Crowding Affects Black-throated Blue Warbler Foraging Behavior
By Gregory Gough
In the forests of the White Mountains of New Hampshire, black-throated blue warbler pairs set up territories in crowded neighborhoods. Typically, they are surrounded by 4 to 6 couples.
Scientists wondered how this concentration of birds might affect their ability to forage for insects. Lots of neighbors could mean smaller territories with less food, and more time spent defending the territory from the encroachments of neighbors.
To answer the question scientists established control plots in sparsely populated areas to compare with the densely populated plots. In the control plots the pairs would likely have larger territories, more food, and need to spend less time in defence.
To see if there were differences between the plots scientists took 3 measurements: how often birds caught insects, how quickly they ate, and how often they hunted aerially versus hopping among branches. Aerial feeding occurs when the warbler flies out from a perch to catch prey.
The results showed that the parents ate and caught prey at the same rates in both the control and experimental plots. However, there was a difference in the type of foraging—those in the control plots spent more time gleaning insects from the foliage as opposed to flying out to catch insects.
It is likely that lesser availability of food in the crowded plots forced the birds to fly out to catch different prey items (flying bugs, for example). This comes at a cost as it is easier to hop among branches to catch prey than to fly out and try to catch it.
This study continues a series of experiments on what happens when an animal's population increases and what subsequently caused the population to stabilize or reduce.
This article summarizes the information in this publication:
Dobbs, Robert C., Sillett, Terence Scott, Rodenhouse, Nicholas L. and Holmes, Richard T. 2007. Population density affects foraging behavior of male Black-throated Blue Warblers during the breeding season. Journal of Field Ornithology, 78(2): 133-139.
Foraging behavior often reflects food availability, a resource that may increasingly limit breeding birds as intraspecific crowding increases.Measuring foraging behavior, therefore, provides a way to investigate effects of population density on food limitation, an important link in understanding how crowding functions to regulate populations.We quantified three components of foraging behavior (prey attack rate, foraging speed, and relative use of morphologically constrained attack maneuvers) for male Black-throated Blue Warblers (Dendroica caerulescens) breeding under experimentally manipulated density conditions. Building on the previous work showing the density of conspecific neighbors affects territory size, reproductive success, and the time budgets of males (Sillett et al. 2004, Ecology 85: 2467–2477), we further show that density affects male foraging strategies. Although not differing in attack rate or foraging speed, male Black-throated Blue Warblers on territories with reduced neighbor densities used energetically expensive aerial attack maneuvers significantly less frequently than males in control (high-density) territories during both the incubation period and when provisioning nestlings and fledglings. We conclude that males altered their foraging behavior to compensate for density-related reductions in time available for foraging and that population density may constrain the time available for foraging.
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