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

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 Swainsoh\u27s Thrush) declined significantly. Others increased, decreased, or re-mained 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 vege-tation structure related to forest succession on the breeding grounds, along with other pro-cesses 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 demo-graphic information on migrant populations in their wintering areas

Consequences of Dominance-Mediated Habitat Segregation in American Redstarts During the Nonbreeding Season

Several species of migratory songbirds exhibit a distinct form of habitat segregation while on their Neotropical wintering grounds in which males and females occupy different habitat types. In the American Redstart (Setophaga ruticilla), that sexual habitat segregation is a result of behavioral dominance of older males. In that study, we examined whether such dominance behavior and the resulting differential habitat segregation has consequences for the condition or survival of excluded individuals. We quantified the physical condition and survival of redstarts (both males and females) occupying two habitat types that differed in the proportion of males and females present in Jamaica. Both sexes of redstarts occupying female-biased habitat lost significantly more mass over-winter and had lower annual survival and longevity compared to individuals in male-biased habitat. These results suggest that nonbreeding habitats differed in suitability, with the least suitable habitat being occupied predominately by females. Because most female redstarts are forced to over-winter in these kinds of habitats, they may often be in poor physiological condition prior to departing on spring migration for the breeding grounds. This in turn may influence dynamics of the breeding period by determining their condition and perhaps reproductive success. Furthermore, because winter habitat segregation appears to lower female survivorship, it may also limit the number and availability of breeding females. These results implicate events that occur during the nonbreeding period as playing a critical role in the annual dynamics of this migratory species

Observations of Musk Oxen on Banks Island, Northwest Territories, Canada

Notes summer 1963 observations of Ovibos moschatus. On a reconnaissance flight over the northern third of the island on Aug. 2 nearly sixty were seen, indicating greater abundance than hitherto supposed. Only two were seen south of the reconnaissance area during a two-month period; apparently they concentrate in the northern part at least during the summer

Foraging Behavior of American Redstarts in Breeding and Wintering Habitats: Implications for Relative Food Availability

We investigated food availability for a long-distance migrant species, the American Redstart (Setophaga ruticilla), in both its summer breeding habitat in New Hampshire and in its winter habitat in Jamaica. We used four components of foraging behavior (prey attack rate, foraging speed, time spent foraging, and foraging maneuver use) as indicators of the relative availability of prey in the two seasons. Redstarts attacked prey at a significantly greater rate in summer than in winter, indicating that foraging birds encountered prey more frequently in summer. The winter prey-encounter rate was low even though redstarts moved almost twice as fast while foraging in winter as in summer. Male redstarts also spent more time foraging in winter (85%) than in summer (43-65%), possibly to balance the low rate at which they encountered prey. In winter, redstarts used more foraging maneuvers that were directed towards small flying prey, whereas in summer they used maneuvers that resulted in the capture of relatively large and presumably energy-rich prey such as lepidopteran larvae. That wintering redstarts foraged faster, attacked prey less often, and spent more time foraging than those in summer indicates that the winter is a period of relative food scarcity for this species, whereas the breeding season is a period of greater resource abundance

Spatial Scaling of Avian Population Dynamics: Population Abundance, Growth Rate, and Variability

Synchrony in population fluctuations has been identified as an important component of population dynamics. In a previous study, we determined that local‐scale (\u3c15‐km) spatial synchrony of bird populations in New England was correlated with synchronous fluctuations in lepidopteran larvae abundance and with the North Atlantic Oscillation. Here we address five questions that extend the scope of our earlier study using North American Breeding Bird Survey data. First, do bird populations in eastern North America exhibit spatial synchrony in abundances at scales beyond those we have documented previously? Second, does spatial synchrony depend on what population metric is analyzed (e.g., abundance, growth rate, or variability)? Third, is there geographic concordance in where species exhibit synchrony? Fourth, for those species that exhibit significant geographic concordance, are there landscape and habitat variables that contribute to the observed patterns? Fifth, is spatial synchrony affected by a species\u27 life history traits? Significant spatial synchrony was common and its magnitude was dependent on the population metric analyzed. Twenty‐four of 29 species examined exhibited significant synchrony in population abundance: mean local autocorrelation (ρ) = 0.15; mean spatial extent (mean distance where ρ = 0) = 420.7 km. Five of the 29 species exhibited significant synchrony in annual population growth rate (mean local autocorrelation = 0.06, mean distance = 457.8 km). Ten of the 29 species exhibited significant synchrony in population abundance variability (mean local autocorrelation = 0.49, mean distance = 413.8 km). Analyses of landscape structure indicated that habitat variables were infrequent contributors to spatial synchrony. Likewise, we detected no effects of life history traits on synchrony in population abundance or growth rate. However, short‐distance migrants exhibited more spatially extensive synchrony in population variability than either year‐round residents or long‐distance migrants. The dissimilarity of the spatial extent of synchrony across species suggests that most populations are not regulated at similar spatial scales. The spatial scale of the population synchrony patterns we describe is likely larger than the actual scale of population regulation, and in turn, the scale of population regulation is undoubtedly larger than the scale of individual ecological requirements

Site-Dependent Regulation of Population Size: a New Synthesis

The nature and extent of population regulation remains a principal unanswered question for many types of organisms, despite extensive research. In this paper, we provide a new synthesis of theoretical and empirical evidence that elucidates and extends a mechanism of population regulation for species whose individuals preemptively use sites that differ in suitability. The sites may be territories, refuges from predation, oviposition sites, etc. The mechanism, which we call site dependence, is not an alternative to density dependence; rather, site dependence is one of several mechanisms that potentially generate the negative feedback required for regulation. Site dependence has two major features: (1) environmentally caused heterogeneity among sites in suitability for reproduction and/or survival; and (2) preemptive site occupancy, with the tendency for individuals to move to sites of higher quality as they become available. Simulation modeling shows that these two features, acting in concert, generate negative feedback when progressively less suitable sites are used as population size increases, reducing average demographic rates for the population as a whole. Further, when population size decreases, only sites of high suitability are occupied, resulting in higher average demographic rates and, thus, population growth. The modeling results demonstrate that this site‐dependent mechanism can generate negative feedback at all population sizes in the absence of local crowding effects, and that this feedback is capable of regulating population size tightly. Operation of site dependence does not rely on the particular type of environmental factor(s) ultimately limiting population size, e.g., food, nest sites, predators, parasites, abiotic factors, or a combination of these. Furthermore, site dependence operates in saturated or unsaturated habitats and over a broad range of spatial scales for species that disperse widely relative to site diameter. A review of relevant field studies assessing the assumptions of the mechanism and its regulatory potential suggests that site dependence may provide a general explanation for population regulation in a wide variety of species

Conserving Migratory Land Birds in the New World: Do We Know Enough?

Migratory bird needs must be met during four phases of the year: breeding season, fall migration, wintering, and spring migration; thus, management may be needed during all four phases. The bulk of research and management has focused on the breeding season, although several issues remain unsettled, including the spatial extent of habitat influences on fitness and the importance of habitat on the breeding grounds used after breeding. Although detailed investigations have shed light on the ecology and population dynamics of a few avian species, knowledge is sketchy for most species. Replication of comprehensive studies is needed for multiple species across a range of areas. Information deficiencies are even greater during the wintering season, when birds require sites that provide security and food resources needed for survival and developing nutrient reserves for spring migration and, possibly, reproduction. Research is needed on many species simply to identify geographic distributions, wintering sites, habitat use, and basic ecology. Studies are complicated, however, by the mobility of birds and by sexual segregation during winter. Stable-isotope methodology has offered an opportunity to identify linkages between breeding and wintering sites, which facilitates understanding the complete annual cycle of birds. The twice-annual migrations are the poorest-understood events in a bird\u27s life. Migration has always been a risky undertaking, with such anthropogenic features as tall buildings, towers, and wind generators adding to the risk. Species such as woodland specialists migrating through eastern North America have numerous options for pausing during migration to replenish nutrients, but some species depend on limited stopover locations. Research needs for migration include identifying pathways and timetables of migration, quality and distribution of habitats, threats posed by towers and other tall structures, and any bottlenecks for migration. Issues such as human population growth, acid deposition, climate change, and exotic diseases are global concerns with uncertain consequences to migratory birds and even less-certain remedies. Despite enormous gaps in our understanding of these birds, research, much of it occurring in the past 30 years, has provided sufficient information to make intelligent conservation efforts but needs to expand to handle future challenges

Lidar Remote Sensing Variables Predict Breeding Habitat of a Neotropical Migrant Bird

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

What role should the commercial food system play in promoting health through better diet?

Martin White and coauthors consider that the commercial food system has the potential to show leadership and support for dietary public health, but systemic change is needed first and this is likely to require governmental actio