Theoretical studies have shown that variation in density regulation strongly influences population dynamics, yet our understanding of factors influencing the strength of density dependence in natural populations still is limited. Consequently, few general hypotheses have been advanced to explain the large differences between species in the magnitude of population fluctuations. One reason for this is that the detection of density regulation in population time series is complicated by time lags induced by the life history of species that make it difficult to separate the relative contributions of intrinsic and extrinsic factors to the population dynamics. Here we use population time series for 23 bird species to estimate parameters of a stochastic density-dependent age-structured model. We show that both the strength of total density dependence in the life history and the magnitude of environmental stochasticity, including transient fluctuations in age structure, increase with generation time. These results indicate that the relationships between demographic and life-history traits in birds translate into distinct population dynamical patterns that are apparent only on a scale of generations.

Comparative analyses of avian population fluctuations have shown large interspecific differences in population variability that have been difficult to relate to variation in general ecological characteristics. Here we show that interspecific variation in demographic stochasticity, caused by random variation among individuals in their fitness contributions, can be predicted from a knowledge of the species' position along a "slow-fast" gradient of life-history variation, ranging from high reproductive species with short life expectancy at one end to species that often produce a single offspring but survive well at the other end of the continuum. The demographic stochasticity decreased with adult survival rate, age at maturity, and generation time or the position of the species toward the slow end of the slow-fast life-history gradient. This relationship between life-history characteristics and demographic stochasticity was related to interspecific differences in the variation among females in recruitment as well as to differences in the individual variation in survival. Because reproductive decisions in birds are often subject to strong natural selection, our results provide strong evidence for adaptive modifications of reproductive investment through life-history evolution of the influence of stochastic variation on avian population dynamics.

We examined 10 characteristics of natural cavities and their influence on reproductive success of Tree Swallows (Tachycineta bicolor) nesting in dead trees in beaver ponds. Large ranges were found for entrance height and area, cavity volume, and nearness to shore of nest sites. Other characteristics were less variable: 46% of cavities were less than 2 m above the pond surface, and 48% had entrance widths of 4-5 cm. Tree Swallow nest sites were uniformly dispersed in the ponds. Two cavity characteristics, cavity height and floor area, influenced reproductive success: Lower nest sites were more frequently preyed on and females laid larger clutches in cavities with a large floor area. Five species larger than Tree Swallows used cavities during the study. Girth of the snag at the base and at the cavity, entrance width, and cavity volume were significantly greater at sites used by these species than those used by Tree Swallows. Nest sites suitable for breeding did not appear to be limiting to Tree Swallows, because characteristics of unused cavities did not differ from those used by Tree Swallows and other species. Intraspecific territoriality was likely responsible for the large number of unused cavities in our populations. Other factors influencing cavity availability in our sites include interspecific competition, predation, snag fall, and continuing woodpecker excavation.

Summary 1.We describe age-related reproductive performance and recapture rates of tree swallows ( Tachycineta bicolor Vieillot) based on a 25-year study of a nestbox population in southeastern Ontario, Canada (1975-99). 2. Performance improved from first-time breeders to intermediate-aged birds. Nest initiation advanced, and clutch size increased in both sexes. In females the number of hatchlings and fledglings increased, and the proportion of nests failing completely declined. Performance declined in females after 'middle-age', in the number of young fledged, and the proportion of young fledged relative to initial clutch and brood size. Also, the proportion of nests that failed completely increased in the oldest birds. Males showed similar patterns.3. An index of performance incorporating clutch size, hatching and fledging efficiency, and two measures of total nest failure increased to, then declined after, 4 years of age in females and 3 years in males. The relationship between this index and age was best predicted by quadratic regression. 4. We found no support for three of four hypotheses to explain improvement in performance with age. Recapture rates declined after age 4Y in males, but remained unchanged in females until age 7Y +, while output decreased in both sexes (Residual Reproductive Value). Birds breeding repeatedly did not perform better during their first attempt compared to birds that bred only once (Selection). Birds with varied breeding experience did not differ in their performance within age-groups (Breeding Experience). We did find support for the Breeding Age hypothesis; in females with no breeding experience, there was a successive advance in laying and increase in clutch size from 2 to 4 years of age. 5. Improved performance may be due to skills acquired with age, such as those devoted to feeding and balancing energy demands, which are necessary to prepare and maintain individual condition prior to, and during, breeding. Senescence in performance after 'middleage' may result from accumulated costs of previous breeding effort which have been identified in this species based on research elsewhere.

Several studies concerned with mate choice, reproductive performance, and life history strategies have been conducted with secondary cavity nesting birds breeding in nest boxes. Although the need for comparative studies has been recognized, populations breeding in nest boxes often have not been compared with those breeding in natural cavities. We compared the ecology of Tree Swallows breeding in nest boxes and natural cavities to determine if nest box populations of Tree Swallows are accurate models of natural populations. Two nest site characteristics, nest site dispersion and cavity height, were similar for birds in both nesting environments. Greater cavity entrance area at natural cavities resulted in increased interspecific competition in natural populations, involving larger competitors, more species, and a greater abundance of each species. Clutch size was smaller in natural cavities compared with nest boxes, likely because floor area was smaller in natural cavities. Fledging success did not differ between populations. Disproportionately more after-second-year females bred in nest boxes, and more second-year females bred in natural cavities, as estimated by a model of Tree Swallow survivorship. Tree Swallows settle at nest boxes before natural cavities in our study area, perhaps as a result of the greater potential for reproductive success and reduced interspecific competition in the nest boxes as opposed to natural cavities. For some aspects of the ecology of secondary cavity nesters, nest boxes do not provide an accurate representation of natural populations. Therefore, evolutionary interpretations of nest box studies should be compared with observations of birds in natural environments.