Two's Company Three's a Crowd: Black-throated Blue Warbler Population Stable
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 scientific paper:
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.
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