Population Regulation of Black-throated Blue Warblers
By Gregory Gough
Intensive and long-term studies of black-throated blue warblers at the Hubbard Brook Experimental Forest in New Hampshire reveal a fairly stable population . What keeps them from becoming too numerous or helping them become numerous after a lean year, is of interest to scientists studying how populations regulate themselves and has long-term implications with regard to climate change and habitat destruction.
Studies of this warbler have shown that crowding and site-dependence are two factors that keep their population in check.
The study was begun in 1969 and continues to the present. Such long-term data studies on birds are rare but useful for understanding population changes.
This article summarizes the information in this publication:
Rodenhouse, N. L., Sillett, Terence Scott, Doran, P. J. and Holmes, R. T. 2003. Multiple density-dependent mechanisms regulate a migratory bird population during the breeding season. Proceedings of the Royal Society (London) B, B270: 2105-2110.
The mechanisms regulating bird populations are poorly understood and controversial. We provide evidence that a migratory songbird, the black-throated blue warbler (Dendroica caerulescens), is regulated by multiple density-dependence mechanisms in its breeding quarters. Evidence of regulation includes: stability in population density during 1969–2002, strong density dependence in time-series analyses of this period, an inverse relationship between warbler density and annual fecundity, and a positive relationship between annual fecundity and recruitment of yearlings in the subsequent breeding season. Tests of the mechanisms causing regulation were carried out within the Hubbard Brook Experimental Forest, New Hampshire, during 1997–1999. When individuals from abutting territories were experimentally removed in a homogeneous patch of high-quality habitat, the fecundity of focal pairs nearly doubled, revealing a locally operating crowding mechanism. A site-dependence mechanism was indicated by an inverse relationship between population size and mean territory quality, as well as by greater annual fecundity on the sites that were most frequently occupied and of highest quality. These site-dependence relationships were revealed by intensive monitoring of territory quality and demography at the landscape spatial scale. Crowding and site-dependence mechanisms, therefore, acted simultaneously but at different spatial scales to regulate local abundance of this migratory bird population.
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