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.