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A topic of recurring interest in ecological research is the degree to which vegetation structure influences the distribution and abundance of species. Here we test the applicability of remote sensing, particularly novel use of waveform lidar measurements, for quantifying the habitat heterogeneity of a contiguous northern hardwoods forest in the northeastern United States. We apply these results to predict the breeding habitat quality, an indicator of reproductive output of a well-studied Neotropical migrant songbird, the Black-throated Blue Warbler (Dendroica caerulescens). We found that using canopy vertical structure metrics provided unique information for models of habitat quality and spatial patterns of prevalence. An ensemble decision tree modeling approach (random forests) consistently identified lidar metrics describing the vertical distribution and complexity of canopy elements as important predictors of habitat use over multiple years. Although other aspects of habitat were important, including the seasonality of vegetation cover, the canopy structure variables provided unique and complementary information that systematically improved model predictions. We conclude that canopy structure metrics derived from waveform lidar, which will be available on future satellite missions, can advance multiple aspects of biodiversity research, and additional studies should be extended to other organisms and regions.

The mechanisms regulating bird populations are poorly understood and controversial. We provide evidence that a migratory songbird, the black-throated blue warbler (Dendroica caerulescens), is regulated by multiple density-dependence mechanisms in its breeding quarters. Evidence of regulation includes: stability in population density during 1969-2002, strong density dependence in time-series analyses of this period, an inverse relationship between warbler density and annual fecundity, and a positive relationship between annual fecundity and recruitment of yearlings in the subsequent breeding season. Tests of the mechanisms causing regulation were carried out within the Hubbard Brook Experimental Forest, New Hampshire, during 1997-1999. When individuals from abutting territories were experimentally removed in a homogeneous patch of high-quality habitat, the fecundity of focal pairs nearly doubled, revealing a locally operating crowding mechanism. A site-dependence mechanism was indicated by an inverse relationship between population size and mean territory quality, as well as by greater annual fecundity on the sites that were most frequently occupied and of highest quality. These site-dependence relationships were revealed by intensive monitoring of territory quality and demography at the landscape spatial scale. Crowding and site-dependence mechanisms, therefore, acted simultaneously but at different spatial scales to regulate local abundance of this migratory bird population.

We examined patterns of habitat use and reproductive performance of a migratory songbird, the black-throated blue warbler (Dendroica caerulescens (Gmelin, 1789)), within a 3160-ha forested landscape. We surveyed 371 sites over a 3-year period. Some sites were never occupied, while others were occupied for 1, 2, or 3 years. For these 3 years we found that warbler abundance increased with frequency of occupancy. Additionally, we found that (i) deciduousness and understory shrub density increased with frequency of occupancy; (ii) in 1 of 3 years, food abundance was higher at the most frequently occupied sites; and (iii) nest predators exhibited predator-specific abundance patterns across occupancy categories. We next used occupancy patterns documented in the first 3 years of the study to predict settlement, age structure, and reproductive performance at a subset of sites in the final year of the study. We found that males settled earlier in the breeding season at sites with a high frequency of occupancy. There were no differences in arrival times of females. Additionally, age structure did not vary for either males or females across sites with different occupancy levels. Although we found no difference in mean reproductive output across sites with different occupancy levels, over 50% of the young produced fledged from territories overlapping the high occupancy sites. [ABSTRACT FROM AUTHOR]

Predator mobbing has been viewed as an adaptation to reduce the risk of predation, however, factors influencing mobbing behaviour are still debated. We report on the results of an experiment with Dendroica caerulescens and Dendroica virens designed to determine (1) whether mobbing response by forest songbirds during the breeding season is restricted by territory boundaries, (2) the distance songbirds will move in response to anti-predator mobbing calls, and (3) whether reproductive status, age, and time of the breeding season determine the distance moved to mob. We did not detect an effect of reproductive status, age, or time of breeding season on the distance moved by birds to mob. All birds responded to the mobbing playback within their territory (defined by territorial defence in relation to specific song playbacks). The maximum distance moved within a territory to engage in mobbing ranged from 25 to 175 m ( = 72 ± 6 m). Three of 37 birds responded to playbacks outside their territory boundaries. In all three cases, maximum movement distances outside territories were short (25 m). Thus, for two species of warblers, mobbing is highly constrained by territory boundaries during the breeding season. This finding is congruent with arguments that mobbing is primarily a selfish behaviour, at least with respect to conspecifics. Our results also provide support for the ‘move-on’ hypothesis. [ABSTRACT FROM AUTHOR]

Estimates of nest survival are commonly used as indices of reproductive productivity in a wide range of studies, including examinations of population dynamics, habitat fragmentation, and wildlife management. However, the strength of the relationship between estimates of nest survival and of seasonal fecundity has rarely been tested. To examine that relationship, we used 16 years of demographic data from an intensively studied population of individually marked Black-throated Blue Warblers (Dendroica caerulescens). We found a positive relationship between Mayfield nest-survival estimates and seasonal fecundity (i.e. number of fledglings produced per female). However, seasonal-fecundity predictions based on nest-survival values underestimated observed seasonal fecundity by an average of 33% and population growth rate by 20%. Our results suggest that, though nest-survival estimates may be useful as indices of pressures on a population (e.g. nest predation), care is needed in using them as the basis of conservation or management strategies or to make inferences about population-level processes. [ABSTRACT FROM AUTHOR]