shell. These patterns were somewhat compensatory, such that proportional protein and energy content did not vary with egg size; however, large eggs contained proportionately less fat than did small eggs. Proportional egg composition varied among wetlands (yolk and energy content), but was not affected by supplemental feeding. In general, egg production by yellow-headed blackbirds was not greatly affected by food availability. This may have been due to any of the following four factors: (1) inaccessibility of food supplements owing to competition between male and female blackbirds, (2) insufficient time for females to respond to food supplements, owing to rapid settlement and nest initiation, (3) a nutritionally inappropriate food supplement (i.e., protein availability may not have been enhanced among fed birds), or (4) superabundance of natural foods such that food availability was not limiting egg production.