Neotropical Migrant Species Research Articles

BREEDING BIRD RESPONSES TO THREE SILVICULTURAL TREATMENTS IN THE OREGON COAST RANGE

Silvicultural alternatives to clear-cutting have been suggested to promote development, retention, or creation of late-successional features such as large trees, mul- tilayered canopies, snags, and logs. We assessed bird response to three silvicultural alter- natives to clear-cutting that retained structural features found in old Douglas-fir (Pseudo- tsuga menziesii) forests and that imitated natural disturbance regimes more closely than did traditional clear-cutting: (1) small-patch group selection treatment representing a low- intensity disturbance; (2) two-story treatment, representing a moderate to high-intensity disturbance; and (3) modified clear-cut treatment, representing a high-intensity disturbance. We counted diurnal breeding birds 1 yr prior to and 2 yr after harvest to estimate effects of the silvicultural treatments on bird communities compared with uncut controls. The small-patch group selection treatment was most similar in species composition to control stands. The two-story treatment was more similar to the modified clear-cut treatment. Ten bird species remained abundant following the small-patch group selection treatment. They declined in abundance in modified clearcuts and two-story stands. These species included four neotropical migratory species and five species with restricted geographic ranges and habitat associations. Nine species increased in response to moderate and/or high-intensity disturbances. This group included a larger proportion of species that were habitat generalists. Silvicultural treatments imitating low-intensity disturbances were most effective in retaining bird communities associated with mature forest; high-intensity disturbances such as the two-story and modified clear-cut treatments greatly altered bird community composition. Bird responses to the silvicultural treatments that we studied indicate that a variety of stand types is needed to meet needs of all species.

Influence of Gambel Oak on Breeding Birds in Ponderosa Pine Forests of Northern Arizona

Gambel oak (Quercus gambelii) is widely distributed in the Southwestern United States, where it frequently occurs in association with ponderosa pine (Pinus ponderosa). Fire suppression and fuelwood harvest likely have reduced oak abundance within the pine-oak type. Gambel oak occurs in multiple age-related growth forms, from small shrubs to large, old trees, and may provide important foraging and nesting habitat for breeding birds. I compared attributes of breeding bird communities in 16 northern Arizona ponderosa pine stands from 1993-1995, that were structurally similar except for the presence or absence of Gambel oak. Overlap in bird species composition was high; pine and pineoak stands had a mean Jaccard similarity value of 0.67. Five species were unique to pine stands, whereas 10 species were largely restricted to or only found in pine-oak stands. Overall bird diversity was significantly higher in pine-oak stands, which also had more species of Neotropical migrants, ground nesters, primary cavity excavators, and secondary cavity users than did pine stands. Pine and pine-oak stands had similar species evenness and similar rates of annual species turnover. Total bird abundance did not differ between cover types; however, primary cavity excavators were more abundant in pine-oak stands. Because of the apparent positive influence of oak on breeding birds, forest managers are encouraged to use treatments that retain and enhance the various growth forms of Gambel oak found in pine-oak stands.

Winter Participation by Neotropical Migrant and Resident Birds in Mixed-Species Flocks in Northeastern Mexico

Mixed-species foraging flocks in northeastern Mexico are large, diverse, and include Neotropical migrant and resident species. I characterized the species composition and social organization of mixed-species flocks in four habitats of the El Cielo Biosphere Reserve during the winters of 1993-1995. Thirty-seven percent of all species found in the Reserve participated in mixed-species flocks, including 16 migrant species. Flocks in the dry pine-oak forest had more migrant individuals than flocks in the humid oak-pine/cloud forest or tropical semi-deciduous forest, but flocks in the tropical semi-deciduous forest had more migrant species than resident species. Migrant species rarely have been reported as nuclear species that attract other species to the flock and maintain flock cohesion, yet both Ruby-crowned Kinglet (Regulus calendula) in the dry pine-oak forest, and Blue-gray Gnat-catcher (Polioptila caerulea) in the tropical semi-deciduous forest, are migrant species that behaved as nuclear flock species. Bridled Titmouse (Parus wollweberi) and Crescent-chested Warbler (Vermivora superciliosa), Mexican resident species, were nuclear species in the dry pine-oak forest and the humid oak-pine/cloud forest, respectively. I found that Ruby-crowned Kinglet and Crescent-chested Warbler were follower species (species that occurred regularly in flocks but did not behave as nuclear species) in habitats where they were not nuclear species, indicating that the social role of a species within a flock may depend on habitat, resource availability, species density, or the other species present in the mixed-species flock.

Age-Related Differences in the Stopover of Fall Landbird Migrants on the Coast of Alabama

We estimated fat load, length of stopover, and rate of mass change for six Neotropical migrant landbird species at a site along the northern coast of the Gulf of Mexico. Swainson's Thrushes (Catharus ustulatus), Gray Catbirds (Dumetella carolinensis), White-eyed Vireos (Vireo griseus), Red-eyed Vireos (V. olivaceus), Magnolia Warblers (Dendroica magnolia), and American Redstarts (Setophaga ruticilla) were captured on Fort Morgan Peninsula in coastal Alabama during fall migration, 1990 to 1992. In Swainson's Thrushes, White-eyed Vireos, and American Redstarts, adults carried significantly higher fat loads than young birds, whereas no age-related differences in fat loads were evident in the other species. The likelihood of staying beyond the day of capture and the rate of change in body mass did not differ between age classes. One consequence of differences in fat load is reflected in flight range in relation to the Gulf of Mexico. On average, adult Swainson's Thrushes, White-eyed Vireos, American Redstarts, and both age classes of Magnolia Warblers carried sufficient energy stores to complete a trans-Gulf flight, whereas young Swainson's Thrushes, White- eyed Vireos, American Redstarts and both age classes of Gray Catbirds carried insufficient stores for such a flight, based on flight-performance simulations. Received 17 October 1996, accepted 13 May 1997. A SUCCESSFUL MIGRATION is dependent on how effectively migrants satisfy energy de- mands and meet en-route contingencies. Prob- lems that arise during passage undoubtedly are magnified for hatching-year birds on their first migration (e.g. Ralph 1978, Gauthreaux 1982, DeSante 1983, Moore 1984, Lindstr6m

Diversity and Abundance of Landbirds in a Northeastern Industrial Forest

We studied abundance and diversity of landbirds in all successional stages of habitat in an industrial forest landscape in northern Maine. Of 9 broad habitat categories, clearcuts and mature hardwood forests had the lowest avian diversity, but these 2 habitats each contained many individual species that showed their maximum abundance in those habitats. Greatest avian diversity was found in 2 young forest types (6-20 yr following harvesting). Young, even-aged regeneration had the greatest abundance of individual birds per unit area, whereas mature hardwood had the lowest total abundance. Ten of 37 Neotropical migrant species had their highest abundance in the 2 youngest stages of forest (clearcuts and regeneration) whereas 13 of 37 species had their highest abundance in the 3 mature forest types. Short-distance migrants had highest abundances in early-successional habitats, and mature softwood stands contained the greatest number of resident species. The presence or absence at a point count station of 10 late-successional species (of 37) and 13 early-successional species (of 35) was related to the amount of similar habitat within 1 km of the point count center (homogeneous landscapes). Few species showed highest abundances in heterogeneous landscapes. Spatially consolidating harvesting might enhance both early- and late-successional species simultaneously. Based on species-habitat associations identified in this study, estimated population trends in a 1,270 km 2 section of our study area did not correlate well with Breeding Bird Survey results for the state of Maine (1981-1992). This discrepancy probably, resulted because few Breeding Bird Survey routes occur in the industrial forest of northern Maine, and because of different trends in habitat abundances inside and outside of the study area. This result indicates there might be important subregional differences in landbird population trends. Current timber harvest rates likely will result in a decrease in abundance of mature forest in Maine's industrial forest. Many early-successional species, including some of conservation concern, will benefit from this trend. Abundances of mature-forest species probably will decrease. The challenge industry and conservation biologists face is how to construct dynamic landscapes that balance the age-class distribution of the forest and maintain mature forest species.

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

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

Parasitism at the Landscape Scale: Cowbirds Prefer Forests

Landscape‐scale examination of parasitism patterns of Brown‐headed Cowbirds (Molothrus ater) revealed heterogeneous parasitism rates across the mosaic of a forest and associated old‐field communities. In a two year study in Dutchess County, New York, we found a significantly higher parasitism rate in the forest‐interior community (n = 301 nests; 17 species) than on the species in the adjacent and nearby old field and edge (n = 328 nests; 15 species; 32.3% versus 6.5%; p < 0.0001). Cowbirds invaded a mature 1300‐ha forest stand even when their traditional host species were available in adjacent old‐field and edge habitats. The forest and old‐field study areas were located in a 38,000‐ha township with 55% forest cover and contained numerous agriculture, dairy, and horse farms that provided favorable habitat for cowbirds. Within‐forest examination of parasitism patterns revealed four aspects of cowbird parasitism that contrasted with patterns described in other regions: (1) parasitism was concentrated significantly more often on ground‐ and low‐nesting (nests ≤ 1 m) forest species than on medium‐ and high‐nesting species (nests> 1 m; 35.01% versus 29.93%; p = 0.0393); (2) parasitism was not significantly greater on Neotropical migrant species than on short‐distance migrants and residents; (3) the parasitism rate was not higher in nests close to edges; and (4) the parasitism level was low on certain forest species (such as Wood Thrush) that have experienced high parasitism levels in the Midwest. From a management perspective these data suggest that cowbirds exhibit regional differences in host and habitat use; the target host community of a particular cowbird population is unpredictable at the landscape scale; and a landscape scale should be used in designing cowbird studies to accurately assess local population dynamics.

Effect of Streamside Management Zone Width on Avifauna Communities

Abstract We analyzed 2 yr of bird population data in streamside management zones (SMZs) of three widths (50 ft, 100 ft, and 164 ft), adjoining ≤5 yr old pine plantations, and mature riparian areas in the Georgia Upper Coastal Plain. We evaluated relationships between SMZ width and bird abundance and density. Breeding and wintering avifaunal abundance were greater in SMZs than in the surrounding pine plantation. Breeding forest interior birds and forest interior neotropical migrant species were abundant in the mature forest control, but were essentially absent in SMZs and pine plantations. However, SMZs did provide habitat for an abundant avifauna community including several interior-edge and edge species not found in the pine plantations. Avian abundance was similar among SMZ widths, although densities were highest in the narrow SMZs. Forest managers should regard SMZs as important for maintaining avifauna abundance and diversity across pine dominated landscapes. South. J. Appl. For. 19(4):166-169.

Breeding Bird Response to Pine-Grassland Community Restoration for Red-Cockaded Woodpeckers

Plans exist to restore the fire-dependent pine (Pinus spp.)-grassland community in Ouachita National Forest and potentially throughout the southeastern United States to benefit the endangered red-cockaded woodpecker (Picoides borealis). Restoration and management techniques include wildlife stand improvement (WSI; thinning of midstory and codominant trees) and prescribed fire. We evaluated how habitat improvement for the red-cockaded woodpecker affected other breeding bird species. We compared avian species frequency of occurrence and abundance during 2 breeding seasons in untreated pine-hardwood stands with that in treated stands after WSI and in 3 growing seasons following WSI and prescribed fire. Total bird densities were highest (P = 0.037) in the second growing season following WSI and fire and lowest in the control, whereas species richness did not differ (P = 0.399) among treatments. Densities of ground/ shrub-foraging and shrub-nesting species increased (P = 0.002 and 0.002, respectively) the most following WSI and fire. Only ground-nesting species were more abundant (P < 0.001) in untreated stands than in treated stands. Restoration efforts may be beneficial to neotropical migrant species such as eastern woodpewee (Contopus virens) and prairie warbler (Dendroica discolor), in addition to declining species of regional interest such as red-cockaded woodpecker, Bachman's sparrow (Aimophila aestivalis), and northern bobwhite (Colinus virginianus) that depend upon pine-grassland habitats

Defining Forest Fragmentation by Corridor Width: The Influence of Narrow Forest‐Dividing Corridors on Forest‐Nesting Birds in Southern New Jersey

In studies of forest fragmentation, a fundamental inconsistency exists in the distance criterion used to define the discreteness of forest fragments. We examined three types of ubiquitous, narrow, forest‐dividing corridors for effects that influence the relative abundance and community composition of forest‐nesting birds. Fixed‐radius (100‐meter) point counts were conducted on 54 transects established along three width classes of corridors: unpaved roads (8 meters wide), paved roads (16 meters wide), and powerlines (23 meters wide). Transect locations were distributed equally among corridor edge, forest margin 100 meters from corridor edge, and forest interior 300 meters from corridor edge. Forest‐interior species of Neotropical migrants had significantly reduced relative abundances on edge transects along 16‐ and 23‐meter corridors, compared with 8‐meter corridors and with forest interior points along all three corridor‐width classes. At a landscape scale, the consequences of apparently small reductions in forest area by the presence of narrow forest‐dividing corridors may be cumulatively significant for abundances of forest‐interior species. Brown‐headed Cowbirds were more abundant than 20 of 21 forest‐interior Neotropical migrants. We found surprisingly high abundances of cowbirds associated with narrow forest‐dividing corridors, especially those with mowed grass. Corridor widths as narrow as 8 meters produce forest fragmentation effects in part by attracting cowbirds and nest predators to corridors and adjacent forest interiors. The most serious implication of this study is that narrow forest‐dividing corridors may function as ecological traps for forest‐interior Neotropical migrants. We suggest that these widespread corridors may be inconspicuous but important contributors to declines of forest‐interior nesting species in eastern North America.

Nest Predation Among Vegetation Layers and Habitat Types: Revising the Dogmas

Greater nest predation rates on ground-nesting birds than on off-ground-nesting birds have long been assumed and used as an explanation for patterns such as greater cryptic and monomorphic coloration of ground-nesting birds and for area sensitivity and population decline of many Neotropical migrant species. I use three independent data sets to show that this assumption is not true in forest habitats, where nest predation is instead least on ground-nesting birds. Larger clutch sizes and longer nestling periods of ground-nesting species in forest habitats are indirect evidence that ground-nesting species in forest habitats have suffered lower nest predation over evolutionary time. In contrast, ground-nesting birds seem to suffer greater predation than off-ground-nesting species in shrub and grassland habitats, but evaluation of predation is complicated by habitat disturbance in many studies. Nesting mortality in general appears to be greater in shrub and grassland habitats, and species in these habitats are showing some of the most consistent long-term population declines. Additional examination of nesting mortality of coexisting species in various ecological conditions is needed to uncover patterns that may influence evolution of life-history traits and population demographies.

Multiple Brooding and Productivity of a Neotropical Migrant, the Black-Throated Blue Warbler (Dendroica caerulescens), in an Unfragmented Temperate Forest

We studied a population of Black-throated Blue Warblers (Dendroica caerules- cens) in the White Mountains of central New Hampshire to determine the reproductive performance of this Neotropical migrant species in an unfragmented north-temperate forest. Over a four-year period (1986-1989), female Black-throated Blue Warblers laid an average of 6.6 eggs per season of which 5.2 hatched and 4.3 fledged. This high annual production of young was due to a combination of overall high nesting success (63%), low nest depredation rates (22%), no brood parasitism, and, most importantly, frequent multiple brooding (48%). Over the four years, 35% (range = 14-50%) of the individually-marked females in this pop- ulation successfully fledged two or more broods; one female fledged three successive broods in one season. Multiple brooding accounted for most of the annual variation in numbers of young fledged per female, and its frequency was related to annual differences in food abun- dance and, potentially, to time constraints imposed by the intensity and timing of nest depredation. Multiple brooding was significantly and positively related to age of the male parent, and possibly the female parent. Older males were also more likely to be polygynous, although such matings were infrequent (4-16% of mated males each year). Higher repro- ductive output by older individuals implies that demographic structure of Neotropical mi- grant populations may have important consequences for maintenance of population size. Recruitment of yearling male Black-throated Blue Warblers into the population was positively related to the number of young fledged per female in the preceeding summer. Although not statistically significant, this finding in combination with the above results and other published information suggests that reproductive success and the factors affecting it may be particularly important in influencing the abundance and population dynamics of this Neotropical migrant passerine, and may overide the impact of events occurring in winter or on migration. Received 6 May 1991, accepted 2 December 1991. RECENT evidence suggests that many North American bird populations are declining, es- pecially those that breed in temperate forests and winter in the Neotropics (Robbins et al.

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.