Long-Term Dynamics of a Wood Thrush Population Breeding in a Forest Fragment

Abstract

ABsmAcT.-The dynamics of a population of Wood Thrushes (Hylocichla mustelina) in a Delaware woodlot from 1974 through 1990 did not meet all predictions of the forest-fragmentation hypothesis, which posits that declining abundance of Neotropical, migratory, forest songbirds results from poor reproduction in, and immigration to, forest fragments. Abundance declined at an average rate of 4% per year over the period 1978 through 1987, equalling the national decline found by the Breeding Bird Survey for those years, but recovered by 1990 to pre-decline levels. Abundance of previous residents followed a similar pattern, the only population component to do so. Abundance of new immigrants and of returning, locally produced young was generally stable. Throughout the study, reproductive rates were high enough to maintain the population at the return rates observed early and late in the study. However, a sustained episode of reduced production per female and of an increased percentage of adults failing to produce any young generally coincided with the decline in abundance. When the failure rate later dropped, return rate and abundance subsequently increased. We conclude that a period of elevated, predation-caused failure prompted greater emigration by an ever-younger, less-site-faithful population. High emigration coupled with stable immigration, stable recruitment of local young, and even normal mortality of residents would yield a declining population. A regional predation episode could cause a broader decline in abundance through several mechanisms that could reduce the number of available immigrants. Received 25 March 1991, accepted 9 November 1992.