Breeding Sex Ratios, Territoriality, and Reproductive Success in the Red‐Winged Blackbird (Agelaius Phoeniceus)
1973
Holm, Celia Haigh
Data on sex rations, territoriality, and reproductive success were obtained from a breeding population of Red—winged Blackbirds, and were used to examine the adaptive significance of polygyny and nest—site selection. All the territorial males on the study area were adults, and the sex ratio of the population was 3.0 females per male in 1966 and 2.7 females per male in 1967, with the range being 0—6 females per male in both years. The number of young fledged per male and per female increased with increasing harem size. Harem size was related to the type and density of the vegetation in a territory and not to the size of the territory or the food supply on it. Harems were larger in cattail than in bulrush territories and larger in areas with scattered cattails than in those with denser cattail growth. Cattail territories fledged more young per female than bulrush territories in 1967, but not in 1966. Nests placed in cattails had a higher fledging success both years than nests in bulrushes, and nests in sparse cattail cover fledged more young per nest than nests in denser growth. Concealment from predators and, to a lesser extent, shelter from wind and rain probably account for the preference for cattails as a nest—site. Nests built in cattails suffered less predation than bulrush nests, and in 1966 nestling starvation was lower in cattails. Nests were generally placed higher above water in cattails than in bulrushes, and predation decreased with increasing nest height.
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