Reproductive Success Of Birds Research Articles

Assessing Population Trends of New Hampshire Forest Birds: Local vs. Regional Patterns

Abstract We examined the changes in abundance between 1969 and 1986 of 19 forest-dwelling, mostly migratory bird species breeding in New Hampshire at 2 different scales: one local (an intensively studied 10-ha plot in unfragmented forest) and the other regional (Breeding Bird Surveys statewide). Twelve of the 19 species exhibited similar trends at both scales. Eight neither increased nor decreased, and 4 (Least Flycatcher, Winter Wren, Wood Thrush, and Swainson's Thrush) declined significantly. Others increased, decreased, or remained steady at one or the other scale. Overall, more species declined than increased both locally (8 vs. 1) and regionally (5 vs. 1). Comparisons of these patterns, combined with results of intensive studies at the local level, suggest that changes in food abundance and in vegetation structure related to forest succession on the breeding grounds, along with other processes that influence bird reproductive success and survivorship, are the most plausible explanations for most of the observed trends. Winter mortality was also identified as affecting breeding abundances, but only in short-distance migrant and permanently resident species. We have no evidence to indicate that the numbers of long-distance migrants were affected by events in their Neotropical wintering areas, although this possibility is difficult to assess from breeding-ground data. We urge caution in attributing declines of breeding forest migrant birds to tropical deforestation or similar causes until we either can eliminate alternate explanations that involve breeding-season events or have available critically needed demographic information on migrant populations in their wintering areas.

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

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.

Multiple Paternity in a Territorial Passerine: The Bobolink

Abstract Electrophoretic and behavioral evidence corroborate the conclusion that Bobolink (Dolichonyx oryzivorus) females occasionally copulate with more than one male, which results in the insemination of a single clutch of eggs by those males. In 2 of 12 families from which blood samples were obtained from the mother, the putative father, and the nestlings, at least 2 males were responsible for fertilization of the clutch. We believe this is only the second bird species in which multiple paternity has been documented conclusively in the wild. Our speculation that it may not be uncommon leads to a caution for evolutionary biologists who use apparent reproductive success in birds to address questions about fitness.

Spatial and Temporal Aspects of Colonial Nesting of White Pelicans

Since Darling's (1938) early observations of gulls, many studies have focused on spatial and temporal patterns of reproduction in seabird colonies. Reproductive success of birds differs among colonies at different sites (Hunt 1972, Harris 1973), among habitats within a colony (Brown 1967, Nettleship 1972), between nests positioned centrally or peripherally in a colony (Patterson 1965, Coulson 1966, Tenaza 1971, Dexheimer and Southern 1974), with the relative timing of breeding (Patterson 1965, Vermeer 1970, Parsons 1975, Hunt and Hunt 1976), and between successive seasons (Fordham 1970). At traditional nesting sites, nests of White Pelicans (Pelecanus erythrorhynchos) are grouped into many spatially and temporally separate colonies (Hall 1925, Low et al. 1950, Behle 1958). Since many colonies occur on a single island, climatic factors influencing reproduction affect all birds similarly. At Gunnison Island, Utah, food is equally available to pelicans from different colonies (see discussion by Orians 1961) at the limited foraging areas along the eastern shore of Great Salt Lake (Behle 1958). Patterns of reproductive success in pelicans at Gunnison Island, show basic aspects of the colony-nesting habit better than at many other places. In 1973 and 1974 I studied colonial nesting of White Pelicans at Gunnison Island. I report here the spatial and temporal distribution of their nesting efforts, how the distribution was established, and variations in reproductive success.

Effects of Weather on Reproductive Success of Birds at Churchill, Manitoba

Describes the effect of a severe wind and rain storm, 8 July 1965, on passerine bird nestlings in this area. Mortality from exposure was high: there was little evidence of starvation. The weather did not effect nests with eggs only. Non-passerine species appeared largely unaffected by the storm. The altricial condition of passerine young may account for paucity of arctic passerine species.