Effects of Population Density and Supplemental Food on Reproduction in Song Sparrows

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Abstract
(1) Population density and reproduction in the song sparrow (Melospiza melodia, Wilson) on Mandarte Island. B. C. [Canada ] varied widely from 1975 to 1986. Several measures of reproductive success declined as population density increased by six times. At peak density, females produced only one-quarter the number of young produced at low densities. (2) We provided supplemental food to sixteen of seventy-two pairs before and during the breeding period in 1985, a year of peak density, to test the hypothesis that the amount of food available per pair declines as density increases and thus regulates reproduction. (3) Supplemental food (i) advanced laying date; (ii) increased clutch size, the number of breeding attempts, nestling weight and the number of independent young produced; and (iii) reduced brood parasitism and the interval between successful nesting attempts. (4) There was no difference in the subsequent survival or reproduction of fed and control young. (5) Supplemental food was associated with reduced adult survival to the next breeding season, perhaps because of increased competition for territories with feeders after the feeding experiment. (6) We conclude that food supply is important in regulating reproduction, and that food availability in the breeding period could limit population size.