Black-throated blue warblers are a common bird in the forests of New England. The factors that keep them from becoming too numerous or help them recover after a lean year intrigues scientists who study how populations regulate themselves. These questions also have long-term implications with regard to climate change and habitat destruction.
Intensive long-term studies of this warbler have shown that site-dependence is one of several factors that keep their population in check. Site-dependence means that the site a bird chooses, in this case a breeding territory, affects its survival or ability to raise a lot of young, and that the best sites are chosen first.
The study area is at the Hubbard Brook Experimental Forest in New Hampshire. Sites range from low to high elevations and in wet and dry areas. During the 3 years of the study some sites were occupied every year while others were only used for a year or 2.
If a site was used all 3 years it had, on average, 82% more food (caterpillars and spiders), 47% more leaves in the understory (better to conceal the nests), and 28% fewer nest predators.
Not surprisingly these factors combined to make some sites better for raising young. For sites used all 3 years the average number of young raised was 4.1 versus only 1.9 for the poorer sites.
Since the warblers only live a few years, and many perish during migrations to and from the wintering areas in the Caribbean, the hiqh quality sites are important as they produce enough young to keep the population going. The poorer sites do not produce enough young to keep the population stable.
Therefore, if one wanted to keep this bird's population stable, it would be important to protect not just the forest in which they live, but the best sites within the forest.
Additionally, the best sites tended to be at the highest elevation. Long-term climate change might cause the lowland vegetation to creep upslope and reduce the number of high-quality nesting sites. It is important to understand the population cycles and balance of birds in order to protect them.
This article summarizes the information in this publication:
Rodenhouse, N. L., Holmes, R. T. and Sillett, Terence Scott 2006. Contribution of site-dependence to regulation of population size: evidence and consequences for biological monitoring of populations. Acta Zoologica Sinica, 52: 457-464.
Site dependence, the pre-emptive use of sites (e.g., breeding territories) that differ in suitability for survival and reproduction, is a little-studied negative feedback mechanism that can potentially regulate population size. Because its operation depends upon heterogeneity among sites, it can operate at both local and landscape scales. Field tests on a population of a parulid warbler (Dendroica caerulescens) showed that sites differed greatly in suitability and were occupied preemptively. More poor sites were also occupied when population size was large. Because site dependence operates across a range of spatial scales it is difficult to test. The challenges it creates for sampling include the following. First, sampling designs need to include a broad range of the heterogeneity in suitability found among sites. This requires sampling across relevant environmental gradients and terrain (e.g., elevation, moisture, etc.) that are not typically found within small study plots. Secondly, direct measures of site suitability, such as both territory-based measures of food abundance and measures of bird population density and site-specific demography, are required to distinguish site-dependence from other potential regulatory mechanisms such as crowding. Given these sampling requirements, it is not surprising that site dependence, as one of the multiple mechanisms regulating bird populations, remains poorly understood. Nevertheless, knowledge of site dependence and other regulatory mechanisms is essential for understanding avian population dynamics and for the conservation of bird populations.
Teachers, Standards of Learning, as they apply to these articles, are available for each state.