Neotropical Migrants Research Articles

Results of the Vermont Forest Bird Monitoring Program, 1989-1996

We conducted a statistical analysis of breeding census data from the first 8 years (1989-1996) of the Vermont Forest Bird Monitoring Program (FBMP). Data were collected at 17 study sites located in large tracts (= 40.5 ha) of mature, forested habitats in Vermont. A route regression model was used to produce population trends for 67 species and 3 groups of species categorized by wintering range. Power analyses conducted for 4 species revealed that the FBMP adequately sampled species which occurred on at least 9 or more of the 17 study sites and that our ability to detect small (=2%) population changes will increase dramatically after 15 years of monitoring. FBMP data showed signifi? cant declines in breeding populations of black-capped chickadee, solitary vireo, and Canada warbler, but a significant increase in rose-breasted grosbeak. When species were categorized by wintering range, neotropical migrants showed a significant overall increase and year-round residents declined. The negative trend for Canada warbler, corroborated by Breeding Bird Survey (BBS) data and migration monitoring data from Long Point Bird Observatory in Ontario, sug? gest that this species is experiencing widespread declines over a significant portion of its breeding range. Rates of population change in FBMP data were poorly correlated to BBS trends. We found significant positive correlations between Vermont's average winter temperatures and the relative abundances of brown creeper, downy woodpecker, and for all resident species combined. We evaluated the number of brown-headed cowbird occurrences by study site and physiographic region. Most cowbirds (95.3%) occurred at FBMP sites located in the predominately agricultural Champlain Lowlands, suggesting that even relatively large forested tracts in this region may be subject to high rates of cowbird parasitism.

Population Limitation in Neotropical Migratory Birds: Comments on Rappole and McDonald (1994)

The importance of breedingand winteringground phenomena in population declines of Neotropical migratory birds is the topic of much debate and discussion (see Martin and Finch 1995, Sherry and Holmes 1995). Recently, Rappole and McDonald (1994) provided and evaluated 14 predictions that they argued should be supported if population declines in migrants are due to changes in North American breeding habitats. They concluded that general declines in Neotropical migrants have resulted from habitat alterations on the wintering grounds. Rappole and McDonald's conclusion is weakened, however, because they did not consider many relevant studies that contradict several of their predictions. In addition, they relied heavily on data from a few relatively unique, migratory species that are specialists on forest-interior habitats during the winter. In this commentary, we present alternative analyses that call into question the validity of some of Rappole and McDonald's (1994) predictions and their conclusion of population limitation on the wintering grounds. Our analyses often provide evidence in support of breeding-season limitation. Our intention is to emphasize the potential influence of events throughout the annual cycle on population trends (Morse 1980, Cox 1985, Sherry and Holmes 1995) and to point out the abundance of literature that supports the hypothesis that events on the breeding grounds are likely to influence population trends of Neotropical migrants. We also identify the need for additional data from wintering populations of many Neotropical migrants. Optimal winter habitat-Several of the predictions of Rappole and McDonald (i.e. nos. 1, 5, 6, and 10) are based on the assumption that we know which wintering habitats are optimal for a given species and that we can rank other habitat types in terms of optimality. Optimal breeding habitat has been identified for some species based on long-term studies and rates of survival and reproduction, but very little is known about habitat use and survival rates of birds on their wintering grounds (but see Conway et al. 1995, Sherry and Holmes 1996). Studies of win-

ARE BOREAL BIRDS RESILIENT TO FOREST FRAGMENTATION? AN EXPERIMENTAL STUDY OF SHORT-TERM COMMUNITY RESPONSES

We studied the effect of habitat fragmentation on the richness, diversity, turnover, and abundance of breeding bird communities in old, boreal mixed-wood forest by creating isolated and connected forest fragments of 1, 10, 40, and 100 ha. Connected fragments were linked by 100 m wide riparian buffer strips. Each size class within treatments and controls was replicated three times. We sampled the passerine community using point counts before, and in each of two years after, forest harvesting, accumulating 21340 records representing 59 species. We detected no significant change in species richness as a result of the harvesting, except in the 1-ha connected fragments, where the number of species increased two years after isolation. This increase was accounted for by transient species, suggesting that the adjacent buffer strips were being used as movement corridors. Diversity (log series alpha index) was dependent on area in the isolated fragments only after cutting, having decreased in the smaller areas. Turnover rates in the isolated fragments were significantly higher than in similar connected or control areas, due to species replacement. Crowding occurred in the isolated fragments immediately after cutting, but two years after fragmentation, the responses in abundance of species varied with migratory strategy. Numbers of Neotropical migrants declined in both connected and isolated fragments, and resident species declined in isolated fragments. Most species in these groups require older forest, many favoring interior areas. Abundance of short-distance migrants, most of which are habitat generalists, did not change. Overall, although there was no decrease in species richness from our recently fragmented areas, community structure was altered; maintaining connections between fragments helped to mitigate these effects. Nevertheless, the magnitude of the fragmentation effects we documented is small compared with those observed elsewhere. Birds breeding in the boreal forest, where frequent small- and large-scale natural disturbances have occurred historically, may be more resilient to human-induced habitat changes, such as those caused by forest harvesting. However, these results should be interpreted with caution. First, they are short-term and address only broad-scale community responses based on species richness and relative abundance. Second, the study area was embedded in a landscape where large areas of old, mixed forest are still available, potentially dampening any local-scale impacts of fragmentation.

Effects of Small Forest Openings on the Breeding Bird Community in a Vermont Hardwood Forest

We examined the response of a forest bird community to the presence of small openings created by patch clear-cutting 0.4-ha plots within an extensive northern hardwood forest. We conducted bird censuses (June) and habitat measurements (July-August) in 102 census plots at four distances from openings in 1991 and 1992. Of 19 habitat variables measured, none differed significantly among forest plots at any distance from clear-cuts. Thirty-five (70%) of 50 bird species encountered during censusing were Neotropical migrants. Bird species richness did not differ as a function of distance from openings. However, species composition in plots within openings was least similar to that in plots farthest into forest, and most similar between the two distance categories farthest from openings. The movement of several forest-interior species away from openings, the addition of early-successional colonists in openings, and a high abundance of interior-edge species near openings contributed to the difference in species composition between openings and forest plots. As a group, Neotropical forest-interior migrants were significantly less abundant in openings than at any distance from them, and less abundant 50 m from openings than 200 m from them. Neotropical interior-edge migrants were significantly more abundant 50 m from openings than at any other distance. Nearctic migrants and nonmigrants did not respond to the presence of small openings. Of three locally common avian nest predators, none became more abundant in the openings. Brown-headed Cowbirds (Molothrus ater), also common locally, were never observed in or near the openings. Overall, bird species diversity increased in forested areas containing small openings due to the addition of edge and open-area nesters, but several forest-interior species were adversely affected by the presence of openings.

Survival, Productivity, and Abundance in a Wilson's Warbler Population

-We analyzed patterns in the abundance, productivity, and survivorship of a coastal California population of Wilson's Warbler (Wilsonia pusilla) using capture-recapture data from 18 years of standardized mist-netting conducted during the breeding season. The numbers of adult and hatching-year birds captured each year showed no consistent trend through time. The total number of adults captured annually, an index of adult abundance, was positively related to productivity in the previous year as indexed by the number of hatching-year birds captured. In addition, the number of adults captured for the first time at the study site, an index of new adults entering the population, was positively related to the previous year's productivity. There was a positive correlation between the numbers of summer residents and transients captured in each year. Estimated annual adult survival of summer residents was 50.3%. The annual recapture probability for summer residents (68.8%) was much greater than for presumed transients (7.2%). When the difference in recapture probability between summer residents and transients was ignored, annual survival was underestimated (30.5 %). Year-to-year variation in survival showed no relationship to annual fluctuations in adult abundance. These patterns of survival and productivity parameters suggest that abundance in this population has been influenced primarily by circumstances on the breeding grounds. Received 17 May 1996, accepted 10 January 1997. EVIDENCE OF DECLINES in North American landbird populations has led to widespread efforts to monitor population trends and to identify the causes of population fluctuations (Robbins et al. 1989, Hagan and Johnston 1992, Finch and Stangel 1993). In theory, fluctuations in population size may be influenced by variation in reproductive success, survival and recruitment of young into the breeding population, adult survivorship, immigration, or emigration (Nur and Geupel 1993a). Variation in reproductive success has been suggested as a major cause of fluctuations in the abundance of Neotropical migrants, and several studies have found evidence that the recruitment of new breeders into populations of long-distance migrants is positively correlated with reproductive success in the previous summer (Nolan 1978, Sherry and Holmes 1991, Holmes et al.1992, Johnson and Geupel 1996). Evidence also suggests that variation in mortality during migration and over winter are important factors influencing the abundance of migratory birds (Holmes et al. 1989, Robbins et al. 1989, Peach et al. 1991, Rappole and McDonald Present address: Department of Biology, University of California, Riverside, California 92521, USA. E-mail: mkchase@citrus.ucr.edu 1994). Clearly, studies examining the demographic parameters of migratory bird populations are critical to identifying the proximate causes of population fluctuations (Temple and Wiens 1989; Sherry and Holmes 1991, 1992; Verner 1992). We studied the dynamics of a coastal California population of Wilson's Warbler (Wilsonia pusilla) using 18 years (1979 to 1996) of standardized mist-net data. We estimated three demographic parameters: abundance, productivity, and adult survival. Using the capture rate of adults as an index of abundance, we looked for evidence of long-term trends. To generate insight into the demographic mechanisms influencing population fluctuations, we investigated the relative influence of productivity and adult survival on adult abundance. To examine the influence of productivity on the variation we observed in abundance, we investigated the relationship between productivity in one year and the number of adults captured in the following year. We estimated annual recapture and survival probabilities of adult Wilson's Warblers by analyzing capture/recapture data (Lebreton et al. 1992). To examine the influence of survival on annual variations in abundance, we investigated the relationship between an-

Reproductive Success of Worm‐Eating Warblers in a Forested Landscape

(Askins et al. 1990; Hagan & Johnston 1992; Finch & Stangel 1993). Although there is debate about the overall direction and geographic extent of these trends (James et al. 1992; Askins 1993), some species have undergone significant population declines (Sauer & Droege 1992; Askins 1993, 1995). One suggested cause of population declines is anthropogenic fragmentation of breeding habitat and consequent lowered reproductive success (Askins et al. 1990; Faaborg et al. 1993). Previous studies have shown that the following were correlated with increased levels of forest fragmentation: (1) reduced pairing success (Wander 1985; Gibbs & Faaborg 1990; Villard et al. 1993); (2) reduced fledging success or increased nest predation (Hoover 1992; Porneluzi et al. 1993; Robinson et al. 1995); and (3) increased brood parasitism by Brownheaded Cowbirds (Molothrus ater, Temple & Cary 1988; Robinson et al. 1995). Furthermore, many species of forest birds are less abundant in small isolated patches of forest than in large continuous forests (Askins 1994). However, forest fragmentation can occur on many spatial scales and on a continuum of severity, and the consequences of fragmentation are highly dependent on the species or ecosystem in question (Pearson 1993; Villard et al. 1992, 1993; Robinson et al. 1995). We investigated the reproductive performance of the Worm-eating Warbler (Helmitheros vermivorus), a single-brooded, ground-nesting species. It is one of many species of forest-breeding Neotropical migrants considered area sensitive (e.g., Hayden et al. 1985; Askins et

Declines in Abundance and Species Richness of Birds Following a Major Flood on the Upper Mississippi River

-We examined the abundance and species richness of birds breeding in floodplain forests of the Upper Mississippi River from 1992 to 1994 to identify effects of a major flood in 1993 on the bird assemblage. Sixty-five study plots were divided into treatments and controls based on whether they were flooded in 1993. Among 84 species observed on all plots, 41 species decreased in abundance from 1992 to 1994, 13 species increased, and numbers of 5 species were unchanged. Sample sizes were inadequate to evaluate trends for 25 species. Species richness declined over the three-year period. The main effect was significant in 20 of the 36 species tested. Cool, wet conditions may have contributed to poor reproductive success in 1993 and resulted in widespread declines in abundance during the year following the flood. Bird abundance increased on most unflooded plots in 1993, probably because birds were displaced from flooded plots. This pattern was most striking for Neotropical migrants, species preferring habitat edges, lower-canopy nesters, and species that forage in the air. We suggest that periodic major flooding maintains suitable floodplain habitat for Prothonotary Warblers (Protonotaria citrea) in the face of competition with House Wrens (Troglodytes aedon) for nest sites. Received 30 November 1995, accepted 22 January 1996. FLOODPLAINS ARE DYNAMIC HABITATS where periodic flooding creates natural disturbances that set back succession and provide new sites for plant establishment (Peck and Smart 1986, Kupfer and Malanson 1993, Jones et al. 1994, Sparks 1995, Yin and Nelson 1995). Large floodplain forests like those of the Upper Mississippi River are uncommon in northern landscapes (Mitsch and Gosselink 1993) and provide unique habitats that support a high abundance and diversity of birds (Grettenberger 1991, Best et al. 1995, Knutson et al. 1996). Few studies have examined the avifauna of large floodplain forests (Emlen et al. 1986, Decamps et al. 1987, Knutson 1995), and studies of the effects of floods that include pre-flood data are even less common (Knopf and Sedgwick 1987). Direct effects of flooding on birds nesting in floodplains tend to be minor because spring floods usually recede by midto late May. Summer floods are rare and usually of short duration. However, major floods of long duration or high amplitude can create large-scale disturbances that dramatically change vegetative I Present address: U.S. Geological Survey, Biological Resources Division, Upper Mississippi Science Center, 2630 Fanta Reed Road, La Crosse, Wisconsin 54602, USA. E-mail: melinda_knutson@nb?.gov cover (Yin et al. 1994). Bird assemblages may respond to changes in availability of rnest sites and food resources caused by these disturbances. Flooding had a major effect or a riparian bird community in Arizona by elirminating cottonwood and willow nesting habitat (Hunter et al. 1987). Depending upon the severity of the disturbance, some species may in(rease in abundance while others decline. In addition, flooding may maintain suitable habitat for some bird species that are uncommon in drier, less-disturbed habitats. Rainfall during May to August 1993 in the Upper Mississippi River basin reached historically high levels (ca. twice normal), zpproximating 75to 300-year recurrence i ztervals (Wahl et al. 1993, Interagency Floodplain Management Review Committee 1994). River water levels were well above normal throughout the breeding season (Parrett et al. 1993). We examined the effects of this major flood on the abundance and species richness of bird,; breeding in floodplain forests by comparing cLata collected from 1992 to 1994. We conside-ed our data in relation to two hypotheses of how the flood and associated weather patterns aLffected the floodplain bird assemblage. The Flood Disturbance Hypothesis proposes that the pattern of change in abundance will differ between flooded and unflooded plots because structural

Empirical Bayes Estimation of Proportions with Application to Cowbird Parasitism Rates

Bayesian models provide a structure for studying collections of parameters such are considered in the investigation of communities, ecosystems, and landscapes. This structure allows for improved estimation of individual parameters by considering them in the context of a group of related parameters. Individual estimates are differntially adjusted toward in overall mean, with the magnitude of their adjustment based on their precision. Consequently, Bayesian estimation allows for a more reliable ranking of parameters and, in particular, a more credible identification of extreme values from a collection of estimates. In Bayesian models, individual parameters are regarded as values sampled from a specified probability distribution, called a prior. The requirements that the prior be known is often regarded as an unattractive feature of Bayesian analysis and may be the reason Bayesian analyses are not frequently applied in ecological studies. Empirical Bayes methods provide an alternative approach that incorporates the structural advantages of Bayesian models while requirng a less stringent specification of prior knowledge. Empirical Bayes methods require only that the prior be in a certain family of distributions, indexed by hyperparameters that can be estimated from the available data. This structur is of interest per se, in addition to its value in allowing for improved estimation of individual parameters; for example, hypothese regarding the existence of distinct subgroups in a collection of paramet ers can be considered under the empirical Bayes framework by allowing the hyperparameters to vary among subgroups. We describe the empirical Bayes approach in application to estimation of proportions, using data obtained in a community—wide study Brown—headed Cowbird paratism rates for illustration. Empirical Bayes estimates identify those species for which there is the greatest evidence of extreme parasitism rates. Subgroup analysis of our data on cowbird parasitism rates indicates that parasitisms rates for neotropical migrants as a group are no greater than those of resident/short—distance migrant in this forest community. Our data and analyses demonstrate that the parasitism rates for certain neotropical migrant species (Wood Thrush and Rose—breasted Grosbeak) are remarkably low while those for others (Ovenbird and Red—eyed Vireo) are remarkably high.

Transboundary Conservation: An Ecoregional Approach to Protect Neotropical Migratory Birds in South America

/ Future conservation efforts will need to transcend geopolitical boundaries in efforts to protect entire landscapes and ecosystems. Neotropical migratory birds are as a group a useful conservation tool for linking diverse landscapes and people due to their dependence on multiple habitats, sensitivity to habitat changes, and universal public appeal. The conservation of neotropical migrants can therefore function as a powerful hemispheric umbrella for ecosystem protection. Efforts to protect neotropical migratory birds on their nonbreeding grounds have traditionally been focused on Mexico, Central America, and the Caribbean. To assess the importance of South America to neotropical migrants, an ecoregional classification system was used to determine species distributions in the Andean/Southern Cone Region (Bolivia, Colombia, Ecuador, Paraguay, Peru, and Venezuela). The occurrence of migrants in protected areas that are part of The Nature Conservancy's Parks in Peril program was also assessed. Of the 406 neotropical migrant species, nearly one third (132) occur as regular nonbreeding residents in the region and for almost half of these species (53), South America is their main nonbreeding ground. All Parks in Peril sites were found to harbor neotropical migrants. Forty-eight species (36%) have declining long-term North American Breeding Bird Survey population trends and/or high Partners in Flight concern scores and thus are of significant conservation concern. Most importantly, 29 species (22%) of conservation concern use South America as their primary nonbreeding ground, indicating a need for focused conservation action. The nature of the ecoregional approach used in this endeavor makes future prioritization of ecoregions and conservation strategies for neotropical migrants across national boundaries possible. The ability to link diverse landscapes using a common element such as migratory birds allows for unique transboundary partnerships and opportunities for habitat conservation, which support the goal of the Conservancy's new Migratory Bird Initiative.KEY WORDS: Neotropical migratory birds; Transboundary conservation; Ecoregions; Protected areas; South America

Winter Surveys of Forest-Dwelling Neotropical Migrant and Resident Birds in Three Regions of Cuba

We used mist nets and fixed-radius point counts to survey the distribution and relative abundance of Neotropical migrant and resident birds at 18 sites, six in each of three regions, in Cuba, representing six forest types. Eighty-two species and one hybrid were recorded, of which 32 species (39.0%) were Neotropical migrants. Strong differences were observed in the numbers of migrant and resident individuals and species detected, and the proportions of migrant individuals and species among regions, but results obtained by mist nets and point counts often differed. Combined results suggest that the western montane region contains the most resident species, and Cayo Coco in eastern Cuba holds the most migrant species. Relative abundances of migrants and residents were compared among forest types within regions. Based on mist net captures, the number of migrant and resident individuals and species at each site were significantly positively correlated. A Principal Components Analysis correlating vegetation structure characteristics with mist net captures suggested that residents occupy a broader spectrum of vegetation structure types than migrants which favor sites with structure typical of secondary forests. We observed habitat segregation among dichromatic migrant warblers both among forest types within regions and among regions within forest types. Generally, Cuba supports greater numbers of migrant individuals and species than other sites in the Caribbean. Compared to the Yucatan Peninsula and Belize, Cuba supports fewer migrant species, but they account for a greater proportion of all species and a lower proportion of all individuals. The efficacy and biases of mist nets and point counts are discussed.

Constraints on Double Brooding in a Neotropical Migrant, the Hooded Warbler

We examined the constraints on double brooding in Hooded Warblers (Wilsonia citrina) to explain why 56% of females with successful first nests did not attempt second broods. Double brooded females fledged on average 1.9 more young than single brooded females. Double brooded females nested significantly earlier than single brooded females, but many females with early nests did not double brood. There were no significant differences among single and double brooded females in age, experience, body condition, or reproductive output at their first nest. Female breeding strategy did not depend on male age or male feeding effort at the first nest. Double brooded individuals were usually feeding fledglings while undergoing their pre-basic molt, and the main cost of being double brooded was a three week delay in molt compared with single brooded birds. This delay in molt could impose a high energetic cost due to the overlap of molt and fledgling care, and a time cost in terms of delaying migration and the acquisition of a winter territory.

Breeding bird communities of the western and northern Canadian boreal forest: relationship to forest type

We examined the distribution of 80 species of breeding birds across 67 census plots from a variety of sources in the boreal forest of western and northern Canada to obtain information on bird habitat associations for forest management. The sites ranged from upland black spruce (Picea mariana) to riverine deciduous forests and wet, marshy bogs. Axis 1 of an ordination (detrended correspondence analysis) demonstrated a gradient in bird communities from dry to wet sites; axis 2 may have been a black spruce (nutrient poor) to mixed deciduous forest gradient (nutrient rich). Hierarchical classification (twinspan) identified five groups of sites according to their bird communities. Despite geographical variation in bird communities and possible geographical variation in habitat associations, sites were classified according to their forest types rather than regional affinities. Yellow-rumped warblers (Dendroica coronata) and dark-eyed juncos (Junco hyemalis) showed a pronounced gradient of increased abundance from deciduous to coniferous sites. White-throated sparrows (Zonotrichia albicollis) and alder flycatchers (Empidonax alnorum) showed a reverse gradient. In 22 sites of known-age aspen (Populus tremuloides) – mixedwood forests from central Saskatchewan, an ordination indicated a strong relationship between stand age (and thus the proportion of coniferous cover) and bird community structure. The highest combined densities of Neotropical migrants occurred in old forests, whereas short-distance migrants were most abundant in young forests. The highest abundance of upper-canopy gleaners was found in old forests, whereas ground foragers were most abundant in early successional forests. These findings have important implications for management of boreal forests.

Modulation of the Adrenocortical Stress Response in Neotropical Migrants during Autumn Migration

easonal changes in corticosterone metabolism have been implicated in the migratory physiology of passerines. To investigate how this hormone might relate to migra- tory condition in free-living birds, we compared plasma levels of corticosterone in response to capture and handling stress in premigratory and migrating Gray Catbirds (Dumetella car- olinensis). Lean, premigratory catbirds exhibited stress responses typical of other nonlipogenic, nonhyperphagic bird species sampled on the breeding or wintering grounds (i.e. low levels of corticosterone at capture followed by significant increases over I h). In contrast, fat, migratory catbirds sampled later in the season showed significantly higher baseline levels of corticosterone at capture and no significant increase in hormone levels with handling time. We also sampled a group of migrating Yellow-rumped Warblers (Dendroica coronata) at a stopover site. Like catbirds, migrating warblers exhibited an absence of a stress response, with no significant change in corticosterone levels with capture and handling stress. Unlike birds sampled on the breeding grounds, there was no relationship between corticosterone levels at capture and fat scores in premigratory and migratory catbirds, or in the migratory warblers. These results support our Migration-Modulation Hypothesis regarding seasonal changes in corticosterone levels, viz.: (1) an absence of a relationship between fat deposition and baseline corticosterone levels illustrates the permissive nature of the hormone, which remains elevated throughout the migratory period to facilitate hyperphagia and lipogenesis independent of short-term changes in energetic condition; and (2) further elevation of cor- ticosterone in response to acute stress is suppressed during migration to protect skeletal muscle needed for flight. Received 25 April 1995, accepted 14 July 1995.

Relationships of bird community structure and species distributions to two environmental gradients in the northern boreal forest

Few studies have explicitly examined avian community structure in the North American northern boreal forest. Herein we report upon the results of bird surveys in mature stands of boreal forest in the Great Clay Belt, Ontario, Canada. We related trends in avian community structure and individual bird species' abundance to two environmental gradients described in a landbase classification scheme called the Forest Ecosystem Classification — a moisture and a nutrient‐richness gradient. Variation in environmental characteristics is limited in the Ontario Clay Belt and this was reflected in short environmental gradient lengths. However, major trends in avian community structure were strongly associated with the nutrient‐richness gradient axis summarized by this scheme. Analyses of avian community composition indicated a continuum from moist, coniferous habitats to drier aspen‐dominated mixed woodlands with several bird species occurring in varying abundances across the width of both gradients. 48 of the 58 species examined showed statistically significant associations with at least one of the two multivariate gradient axes. A smaller proportion of short‐distance migrant species were associated with these gradients than were species in either the neotropical migrant or resident categories. We used a multivariate variable (“habitat breadth”) to compare degree of habitat specialization across different migratory groups. A large proportion of neotropical migrants showed a high degree of habitat specialization in mature Clay Belt forests, and neotropical species with small habitat breadths were more commonly associated with habitats dominated by broad‐leaved deciduous tree species than either short‐distance or permanent resident species. We discuss our findings in relation to the post‐glacial history of the Clay Belt region.

Using Landscape Ecology to Test Hypotheses About Large‐Scale Abundance Patterns in Migratory Birds

The hypothesis that Neotropical migrant birds may be undergoing widespread declines due to land use activities on the breeding grounds has been examined primarily by synthesizing results from local studies. Growing concern for the cumulative influence of land use activities on ecological systems has heightened the need for large—scale studies to complement what has been observed at local scales. We investigated possible landscape effects on Neotropical migrant bird populations for the eastern United States by linking two large—scale inventories designed to monitor breeding—bird abundances and land use patterns. The null hypothesis of no relation between landscape structure and Neotropical migrant abundance was tested by correlating measures of landscape structure with bird abundance, while controlling for the geographic distance among samples. Neotropical migrants as a group were more sensitive to landscape structure than either temperate migrants or permanent residents. Neotropical migrants tended to be more abundant in landscapes with a greater proportion of forest and wetland habitats, fewer edge habitats, larger forest patches, and with forest habitats well dispersed throughout the scene. Permanent residents showed few correlations with landscape structure and temperate migrants were associated with habitat diversity and edge attributes rather than with the amount, size, and dispersion of forest habitats. The association between Neotropical migrant abundance and forest fragmentation differed among physiographic strata, suggesting that landscape context affects observed relations between bird abundance and landscape structure. Finally, associations between landscape structure and temporal trends in Neotropical migrant abundance were counter to those observed in space. Trends in Neotropical migrant abundance were negatively correlated with forest habitats. These results suggest that extrapolation of patterns observed in some landscapes is not likely to hold regionally, and that conservation policies must consider the variation in landscape structure associations observed among different types of bird species and in physiographic strata with varying land use histories.

New Approaches to the Analysis of Population Trends in Land Birds

Although there is strong evidence that a few Neotropical migrant land birds have severely declining populations, we know of no evidence of general overall declines. One widely cited analysis of data from the Breeding Bird Survey (BBS), the only continental—scale monitoring program for land birds in the breeding season, indicates declining populations in eastern forests in the 1980s, but other analyses conclude that most species have had stable or increasing populations in the last 25 yr. These conflicting results call for a new perspective. Here, we propose two approaches to the analysis of BBS data that emphasize the detection of temporal and geographic variation in population trends. The first approach uses nonlinear regression to detect temporal changes in population trends, and permits the construction of graphs that compare nonlinear trends among regions. The second approach provides statistical tests of multispecies patterns of geographic variation in population trends. We report analyses of BBS data for 1966 to 1992 for the 26 species of wood warblers (Parulinae) for which the most data are available and the chance of obtaining reliable estimates is best. Using pooled data for all 26 species for eastern and central North America, nonlinear regression indicated declining populations in the Appalachian Mountains and the Eastern Foothills in the 1970s, and stability elsewhere. As examples of the use of this first approach in single—species analyses, we give maps, graphs, and statistical tests to illustrate variation in population trends by physiographic strata in the Blue—winged Warbler (Vermivora pinus) and the Ovenbird (Seiurus aurocapillus). As an example of the second approach, we applied a probit—normal model to a matrix of increases and decreases in the 26 species in 37 geographic strata. The analysis identified specific highland areas as having unusually high proportions of declining populations. These results suggest that correlates of elevation should be considered as factors possibly regulating populations of warblers and other land birds. Our perspective is that focus on studies of temporal and geographic variation in population trends of all birds, including Neotropical migrants, could help conservation biologists identify where species and groups of species are in the most trouble.

Geostatistics as a Tool for Examining Hypothesized Declines in Migratory Songbirds

Data from the North American Breeding Bird Survey (BBS) indicate significant declines in the populations of several species of songbirds, including several Neotropical migrants. These declines have been attributed to habitat destruction and fragmentation on the breeding grounds, in strategic migratory stopover sites, and on the wintering grounds. Using BBS data from the 1967—1989 period and universal kriging, we produced maps of abundance change for two declining species of wood warblers to test hypothetical spatial scenarios of decline over entire breeding ranges. These species were the Cerulean Warbler (Dendroica cerulea) and the Prairie Warbler (D. discolor). We found considerable variability in the location of areas of decline when comparing successive 5—yr periods. In some comparisons, areas of decline were concentrated in the centers of abundance of these species, and in others, they were scattered throughout their range. We also found that the direction and intensity of population trends was quite sensitive to the methods used for calculating abundance. Our results indicate that, even for species where significant long—term declines have been reported, considerable variation exists in the direction of abundance change, both geographically and temporally. Although most of the long—term declines reported in particular species at local and regional scales are undeniable, gaps in our knowledge still prevent us from incorporating these trends into a global model of the annual cycle of Neotropical migrants.

Source‐Sink Population Dynamics may Complicate the Interpretation of Long‐ Term Census Data

We examined the reproductive success and long—term population dynamics of Neotropical migrant birds in the fragmented landscapes of Illinois. Our primary objective was to assess whether annual variation in abundances and persistence of populations within woodlots was related to variation in reproductive success. Nesting success of migrants was so low in the woodlots where we sampled that it is unlikely that these populations are self—sustaining. Moreover, populations frequently disappear from and recolonize these woodlots. These data suggest that the source—sink metaphor for population dynamics within spatially structured populations is applicable to migratory birds in Illinois. The scale of this dynamic is unknown, but may be regional (i.e., midwestern U.S.). An important implication of this result is that census data must be interpreted carefully. For one woodlot, we detected no systematic trend in the overall abundances of migratory birds over much of the 20th century. Yet the viability of populations within that woodlot likely decreased dramatically over that time owing to increasing rates of nest predation and brood parasitism. Lack of data on dispersal is a major gap in understanding the population dynamics of Neotropical migrants and prescribing effective conservation measures.

Neotropical Bird Migration during the Ice Ages: Orientation and Ecology

ABsTRAcr.-Reconstruction of breeding habitat of North American Neotropical migrants 18,000 years ago and 9,000 years ago indicated major shifts in both location and composition of plant communities relative to present conditions. Increased vegetation in xeric areas may have compensated, at least in part, for the reduction in breeding habitat due to glaciation. Autumnal flights of Neotropical passerine migrants flying on constant headings from North America to Central and South America were simulated under present wind conditions and for winds during periods of glaciation at 18,000 and 9,000 years ago. The 1550 average headings currently observed for Atlantic migrants were found to function well during periods of glaciation and may have been more generally useful during those times than at present. Received 15 June 1994, accepted 27 January 1995.

Conservation and Management of Neotropical Migrant Landbirds in the Northern Rockies and Great Plains

Conservation and Management of Neotropical Migrant Landbirds in the Northern Rockies and Great Plains