Effects of Supplemental Feeding on Timing of Breeding, Clutch-Size and Polygyny in Red-Winged Blackbirds Agelaius phoeniceus

Abstract

Abundance of a protein-rich food was experimentally increased in territories of red-winged blackbirds during the 1977 and 1978 breeding seasons in Washington state, USA. Supplemental feeding advanced the onset of nesting. The effect was increased when food was provided earlier in the prebreeding season. Within experimental territories the frequency distribution of nesting activity as a function of season was skewed to the right; in controls the distribution was more symmetrical. Offspring mortality rates increased as the season progressed and were higher in experimental territories than in controls. Frequencies of chicks found dead in their nests and frequencies of complete brood loss indicate that the increased rates were due to predation rather than starvation. Clutch-size decreased significantly over the season only in artificially enriched territories in 1977. In 1978, when breeding was advanced more substantially, clutches were smaller early in the season, increased rapidly during the normal onset of the breeding season and declined thereafter. Mean sizes of clutches in enriched territories were not larger than in controls. When a single feeder was provided per territory, numbers of females nesting simultaneously in enriched territories were greater than in controls. When density of feeders was increased by approximately an order of magnitude, intrusion rate increased, density of territorial males increased and enriched territories did not have significantly more females nesting simultaneously than did controls.