The Influence of Drought on Reproduction in a Breeding Population of Redwinged Blackbirds

Abstract

The history of a breeding population of Redwinged Blackbirds (Agelaius phoeniceus) in Centre Co., Pennsylvania, recorded from 1960 through 1964, was analyzed to gain a better understanding of the effects of drought on a population. The average air temperature in the study area during the nesting season in May and June was 15.8 C in 1962 and 17.1 C in 1964 During the 4 years of study (1960-63), the average annual precipitation was 6.55 inches below the normal annual precipitation of 39.15 inches, and the vegetation changed from a cattail (Typha latifolia)-sedge (Carex spp.) marsh to predominantly a sedge marsh. The number of males that established permanent territories was essentially the same in each of the 5 years of study. However, the female portion of the population decreased from 42 in 1960 to 7 in 1963, but increased to 17 in 1964 The total number of nestlings decreased from 100 in 1960 to 15 in 1963, increasing to 40 in 1964 The number of young lost to the population through mortality resulted in a variation in the number of young fledged. The size of the population appears to correspond to the energy available to the population as insects. The relationship between rainfall and reproduction in birds is discussed. In contrast to tropical and xerophilic species, rainfall does not appear to be a stimulus for the timing of nesting activity in birds in the temperature deciduous forest biome. However, rainfall and its effects on the type of vegetation and insect biomass may regulate the size of the population.