Energy Flow in Two Breeding Populations of Redwinged Blackbirds

The energy requirements of nestlings and adults in two breeding populations of Redwinged Blackbirds (Agelaius phoeniceus) were studied from 1961 through 1964. The energy at several trophic levels was determined and a study undertaken of environmental factors that may influence energy flow throughout the ecosystem. Two marshes in Center Co., Pennsylvania, were chosen as study areas. The adult and nestling population on one study area (Duck's Marsh) was the same during 1961 and also in 1962; hence, the energy requirements of the population were the same both years. The number of males that established permanent territories on the other (Millbrook Marsh) was essentially the same each year; however, the females varied during the four years of study. The variation in the size of the breeding population and mortality rate of nestlings in the different years resulted in fluctuations in the total energy requirements of the population in Millbrook Marsh. However, the percent utilization of the available energy supply (insects) did not vary significantly throughout the four years of study (P > .15). The size of the population on both marshes appears to correspond to the available energy supply. A correlation was found between the insect biomass and the size of the breeding population. The amount of energy available on the marshes as insects, in conjunction with the type of vegetation preferred for nesting, may be a factor in regulating the size of the breeding population. The Redwinged Blackbird populations on both marshes appeared to correlate their breeding activity with a period when an abundant energy supply (insects) was available for feeding the young. The environmental temperature is probably a common stimulus for the development of insects and for the nesting activity of birds.