Shifting Songbirds

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Male Black-throated Blue Warbler feeding nestlings. Nests of this species are built typically less than one meter above ground in a shrub such as hobblebush (Viburnum alnifolium)
N. Rodenhouse

Forest bird populations of northeastern North America are being increasingly affected by environmental challenges, including habitat loss and degradation, forest disturbances such as ice storms, atmospheric pollutants such as acid deposition, pathogens that enhance tree mortality and climate change. To develop conservation and management plans that might mitigate such impacts, a mechanistic understanding of the ecology of forest birds, specifically factors affecting their reproductive success, survival and recruitment is essential. Since 1969, Richard T. Holmes (Dartmouth College) and co-workers have been conducting research on the population and community ecology of birds inhabiting northern hardwoods forests within the Hubbard Brook Experimental Forest, New Hampshire, USA. These investigations, largely funded by NSF programs targeting long-term research (e.g., LTER and LTREB programs) have involved long-term measurements of bird abundance and distribution at small plot and landscape scales; intensive field studies of habitat use, feeding behavior, annual production of young, and the survival and behavior of individually-marked birds. Simultaneously, the resources and conditions in which the birds live were monitored, including: the abundance of food for birds, the number of nest predators and the structure, composition and dynamics of the vegetation in which they nest and forage. Because most of the bird species breeding in these forests are Neotropical migrants (i.e., they migrate long distances to winter in tropical habitats), the habitat use and survival of selected species in their tropical winter quarters has also been studied.

Forty years of research on avian ecology within the Hubbard Brook Experimental Forest has produced numerous major findings. The distributions and abundances of all bird species are surprisingly dynamic, even within well-developed and mature forests, and each species responds differently to changes in vegetation, food availability and other features of the forest environment such as the presence of members of the same species. However, nearly all species show aggregated distributions with the forest suggesting conspecific attraction. At the local scale, bird population dynamics are most affected by factors that influence fecundity and recruitment, particularly food availability, nest predators and weather. Food abundance limits the production of young in most years and the effects of food availability but not nest predators are dependent on the density of breeding pairs. Annual fecundity differs greatly among years due to differences in food abundance, which is strongly affected by weather, and nest predation that is largely caused by small mammals. Annual fecundity is correlated significantly with subsequent recruitment and is therefore critical for maintaining the abundance of breeding birds. Events in the non-breeding season, however, can also influence the bird abundance and dynamics, indicating connectivity of events and conditions throughout the species' annual cycles. At the landscape scale, populations are spatially structured by species' differential responses to vegetation, climate and social interactions such as competition and conspecific attraction. Results from these long-term studies have provided a mechanistic understanding of avian population and community dynamics, and they are being used to predict how future changes in habitat quality, climate, and other environmental changes may affect bird populations in north-temperate forests.

Forty-year of bird abundance on the long-term study plot at the Hubbard Brook Experimental Forest, New Hampshire, 1969-2008, and 15-year trends on three replicate study plots elsewhere in the White Mountains National Forest, 1986-2000. Y- axis shows total adults (males plus females for all species combined) occupying the 10-ha study plots.
Holmes, R. T. 2010. Birds in northern hardwoods ecosystems: Long-term research on population and community processes in the Hubbard Brook Experimental Forest. Forest Ecology and Management doi:10.1016/j.foreco.2010.06.021 Groffman, P.M., C.T. Driscoll, T.J. Fahey, J.P. Hardy, R.D. Fitzhugh, G.L. Tierney. 2001. Colder soils in a warmer world: A snow manipulation study in a northern hardwood forest ecosystem. Biogeochemistry 56:135-150. Fitzhugh, R.D., C. T. Driscoll, P. M. Groffman, G. L. Tierney, T. J. Fahey and J. P. Hardy. 2001. Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem. Biogeochemistry 56:215-238.
For further reading: 
Holmes, R. T. 2010. Birds in northern hardwoods ecosystems: Long-term research on population and community processes in the Hubbard Brook Experimental Forest. Forest Ecology and Management doi:10.1016/j.foreco.2010.06.021
Holmes, R.T., 2007. Understanding population change in migratory songbirds: long-term and experimental studies of Neotropical migrants in breeding and wintering areas. Ibis 149 (Suppl. 2), 2-13.
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