Mechanisms regulating bird populations are poorly understood and controversial. Here, we provide evidence that a migratory songbird, the black-throated blue warbler (Dendroica caerulescens), is regulated by multiple density-dependence mechanisms on its breeding quarters.
We are investigating the factors that determine the size of migratory songbird populations. Our focal species for this research is the black-throated blue warbler (Dendroica caerulescens).
Scientists have long been interested in how animal populations are controlled, or regulated. A fundamental question in ecology is: what keeps populations from becoming too numerous or from going extinct? The size of regulated populations tend to grow when the density of individuals is low, and tend to decline when population density is high. This phenomenon is called “density dependence.”
Although many animal populations fluctuate as if regulated, the mechanisms that generate regulation in nature are poorly understood. Elucidating these mechanisms is essential for predicting how populations will respond to environmental perturbations such as habitat fragmentation or climate change.
Unlike many migratory songbird species, black-throated blue warbler populations have remained relatively stable since the 1960s. This stability allows us to examine the natural processes that cause the warbler’s population to vary over time.
We have monitored black-throated blue warblers at Hubbard Brook Experimental Forest in the White Mountains of New Hampshire for over 30 years. Our research has shown that breeding habitat quality influences this species':
Although the latter relationship is strong evidence for regulation, the mechanism(s) underlying regulation in this population, as well as for those of any migratory songbird, remains elusive.
For instance, the pattern shown in Figure 2 could be explained by the following mechanisms:
Recently, we tested for the effects of crowding and site dependence in our study population.
This density reduction experiment yielded clear demographic and behavioral differences in the two treatments.
Birds in the reduced-density treatment:
This experiment demonstrated that neighboring conspecifics can affect warbler reproductive success and behavior, and therefore that crowding is an important regulatory mechanism operating in this population.
We randomly selected 50 black-throated blue warbler territories on our study site and quantified the following variables over a 3 year period:
The territory sites differed markedly in all measures of suitability; these differences among sites were predictable, and black-throated blue warblers responded to these differences (Figure 4).
These differences in site characteristics corresponded with significant differences in annual production of young. Poor sites, when occupied, produced only about half the number of young on average when compared to good sites. Thus, quality of individual sites (territories) affects reproductive performance in this species, which is a key condition for a site dependent regulatory mechanism.
Our results, combined with a computer simulation model, demonstrate that songbird populations can be regulated solely by the negative feedback on reproductive success generated by warbler density. This density-dependent negative feedback seems to be caused by both crowding and site dependence mechanisms.
To date, therefore, the results from our long-term demographic monitoring, the density reduction experiment, site suitability measures, and computer models suggest that population regulation in our study species, and probably other songbirds, is is the result of multiple negative mechanisms.