Deanna K. Dawson

Modeling abundance effects in distance sampling

Distance-sampling methods are commonly used in studies of animal populations to estimate population density. A common objective of such studies is to evaluate the relationship between abundance or density and covariates that describe animal habitat or other environmental influences. However, little attention has been focused on methods of modeling abundance covariate effects in conventional distance-sampling models. In this paper we propose a distance-sampling model that accommodates covariate effects on abundance. The model is based on specification of the distance-sampling likelihood at the level of the sample unit in terms of local abundance (for each sampling unit). This model is augmented with a Poisson regression model for local abundance that is parameterized in terms of available covariates. Maximum-likelihood estimation of detection and density parameters is based on the integrated likelihood, wherein local abundance is removed from the likelihood by integration. We provide an example using avian point-transect data of Ovenbirds (Seiurus aurocapillus) collected using a distance-sampling protocol and two measures of habitat structure (understory cover and basal area of overstory trees). The model yields a sensible description (positive effect of understory cover, negative effect on basal area) of the relationship between habitat and Ovenbird density that can be used to evaluate the effects of habitat management on Ovenbird populations.

Effect of distance-related heterogeneity on population size estimates from point counts

Abstract.— Point counts are used widely to index bird populations. Variation in the proportion of birds counted is a known source of error, and for robust inference it has been advocated that counts be converted to estimates of absolute population size. We used simulation to assess nine methods for the conduct and analysis of point counts when the data included distance-related heterogeneity of individual detection probability. Distance from the observer is a ubiquitous source of heterogeneity, because nearby birds are more easily detected than distant ones. Several recent methods (dependent double-observer, time of first detection, time of detection, independent multiple-observer, and repeated counts) do not account for distance-related heterogeneity, at least in their simpler forms. We assessed bias in estimates of population size by simulating counts with fixed radius w over four time intervals (occasions). Detection probability per occasion was modeled as a half-normal function of distance with scale parameter &sgr; and interceptg(0) = 1.0. Bias varied with &sgr;/w; values of &sgr; inferred from published studies were often <25 m, which suggests a bias of >50% for a 100-m fixed-radius count. More critically, the bias of adjusted counts sometimes varied more than that of unadjusted counts, and inference from adjusted counts would be less robust. The problem was not solved by using mixture models or including distance as a covariate. Conventional distance sampling performed well in simulations, but its assumptions are difficult to meet in the field. We conclude that no existing method allows effective estimation of population size from point counts.

Radar analysis of fall bird migration stopover sites in the northeastern U.S.

ABSTRACT The national network of weather surveillance radars (WSR-88D) detects flying birds and is a useful remote-sensing tool for ornithological study. We used data collected during fall 2008 and 2009 by 16 WSR-88D radars in the northeastern U.S. to quantify the spatial distribution of landbirds during migratory stopover. We geo-referenced estimates based on radar reflectivity, of the density of migrants aloft at their abrupt evening exodus from daytime stopover sites, to the approximate locations from which they emerged. We classified bird stopover use by the magnitude and variation of radar reflectivity across nights; areas were considered “important” stopover sites for conservation if bird density was consistently high. We developed statistical models that predict potentially important stopover sites across the region, based on land cover, ground elevation, and geographic location. Large areas of regionally important stopover sites were located along the coastlines of Long Island Sound, throughout the Delmarva Peninsula, in areas surrounding Baltimore and Washington, along the western edge of the Adirondack Mountains, and within the Appalachian Mountains of southwestern Virginia and West Virginia. Locally important stopover sites generally were associated with deciduous forests embedded within landscapes dominated by developed or agricultural lands, or near the shores of major water bodies. Preserving or enhancing patches of natural habitat, particularly deciduous forests, in developed or agricultural landscapes and along major coastlines could be a priority for conservation plans addressing the stopover requirements of migratory landbirds in the northeastern U.S. Our maps of important stopover sites can be used to focus conservation efforts and can serve as a sampling frame for fieldwork to validate radar observations or for ecological studies of landbirds on migratory stopover.

Artificial light at night confounds broad‐scale habitat use by migrating birds

With many of the worlds migratory bird populations in alarming decline, broad-scale assessments of responses to migratory hazards may prove crucial to successful conservation efforts. Most birds migrate at night through increasingly light-polluted skies. Bright light sources can attract airborne migrants and lead to collisions with structures, but might also influence selection of migratory stopover habitat and thereby acquisition of food resources. We demonstrate, using multi-year weather radar measurements of nocturnal migrants across the northeastern U.S., that autumnal migrant stopover density increased at regional scales with proximity to the brightest areas, but decreased within a few kilometers of brightly-lit sources. This finding implies broad-scale attraction to artificial light while airborne, impeding selection for extensive forest habitat. Given that high-quality stopover habitat is critical to successful migration, and hindrances during migration can decrease fitness, artificial lights present a potentially heightened conservation concern for migratory bird populations.

Land-use planning to conserve habitat for area-sensitive forest birds

Models predicting the occurrence of area-sensitive bird species in forests were developed from bird survey data from 499 forests in Prince Georges County, Maryland. The predicted probabilities of occurrence for species were integrated with forest cover data for the County in a Geographic Information System (GIS). This information was used in combination with local zoning and forest conservation requirements to develop a preliminary forest conservation plan for the watershed of the Western Branch of the Patuxent River. We identify forests patches most likely to support breeding populations of area-sensitive birds, and use the GIS to devise ways to consolidate and link them. Forests that do not contribute significantly to the integrity of these priority forests are designated as sites in which development could occur. The resulting conservation plan will maintain and enhance breeding habitat for area-sensitive forest birds, while still allowing for additional development as human populations increase.

Chandler S. Robbins, 1918–2017

The first day of spring 2017 marked the passing, at the age of 98, of Chandler S. Robbins, one of the great American ornithologists. Chan, as he was called, was a man of boundless energy, unflagging positivity, and exceptional intellectual capabilities. He spent his long career entirely with the U.S. government, which provided him with a forum that perfectly complemented his skills. As a field ornithologist arriving at the Patuxent Research Refuge in Maryland in the 1940s, he gathered the threads of a nascent paradigm in ornithology that combined the emerging need for information on bird distribution and population change with publicly collected bird data. Frank Chapman, Frederick Lincoln, and Wells Cooke had defined the essential ideas of citizen science for the Christmas Bird Count (CBC), bird banding, and phenology data, but Chan brought an appreciation for the needs and capabilities of citizen scientists that transformed our way of gathering bird population data. He connected with birders individually and collectively through bird clubs, organized counts, and a field guide that was focused on the development of the song identification skills required for scientific bird counting. Combined with a superhuman tolerance for the bookkeeping aspects of bird counting and a love of being in the field, Chan’s bully pulpit in public service enabled him to launch monitoring programs and research initiatives that have formed the basis of North American bird conservation. Chandler Seymour Robbins was born on July 17, 1918, in Belmont, Massachusetts. His family has deep New England roots. Chan’s parents and grandparents were all interested in natural history, and his curiosity about birds started before he was a teenager. At an early age, he met some of the prominent ornithologists of the day, including William Brewster and Ralph Hoffman. At the age of 16, he organized and compiled his first CBC, which Ludlow Griscom questioned for its report of Barrow’s Goldeneyes from a local freshwater pond. (The identification was later confirmed.) Chan graduated from Harvard in 1940 with a degree in physics. Ironically, Griscom, Chan’s mentor at Harvard, advised him not to get a degree in biology, as ornithologists had limited employment opportunities! While at Harvard, Chan saw his cousin, Franklin D. Roosevelt, receive an honorary doctorate and graduated in the same class as John F. Kennedy. Chan received his M.Sc. from George Washington University in 1950, and the University of Maryland awarded him an Honorary Doctorate of Science in 1995. In 1940, Chan began teaching math and science at a school in Vermont and was nature director at a summer camp in New Jersey. Influenced by an uncle who became a minister after serving inWorld War I, Chan worked in the Civilian Public Service during World War II. He transferred to Patuxent in 1943 and was hired by the U.S. Fish Chandler S. Robbins in Panama in 2012. Photo credit: Carlos Bethancourt

Linking animals aloft with the terrestrial landscape

Despite using the aerosphere for many facets of their life, most flying animals (i.e., birds, bats, some insects) are still bound to terrestrial habitats for resting, feeding, and reproduction. Comprehensive broad-scale observations by weather surveillance radars of animals as they leave terrestrial habitats for migration or feeding flights can be used to map their terrestrial distributions either as point locations (e.g., communal roosts) or as continuous surface layers (e.g., animal densities in habitats across a landscape). We discuss some of the technical challenges to reducing measurement biases related to how radars sample the aerosphere and the flight behavior of animals. We highlight a recently developed methodological approach that precisely and quantitatively links the horizontal spatial structure of birds aloft to their terrestrial distributions and provides novel insights into avian ecology and conservation across broad landscapes. Specifically, we present case studies that (1) elucidate how migrating birds contend with crossing ecological barriers and extreme weather events, (2) identify important stopover areas and habitat use patterns of birds along their migration routes, and (3) assess waterfowl response to wetland habitat management and restoration. These studies aid our understanding of how anthropogenic modification of the terrestrial landscape (e.g., urbanization, habitat management), natural geographic features, and weather (e.g., hurricanes) can affect the terrestrial distributions of flying animals.