Black-throated blue warblers have a fairly stable population at the Hubbard Brook Experimental Forest in New Hampshire. 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.
Intensive and long-term studies of this warbler have shown that crowding is one of several factors that keep their population in check. Crowding occurs when there are so many pairs of warblers in an area that the territories are smaller than normal.
Scientists compared territories with few neighbors to those with a lot of neighbors. They found that the uncrowded territories were larger and the males spent more time looking for food (as opposed to mate guarding or defending the territory). This resulted in more young fledging than in the crowded territories.
So as the warbler population gets bigger, the territories get smaller, fewer young are produced, and the population falls. As the population falls, the territories get bigger, more young are produced, and the population grows. This loop keeps the population stable over many years.
The study was conducted over 3 years and the results were not uniform, it appears that crowding is most influential in years when food is scarce, for example, an El Niño year.
This article summarizes the information in this publication:
Sillett, Terence S., Rodenhouse, N. L. and Holmes, R. T. 2004. Experimentally reducing neighbor density affects reproduction and behavior of a migratory songbird. Ecology, 85(9): 2467-2477.
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 Nino 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.
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