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Because populations of territorial birds are relatively stable compared to those of other animal taxa, they are often considered to be tightly regulated. However, the mechanisms that produce density-dependent feedbacks on demographic rates and thus regulate these populations are poorly understood, particularly for migratory species. We conducted a three-year density-reduction experiment to investigate the behavioral mechanisms that regulate the abundance of a Nearctic–Neotropical migrant passerine, the Black-throated Blue Warbler (Dendroica caerulescens), during the breeding season. We found that the number of young fledged per territory, territory size, and the proportion of time males spent foraging were significantly greater on territories around which neighbor density was experimentally reduced compared to control territories. Territory quality, proportion of nests depredated per territory, and male countersinging rates were not statistically different between treatments. These results indicate that individuals with more neighbors (i.e., in neighborhoods with greater conspecific density) have reduced breeding productivity. The results also suggest that a crowding mechanism that mediates interactions among territory-holders could generate the density dependence needed to regulate local abundance, at least in areas of homogeneous, high-quality habitat. The effect of the neighbor-density reduction on warbler fecundity and behavior varied with annual fluctuations in weather and food availability, and was strongest in 1997, an El Niño year, when conditions for breeding were least favorable. This variation in our experimental results among years implies that density dependence due to crowding may have its strongest impact on local abundance when environmental conditions are relatively poor.
Sillett et al. (Wed,) studied this question.