Bird Community Dynamics in a Temperate Deciduous Forest: Long‐Term Trends at Hubbard Brook

Abstract

Changes in species composition and abundance of birds breeding in an unfragmented temperate deciduous forest in New Hampshire, USA, were studied intensively during 16 consecutive breeding seasons, 1969—1984. The number of species breeding in the 10—ha study area in any one year varied from 17 to 28, and averaged 24. Total numbers of individuals breeding on the 10—ha plot ranged from 214 to 89, with many species (70%) declining during the 16—yr period. Overall, there was significant positive covariation among population trends of all species, suggesting a major, perhaps single, factor affecting population levels, such as weather or food supply. No two species, however, had identical patterns of change across all 16 yr. Thus, populations in this forest fluctuated largely independently of one another, suggesting a different combination of regulatory factors for each species. The major factors pulsing or stressing bird populations in the field included: (1) changes in food abundance due largely to irruptions of defoliating Lepidoptera (most bird species, but especially vireos and warblers), (2) harsh late spring and summer weather (Scarlet Tanager, American Redstart), (3) changes in habitat structure related to forest succession (Least Flycatcher, Philadelphia Vireo), (4) interspecific interactions, particularly interference competition (Least Flycatcher—American Redstart, Red—eyed Vireo—Philadelphia Virreo), and (5) mortality during winter for both resident and migrant populations (Hermit Thrush, Dark—eyed Junco, permanent resident species such as woodpeckers and nuthatches. Some, but not all, irruptions of defoliating Lepidoptera significantly influenced many bird populations in this forest and contributed to the observed positive covariation in abundances. These irruptions produced pulses of food for breeding birds, but occurred at long and variable time intervals. Between outbreaks, food may regularly limit reproductive output of these forest birds, and low food abundance, along with the effects of predators, weather, and other mortality factors, contributes to high variability in bird reproductive success, which ultimately influences population size. Competition may also become important during these periods of food scarcity. We therefore propose that birds in these temperate deciduous forests experience periods of prolonged food limitation, interrupted by relatively brief periods of superabundant food, a pattern contrasting with that described for other temperate systems (e.g., by Wiens 1977). From this examination of bird community dynamics on one site over many years, it is apparent that each species responds to its environment in a unique way, as determined by a variety of influences on its populations. Some of these operate on a local scale (e.g., vegetation structure, food abundance, interspecific competitors), while others function at regional (e.g., some weather effects) and global (e.g., winter events) geographic scales. These influences also differ as to their temporal persistence and predictability, and thus are difficult to detect in short—term studies. This pluralistic view of community structure for birds in temperate forests represents a compromise between nonequilibrial and competitionist models, and argues against the existence of a tightly organized community at any one spatial or temporal scale.