Neotropical Migrants Research Articles

Geographic Range Fragmentation and Abundance in Neotropical Migratory Birds

Populations of neotropical migrant landbirds have experienced significant declines in recent years. We investigated potential consequences of these declines by examining the relationship between abundance and fragmentation of geographic ranges of species on the North American breeding grounds. We estimated areograpbic fragmentation using the box dimension of a species’ geographic range and demographic fragmentation using the fractal dimension of the semivariance function calculated from samples of population abundance across species’ geographic ranges. We found a negative relationship between average abundance and demographic fragmentation for neotropical migrants, but not for residents. We also showed that demographic fragmentation and areographic fragmentation are inversely related for residents, but not for neotropical migrants. These results imply that neotropical migrants may be more sensitive to extinction than are residents.

Toward Conservation of Midcontinental Shorebird Migrations

Shorebirds represent a highly diverse group of species, many of which experience tremendous energy demands associated with long‐distance migratory flights. Transcontinental migrants are dependent upon dynamic freshwater wetlands for stopover resources essential for replenishment of lipid reserves and completion of migration. Patterns of shorebird migration across midcontinental wetlands were detected from migration reports to American Birds and information provided by U.S. Fish and Wildlife Service national wildlife refuges. Patterns in species composition and abundance varied geographically, emphasizing the uniqueness of different regions to migrating shorebirds. Smaller species and neotropical migrants moved primarily across the Great Plains, whereas larger species and North American migrants predominated in assemblages in the intermountain west Shorebirds were broadly dispersed in wetland habitats with dynamic water regimes. Whereas populations of shorebirds in coastal systems appear to concentrate at sites of seasonally predictable and abundant food resources, we propose that transcontinental shorebirds disperse and use wetlands opportunistically. This migration system exemplifies the need for large‐scale, coordinated regional management efforts that recognize the dynamic nature of ecosystem processes.

Status of Neotropical Migrant Songbirds in Manitoba – Time for a Second Look

Status of Neotropical Migrant Songbirds in Manitoba – Time for a Second Look

Avian communities in riparian forests of different widths in Maryland and Delaware

In agricultural landscapes, much of the remaining forest is in linear tracts along streams. These riparian forests provide habitat for forest birds, but their use by forest interior birds may depend on forest width. We conducted point-count surveys of birds in riparian forests on the Eastern shore of Maryland and Delaware to assess whether the presence of any species was dependent on corridor width. We surveyed 117 corridors that ranged from 25- to 800-m wide. Several area-sensitive neotropical migrants were encountered more frequently in wider riparian forests, and probabilities of occurrence increased most rapidly between 25 and 100 m. Based on these surveys, we recommend that riparian forests be at least 100-m wide to provide some nesting habitat for area-sensitive species. Wider riparian forests would be preferable and should be preserved.

Forty-Year Changes in the Avifauna of Grand Teton National Park, Wyoming, with Emphasis on Neotropical Migrants

Ornithologists and wildlife biologists have always been interested in documenting long-term changes in bird populations (e.g., Temple and Temple 1976, Kendeigh 1982). Long-term comparisons can assist in identifying patterns of change. These patterns, in turn, provide assistance in defining human impacts that may lead to some species or groups declining towards extirpation. One study specifically (Wilcove 1988) has had a major impact upon current resource issues within the Fish and Wildlife Service. That study used 40-year comparisons to confirm that fragmentation of the eastern deciduous forest had led to declines of neotropical migrants. The Wilcove (1988) study along with an essay (Hutto 1988) and an ecological study in New Hampshire (Holmes and Sherry 1988) were fundamental in identifying that declines in this group of birds cannot be blamed solely upon changes on Latin American wintering grounds. Missing from the neotropical migrant story, however, is an image of how neotropical migrants from western North America have changed in recent decades. An historical data set for comparison is available for seven vegetative associations in the vicinity of Jackson, Wyoming (Salt 1957). This project was begun in 1993 to replicate Salt's 1957 study in Grand Teton National Park. The work in 1993 was a pre-study to evaluate the potential for replicating Salt's study in the mid 1990's, 40 years after the original work.

Ecology and Conservation of Neotropical Migrant Landbirds, by John M. Hagan III an David W. Johnson [Review

Ecology and Conservation of Neotropical Migrant Landbirds, by John M. Hagan III an David W. Johnson [Review

Visible Morning Flight of Neotropical Landbird Migrants at Cape May, New Jersey

-We studied the autumn morning flight of Neotropical migrants for two years at Cape May, New Jersey. A majority (86%) of the 24,378 identified Neotropical migrants were warblers, while thrushes (0.01%) rarely made morning flights. A disproportionately large number of migrants was seen on the day of and the day following cold fronts. Only 23% of all Neotropical migrants flew on days not associated with cold fronts, and nearly 12% flew in rain. Morning flights of Neotropical migrants commenced in late July, and 72% passed before 15 September. All migrants flew north. At Cape May, 87% of visible morning flight occurred within 2 h after sunrise. We examine several hypotheses advanced to explain morning flight and reverse migration, and conclude that the morning flight of Neotropical landbird migrants at Cape May probably consists of a combination of birds returning to land from the ocean, birds compensating for lateral drift incurred during the previous night's migration, and birds seeking habitat in which to rest and forage. Topography may dictate the direction of morning flight. We suggest that researchers in North America commence studies of autumn passerine migration in late July and that researchers studying habitat selection by migrant passerines wait 2 to 3 h after sunrise before conducting surveys to avoid sampling birds undertaking morning flight. Received 18 March 1991, accepted 13 January 1992. THE MORNING FLIGHT of nocturnally migrating songbirds occurs in several locations in North America (Gauthreaux 1978, Bingman 1980, Hall and Bell 1981) and Europe (Alerstam 1978, Lindstrom and Alerstam 1986). In the autumn of 1988 and 1989, we studied the morning flight of Neotropical migrant songbirds at Cape May Point, New Jersey (38?56'N, 74?58'W). We sought to determine which species undertook morning flight, study their flight behavior, and describe the seasonal and diel timing of their morning flight. In addition, we examined the influence of weather frontal systems on the number of birds observed flying. In 1988, we conducted a daily transect survey through representative vegetation types to compare the species composition of birds remaining in the habitat to that of birds migrating out. This comparison allowed us to determine which of the Neotropical migrants undertook morning flight and which did not. With our data, we examine seven of the hypotheses Richardson (1982) listed to explain northbound migration in the fall, as well as Bingman's (1980) onward-migration hypothesis for diurnal passerine migration. I Present address: Department of Ornithology, The Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, Pennsylvania, 19103, USA. The fact that so many species and such large numbers of migrants participate in morning flight strongly suggests that this behavior is an integral part of migration. Our study examines the function of morning flight. In addition, morning flight has important implications for researchers studying both seasonal timing of passerine migration and use of stopover habitat by passerine migrants.

Population Densities and Community Structure of Birds in Jackson Hole: A Reassessment After 25 Years

There is a wide literature in bird community ecology that bears on questions of species composition and species densities, and the variations in these attributes within habitats between years and between different geographic regions, and between habitat types both locally and regionally (Cody 1975, 1985; Diamond & Case 1986). While there has always been considerable debate on the constancy of bird communities within habitats and among years, and the extent to which community attributes are predictable and deterministic, rather than variable, stochastic or even chaotic (e.g. Wiens 1985, 1988), recently other developments have accentuated the need for measuring and evaluating bird distributions and densities. There is a fast-developing literature that documents recent (ca. the last decade or two) declines in bird species' distributions and densities at both local and regional scales, and emphasizes in particular evidence for recent reductions in the ranges and densities of bird species that are neotropical migrants (e.g. Hutto 1980, 1986; Keast & Morton 1980; Terborgh 1989; Smithsonian 1991). Given especially the concern that bird species breeding in North American sites and overwintering at lower latitudes (where habitat destruction and fragmentation are particularly rapid), data on status changes in breeding bird communities need to be carefully monitored. The best, perhaps the only, way of doing this is to collect current data, to contrast with comparable data collected in a similar fashion in earlier periods. Thus the rationale for this study is apparent: during 1966-68 I obtained extensive data on the bird communities at two sites in Jackson Hole within Grand Teton National Park, located near the site of the old Research Station on the north side of the Snake River below the dam at Jackson Lake. During two field seasons 1991-92 I reassessed the bird communities at these two sites, with the major objective being a documentation of whether, in which ways, and to what extent, the bird communities of the two sites had changed over the 25-year period. The birds in Jackson Hole are particularly appropriate for this 25-year, then-and-now comparison, since none of the dominant species at the two study sites is resident. But while some species winter almost wholly within the United States (e.g. Fox sparrow (Passerella ilaca), other species winter in northern Mexico in desert habitats (e.g. Brewer's sparrow Spizella breweri, Chipping sparrow Spizella passerina) or west-coastal second growth habitats (e.g. Yellow warbler Dendroica petechia, Common Yellowthroat Geothlypis trichas), and yet others winter much further south into Central America (e.g. MacGillivray's warbler Oporornis tolmei, Wilson's warbler Wilsonia pusilla). The results of the comparison are presented in this report, where differences in species composition and density are revealed in both the short-term (adjacent years) and the longer term (between censuses over 25 years apart). But despite such variations, it will be noted that the overall community structure and composition of the sites has changed little over the quarter century.

Use of avian and mammalian guilds as indicators of cumulative impacts in riparian-wetland areas

A new method of assessing cumulative effects of human activities on bird and mammal communities of riparian-wetland areas was developed by using response guilds to reflect how species theoretically respond to habitat disturbance on a landscape level. All bird and mammal species of Pennsylvania were assigned values for each response guild using documented information for each species, to reflect their sensitivity to disturbances; high guild scores corresponded to low tolerance toward habitat disturbance. We hypothesized that, given limited time and resources, determining how wildife communities change in response to environmental impacts can be done more efficiently with a response-guild approach than a single-species approach. To test the model, censuses of birds and mammals were conducted along wetland and riparian areas of a protected and a disturbed watershed in central Pennsylvania. The percent of bird species with high response-guild scores (i.e., species that had specific habitat requirements and/or were neotropical migrants) remained relatively stable through the protected watershed. As intensity of habitat alteration increased through the disturbed watershed, percentage of bird species with high response-guild scores decreased. Only 2%–3% of the neotropical migrants that had specific habitat requirements were breeding residents in disturbed habitats as compared to 17%–20% in reference areas. Species in the edge and exotic guild classifications (low guild scores) were found in greater percentages in the disturbed watershed. Composition of mammalian guilds showed no consistent pattern associated with habitat disturbance. Avian response guilds reflected habitat disturbance more predictively than mammalian response guilds.

The status of the Black CatbirdMelanoptila glabrirostrison Caye Caulker, Belize

SummaryAn evaluation of the relative abundance of the Black CatbirdMelanoptila glabrirostriswas undertaken on Caye Caulker, an offshore island along the barrier reef of Belize, Central America. Although the species is locally abundant in part of the Sian Kaàn Biosphere Reserve in Mexico, the species is facing habitat fragmentation and loss due to development, both along the Mexican coastal areas and on the island habitats in Belize. The Caye Caulker population appears to be the largest in Belize. Transects of fixed radius point counts resulted in 15.5% and mist-netting resulted in 22.4% of all birds recorded being Black Catbirds. The Neotropical migrant Grey CatbirdDumetella carolinensisapparently shares the same habitat on Caye Caulker and comprised 11.2% of all birds netted. The netted Black Catbirds were noted to have substantial fat deposits. The habitat on Caye Caulker is facing probable fragmentation from recent accelerated land speculation and development of an airstrip.

Estimating the Viability of Ovenbird and Kentucky Warbler Populations in Forest Fragments

Breeding habitats for birds in deciduous forests of the eastern and central United States are becoming increasingly fragmented by human activities. Avian communities in small, isolated forest tracts are generally subsets of the communities present in larger forests (Blake & Karr 1987; Hayden et al. 1985); species that are intolerant of habitat reduction are frequently termed area-sensitive. Severe declines in songbird populations, particularly those of forest-interior-breeding neotropical migrants, have been linked to the destruction and fragmentation of natural habitats (Lynch & Whigham 1978; Robbins 1979; Whitcomb et al. 1981). Detailed studies of the dynamics and constitution of small populations in relict habitats are needed (Lande 1988). Data on the presence of species over a range of patch sizes of suitable habitat have been analyzed to estimate the patch size at which an area-sensitive species commonly occurs (Blake 1983; Diamond 1978; Galli et al. 1976), which is known as its minimum area requirement (Robbins 1979). However, tract sizes corresponding with incidence-curve inflection points (Diamond 1978) or areas of occurrence (Blake 1983; Galli et al. 1976) may fall in the range of marginal rather than preferred habitats for area-sensitive species. Forest fragments may represent marginal habitat for

Population declines in North American birds that migrate to the neotropics.

Using data from the North American Breeding Bird Survey, we determined that most neotropical migrant bird species that breed in forests of the eastern United States and Canada have recently (1978-1987) declined in abundance after a period of stable or increasing populations. Most permanent residents and temperate-zone migrants did not show a general pattern of decrease during this period. Field data from Mexico were used to classify a subset of the neotropical migrants as using forest or scrub habitats during winter. Population declines during 1978-1987 were significantly greater among the forest-wintering species, while populations of scrub-wintering species increased. The same subset of neotropical migrants also showed overall declines in forest-breeding species, but no significant differences existed between species breeding in forest and scrub habitats. Neotropical migrant species that primarily use forested habitats in either wintering or breeding areas are declining, but a statistically significant association between habitat and population declines was detected only in the tropics.

Long-term changes in the breeding bird populations of a New Jersey forest

The breeding bird populations at Hutcheson Memorial Forest in central New Jersey, USA, were sampled by mist-netting from 1959 through 1985. Many of the long-distance migrant species, wintering in the neotropics, declined sharply (virtually to zero) during the 1960s. One long-distance migrant species increased as a result of geographical range expansion. One short-distance migrant species, wintering mainly in the southeastern USA, has also increased, whereas resident species (never abundant) seem to have declined slightly. Annual return rates in the first year after banding of long-distance migrants were higher than those for short-distance migrants or resident species. These data are discussed with regard to hypotheses accounting for the declines of neotropical migrants in the northeastern United States. The declines at HMF seem to be a result of ‘leakage’, emigration exceeding immigration of young birds.

Predicting Avian Community Response to Lakeshore Cottage Development

Breeding songbird species were studied in both developed and undeveloped lakeshore habitats of central Ontario. Information is presented for managing lake shorelines for human recreational development without significant loss of wildlife values. The disturbed ground vegetation layer was significantly correlated with a number of habitat variables deemed important in describing avian habitats. Avian species were placed in one of three groups: the ovenbird (Seiurus aurocapillus), an example from the group found primarily in undeveloped habitats; the eastern phoebe (Sayornis phoebe), one of the group found in highly developed habitats; and the veery (Catharus fuscescens), from the group that was not significantly affected by development. These findings are presented in a simple model of breeding bird response to disturbance that can be used to assess the negative impact of cottage development. J. WILDL. MANAGE. 48(4):1239-1247 Human encroachment on undeveloped areas often impacts the plant and animal communities located there. Avian species respond as their breeding habitat is altered or destroyed. Neotropical migrants may be particularly affected by development because not only is their breeding habitat being altered, but wintering grounds are being converted to pasture and farmland (Terborgh 1974). In the last 50 years, many lakes in south-central Ontario have become popular for recreational activity. Cottages have been built along many lakeshores with associated roads and transmission line corridors nearby. Unlike successional sequences, cottage construction tends to impose permanent change in the vegetation and associated environment. Resource managers need information to predict how changes imposed by this development affect wildlife. Robertson and Flood (1980) found an increase in diversity of the songbird community along portions of cottage lakesh re in the nearby Rideau Lakes. Martin (1960) investigated changes in bird populations in relation to forest succession in the Algonquin Park area. James and Wamer (1982) and others have demonstrated that bird species diversity depends on vegetation structure and composition. The mportance of large blocks of unbroken for st was reviewed by Robbins (1979). We have added to these studies to provide resource managers with a tool to predict how future cottage development m y affect bird communities along lakeshores. Songbird populations are notoriously diffic lt to measure, and the populations themselves fluctuate widely. Preston (1960) noted that for 57 breeding species in Ohio, based on 10 years of survey data, nly a out half (28 species) bred every year. James and Wamer (1982) showed that study plots of less than 10 ha often have less than the total species breeding in an area. In our work, based on 2 years of data and small sample plots, we cannot hope to show the long-term comprehensive impact of cottage construction on songbird populations. However a general model of cottage development and the re'Present address: Ontario Ministry of Environment, Box 21, Dorset, Ontario POA 1EO, Canada. 2 Present address: Ontario Ministry of Natural Resources, Cochrane, Ontario, POL 1CO, Canada. J. Wildl. Manage. 48(4):1984 1239 This content downloaded from 157.55.39.70 on Tue, 06 Sep 2016 05:51:31 UTC All use subject to http://about.jstor.org/terms 1240 AVIAN COMMUNITY RESPONSE * Clark et al. suiting impact on the wildlife community is needed to provide planners with guidance in approving or disapproving plans for cottage subdivisions. A concurrent study described the habitat associations of 19 species of breeding birds at varying levels of cottage development and determined that tree density, shrub coverage, conifer composition, and canopy volume were important in describing the habitat associations (Clark et al. 1983). The purpose of this study was to develop a method to predict changes in avian species composition caused by cottage development that could be used to model the impact of habitat disturbance on wildlife commu-

Sexual Selection and Variance in Reproductive Success

Sexual Selection and Variance in Reproductive Success

Maximum oxygen consumption and heat loss facilitation in small homeotherms by He-O2.

ROSENMANN, MARIO, AND PETER MORRISON. Maximum oxygen consumption and heat loss facilitation in small homeotherms by He-02 . Am. J. Physiol. 226(3) : 490495. 1974.-The high thermal conductance of an 80% He-20y0 02 atmosphere was used to elicit maximum metabolism (Mmax) in moderate cold in species ranging from 7-g pygmy mice (Baiomys taylori) to 250-g white rats, including redpolls (Acanthis J?ammea), two vesper mice (Calomys d&la, C. callosus), tundra voles (Microtus oeconomus), and four strains of Mus musculus. Values slightly exceeded those in similar animals using other methods to confirm the low metabolic ratio (Mmax/M, in) in rodents (4-8 X). Submaximal values at higher temperatures defined thermal conductance in He-02 and air. In different species the ratios of these conductances ranged from 1.4 to 2.6, differences which relate to the extent and quality of the respective insulation. M,,, was obtained at 13-70°C warmer in He-02 than required in air for the same metabolic effort. Avoidance of low-temperature technology and freezing injury, elimination of treadmills and training in running, prompt attainment of M,,., (3-10 min after He-02 exposure), and obviation of shaving or wetting procedures are advantages of the present technique.