David A. Buehler

Assessing bias in studies of bald eagle food habits

Although studies of bald eagle (Haliaeetus leucocephalus) food habits are numerous, few authors have quantified biases inherent in the techniques used. In our study of food habits of nonbreeding bald eagles on the northern Chesapeake Bay, we examined biases associated with pellet analysis, food remains analysis, and direct observation. We assessed these biases through controlled feedings of 2 captive bald eagles and through observations of free-ranging eagles

STATUS AND CONSERVATION PRIORITIES OF GOLDEN-WINGED WARBLER (VERMIVORA CHRYSOPTERA) IN NORTH AMERICA

department of Forestry, Wildlife and Fisheries, University of Tennessee, Knoxville, Tennessee 37996, USA; 2School of Forest Resources and Environmental Science, Michigan Technological University, Houghton, Michigan 49931, USA; Evolutionary Biology Program, Cornell Lab of Ornithology, Ithaca, New York 14850, USA; 4 U.S. Fish and Wildlife Service, Region 3, Nongame Migratory Bird Office, Fort Snelling, Minnesota 55111, USA; ^Department of Biology, Ithaca College, Ithaca, New York 14850, USA; department of Biological Sciences, University of Cincinnati, Cincinnati, Ohio 45221, USA; and 7Conservation Science Program, Cornell Lab of Ornithology, Ithaca, New York 14850, USA

Influence of point count length and repeated visits on habitat model performance

Point counts are commonly used to monitor bird populations, and a substantial amount of research has investigated how conducting counts for different lengths of time affects the accuracy of these counts and the subsequent ability to monitor changes in population trends. However, little work has been done to assess how changes in count duration affect bird-habitat models developed from point count data. In this paper, we present an empirical comparison of the performance of bird-habitat models, which were developed via logistic regression analyses based on point count data from 3-, 5-, 10-, and 20-min counts. We also investigated the effect of the number of visits to each survey point on model performance. We assessed model performance on the basis of R 2 -values and percent concordant pairs. A positive relation between model performance and count duration was most apparent for species with relatively low detection probabilities, whereas performance of models for species with relatively high detectability was fairly consistent or even decreased as count duration increased. Our results suggest that while some improvement in bird-habitat models for species with low detection rates can be achieved via longer point counts, the modest gains in model performance should be weighed against the increased time and effort required to conduct longer counts. Models based on data from a single visit to each point did not performed as well as models based on multiple visits. However, we found little or no improvement in model performance when the number of visits per point increased from 2 to 3. We suggest that current recommendations on point count durations (5 or 10 min) will provide adequate data for modeling bird-habitat relations.

Effects of human activity on bald eagle distribution on the northern Chesapeake Bay

We determined the relationship between bald eagle (Haliaeetus leucocephalus) distribution and human activity on the northern Chesapeake Bay shoreline during 1985-89. Only 55 of 1,117 locations of radio-tagged eagles (4.9%) occurred in the developed land-cover type (≥4 buildings/4 ha), although 18.2% of potential eagle habitat was developed (χ 2 =428.9, 4 df, P<0.001). Eagle use of the shoreline was inversely related to building density (χ 2 =22.1, P<0.001) and directly related to the development set-back distance (χ 2 =5.3, P=0.02). Few eagles used shoreline segments with boats or pedestrians nearby (P<0.001). Only 360 of 2,532 segments (14.2%) had neither human activity nor shoreline development

GOLDEN-WINGED WARBLER BREEDING HABITAT CHARACTERISTICS AND NEST SUCCESS IN CLEARCUTS IN THE SOUTHERN APPALACHIAN MOUNTAINS

Abstract Despite widespread population declines, few studies have sought to quantify nesting habitat and reproductive characteristics of Golden-winged Warblers (Vermivora chrysoptera). We characterized territory and nesting habitat and measured productivity in forest regeneration areas in the Nantahala National Forest, North Carolina, and Cherokee National Forest, Tennessee, during 1997–1998. We located 82 territories and monitored 23 nests. Golden-winged Warblers occupied young stands with low basal area and annually mowed daylit roads (roads mowed wider than the track to facilitate drying), and nested in sites with high herbaceous density compared to unoccupied regeneration areas. Nest sites had fewer saplings and less canopy cover than the surrounding territory. Overall nest success was 72.5% and nests fledged an average of 3.65 young. Recently harvested forest stands (age ;le13 yr) with herbaceous cover on logging roads and log landings provided habitat capable of supporting Golden-winged Warbler reproduction at levels equal to or greater than productivity reported from other study sites across the range of this species.

Survival Rates and Population Dynamics of Bald Eagles on Chesapeake Bay

Survival of 39 radio-tagged bald eagles (Haliaeetus leucocephalus) in the Chesapeake Bay region was 100% in the first year of life. Mean minimum survival per year of all eagles was 91% (95% CI=86-96%); mean maximum survival was 98% (95% CI=96-100%). A deterministic life-table model predicted a finite growth rate of 5.8% per year, whereas the growth rate based on the maximum survival estimates was 16.6% per year. The breeding population actually increased 12.6% per year from 1986 to 1990. We estimated the intrinsic growth rate at 6.9% based on natality and minimum survival data and 19.2% based on maximum survival data. Because eagle habitat is being converted to human developments at a rapid rate on the Chesapeake, models incorporating these habitat losses are needed to accurately predict future population trends

Factors Influencing Golden-Winged Warbler (Vermivora Chrysoptera) Nest-Site Selection and Nest Survival in the Cumberland Mountains of Tennessee

Abstract Studies of reproduction and habitat use are essential for any species assessment, especially for species with declining populations. We compared habitat in nest sites and randomly selected sites within Golden-winged Warbler (Vermivora chrysoptera) territories. We also modeled the effects of temporal and biotic factors on daily nest survival in relation to the constant-survival model. The percent cover of herbaceous vegetation was greater at nest sites, and that of woody vegetation was greater at non-nest sites. There was support for models with annual variation and a decline in nest survival throughout the nesting season, but the constant-survival model performed equally well. One parameter performed marginally better than the constant-survival model: nests with a woody stem in the substrate had lower nest-survival rates. We conclude that nest-site selection was nonrandom, such that females use specific criteria to select nest sites. However, habitat characteristics did not appear to significantly affect daily nest survival or, therefore, predation rates. Until factors that affect predation rates are better understood, conservation strategies that increase breeding habitat with specific nest-site features may be more successful than attempts to directly control nest survival.

Cerulean warbler reproduction, survival, and models of population decline

Abstract We present and compare demographic data for cerulean warblers (Dendroica cerulea) from 5 study sites across the range of the species from 1992 to 2006. We conducted field studies to collect data on daily nest survival, nest success, and young fledged per successful nest, and we used data to estimate fecundity. Daily nest survival, nest success, young fledged, and fecundity varied widely across the cerulean range and among years. Study sites in agriculture-dominated landscapes (Mississippi Alluvial Valley, IN, and MI, USA) had negative growth rates in all years monitored because measured values of nest success and young produced per successful nest were incapable of offsetting apparent mortality. Ontario (Canada) and Tennessee (USA) populations had greater nest success and fecundity but still appeared to be incapable of producing stable populations (λ = 1) under field-measured and assumed conditions. We had survival data only for one site (Ontario); thus, additional survival data are greatly needed to enable more reliable estimates of population growth. Conservation strategies for cerulean warblers in agriculture-dominated landscapes (e.g., Mississippi Alluvial Valley, IN, and MI) may require major landscape-level habitat reconfiguration to change agriculture-dominated landscapes to forest-dominated landscapes to increase fecundity. Conservation strategies in predominantly forested landscapes in the core of the range (e.g., TN) require a focus on minimizing habitat loss and developing management prescriptions capable of improving fecundity. In both cases, based on sensitivity and elasticity analyses, efforts to improve survival during the nonbreeding season would have the greatest positive effect on population growth.

Emulating Natural Disturbances for Declining Late-Successional Species: A Case Study of the Consequences for Cerulean Warblers (Setophaga cerulea)

Forest cover in the eastern United States has increased over the past century and while some late-successional species have benefited from this process as expected, others have experienced population declines. These declines may be in part related to contemporary reductions in small-scale forest interior disturbances such as fire, windthrow, and treefalls. To mitigate the negative impacts of disturbance alteration and suppression on some late-successional species, strategies that emulate natural disturbance regimes are often advocated, but large-scale evaluations of these practices are rare. Here, we assessed the consequences of experimental disturbance (using partial timber harvest) on a severely declining late-successional species, the cerulean warbler (Setophaga cerulea), across the core of its breeding range in the Appalachian Mountains. We measured numerical (density), physiological (body condition), and demographic (age structure and reproduction) responses to three levels of disturbance and explored the potential impacts of disturbance on source-sink dynamics. Breeding densities of warblers increased one to four years after all canopy disturbances (vs. controls) and males occupying territories on treatment plots were in better condition than those on control plots. However, these beneficial effects of disturbance did not correspond to improvements in reproduction; nest success was lower on all treatment plots than on control plots in the southern region and marginally lower on light disturbance plots in the northern region. Our data suggest that only habitats in the southern region acted as sources, and interior disturbances in this region have the potential to create ecological traps at a local scale, but sources when viewed at broader scales. Thus, cerulean warblers would likely benefit from management that strikes a landscape-level balance between emulating natural disturbances in order to attract individuals into areas where current structure is inappropriate, and limiting anthropogenic disturbance in forests that already possess appropriate structural attributes in order to maintain maximum productivity.

Tornadic Storm Avoidance Behavior in Breeding Songbirds

Migration is a common behavior used by animals of many taxa to occupy different habitats during different periods. Migrant birds are categorized as either facultative (i.e., those that are forced to migrate by some proximal cue, often weather) or obligate (i.e., those that migrate on a regular cycle). During migration, obligate migrants can curtail or delay flights in response to inclement weather or until favorable winds prevail, and they can temporarily reorient or reverse direction when ecological or meteorological obstacles are encountered. However, it is not known whether obligate migrants undertake facultative migrations and make large-scale movements in response to proximal cues outside of their regular migration periods. Here, we present the first documentation of obligate long-distance migrant birds undertaking a facultative migration, wherein breeding golden-winged warblers (Vermivora chrysoptera) carrying light-level geolocators performed a >1,500 km 5-day circumvention of a severe tornadic storm. The birds evacuated their breeding territories >24 hr before the arrival of the storm and atmospheric variation associated with it. The probable cue, radiating >1,000 km from tornadic storms, perceived by birds and influencing bird behavior and movements, is infrasound (i.e., sound below the range of human hearing). With the predicted increase in severity and frequency of similar storms as anthropogenic climate change progresses, understanding large-scale behavioral responses of animals to such events will be an important objective of future research.