Multiple density–dependence mechanisms regulate a migratory bird population during the breeding season

Abstract

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.

Social information trumps vegetation structure in breeding-site selection by a migrant songbird

“…We chose black-throated blue warblers as our study species because their habitat requirements have been studied extensively (Holmes et al 1996;Rodenhouse et al 2003), they are specialized in their habitat use (Doran & Holmes 2005) and the structure of their mature hardwood forest habitat is relatively stable between years (Holmes et al 1996). Also, there is correlative evidence that this species aggregates, which suggests that individuals may use others as cues to locate suitable habitat (Betts et al 2006).…”

“…To test the hypothesis that male song in the post-breeding season is correlated with reproductive success, we randomly selected 60 warbler territories from a concurrent study on the demography of black-throated blue warblers (Rodenhouse et al 2003) that either successfully produced fledglings (nZ30) or failed (nZ30). We used 20 min point counts, instead of 10 min as in our previous sampling, to increase detection probability of infrequent songs.…”

Experimental food supplementation reveals habitat-dependent male reproductive investment in a migratory bird

“…The dominant vegetation in the understory is hobblebush, striped maple (Acer pensylvanicum) and saplings of canopy species. The abundance of Lepidoptera larvae, the primary food source and important predictor of territory quality for black-throated blue warblers [27], is positively correlated with elevation at Hubbard Brook [24,34]. Natural variation in food availability is similar between the mid and high elevations, relative to the low elevations [24,34].…”

Inadvertent social information in breeding site selection of natal dispersing birds

“…The way distribution patterns arise have been the subject of theories such as isoleg models (Rosenzweig 1981;Abramsky et al 1991), species assembly models (Fox 1987;Brown et al 2000a) and a family of 'ideal' distribution models: ideal free (Fretwell & Lucas 1970), ideal despotic (Fretwell 1972), ideal pre-emptive (Pulliam 1988) and ideal interference (Sutherland 1983;Lessells 1995) that are perhaps the best known among sitedependent theories (Rodenhouse et al 2003).…”