Migratory Landbirds Research Articles

SCALE-DEPENDENT HABITAT USE BY FALL MIGRATORY BIRDS: VEGETATION STRUCTURE, FLORISTICS, AND GEOGRAPHY

Animal habitat selection is a central focus of ecology and conservation biology. Understanding habitat associations in migratory animals is particularly complicated because individuals have variable habitat requirements during the annual cycle, across their geographic range, along migratory routes, and at multiple spatial scales. We studied habitat associations of 16 fall Nearctic–Neotropical migratory land birds at two spatial scales at a stopover site along the northern Yucatan coast to examine scale-dependent habitat use, identify proximate cues shaping birds' distributions, and evaluate similarities in habitat use between our tropical stopover site and temperate sites. We addressed scale-dependent habitat associations in two ways, by (1) quantifying species' distributions among and within broad vegetation types and (2) comparing migrants' associations with architectural gradients between the two spatial scales. We also evaluated the relative importance of vegetation architecture and floristics in explaining migrants' distributions within broad coastal vegetation types. Bird species were nonrandomly distributed among broad- and fine-scale vegetation types, and patterns of habitat use varied between the two scales. Moreover, birds had different preferences for vegetation architecture at the two scales, which may reflect trade-offs between competing demands and/or reduced variation in vegetation at the small scale in our study. These findings illustrate the manner in which spatial scale and range of vegetation variation influence perceptions of animal–habitat associations. Within broad coastal vegetation types birds refined their distributions in relation to architectural and floristic attributes, which provided them with redundant and/or complementary information regarding the distribution of suitable habitat. We suggest that the relative importance of architecture and floristics is likely scale-dependent. Habitat use at our site was similar to that observed at temperate stopover sites for almost all species, indicating that habitat affinities are maintained along the migratory route for these eastern populations despite latitudinal changes in environmental factors. We highlight examples of similar patterns observed in other migratory and nonmigratory animals to illustrate the generality of these patterns beyond en route land birds and our tropical site. We also indicate where we expect to see differences and outline areas of research that merit greater attention in order to advance our understanding of animal habitat selection.

NEOTROPICAL AUSTRAL MIGRANT LANDBIRDS: POPULATION TRENDS AND HABITAT USE IN THE CENTRAL MONTE DESERT, ARGENTINA

Abstract We studied density changes of two groups of Neotropical austral migrant landbirds—the South American temperate-tropical (SATT) and cold-temperate (SACT) migratory systems—in the main habitat types of the central Monte desert of Argentina (open Prosopis flexuosa woodland and Larrea cuneifolia shrubland) over 10 years. Five species, all tyrant flycatchers (Tyrannidae) made up SATT, whereas only two of the seven species of SACT were tyrannids. Densities of both SATT and SACT were higher in open woodland than in shrubland. SATT density did not differ among years, but SACT density did, having lower values in 1994 in both habitats. In subsequent years, SACT densities increased but did not reach values similar to those previous to 1994. The decline in 1994 coincided with a two-year drought period that began in 1993, but lower density in the following years did not appear to be related to climatic conditions in the study area, suggesting a low capacity of SACT species to recover population abundance afte...

Conservation Strategies and Vegetation Characterization in the Columbia Bottomlands, an Under-recognized Southern Floodplain Forest Formation

Abstract The Columbia Bottomlands, a Southern floodplain forest formation on the upper Texas coast, historically covered over 283,000 ha but has since been reduced to 25% of its former extent. The importance of this regional ecosystem as critical stopover and staging habitat for Nearctic-Neotropical migratory landbirds gave rise to the Columbia Bottomlands Conservation Plan, an active land acquisition and conservation program administered by the U.S. Fish and Wildlife Service and its governmental and non-governmental partners. The Plan seeks to establish an integrated network of protected tracts as representative examples of the regional landscape, and thus conserve ecosystem integrity, function, heterogeneity, and biological diversity. We describe the Conservation Plan and its progress to date, and we summarize data on the plant composition of a typical preserved tract. Vegetation sampling at the Dance Bayou Unit, a mature forest remnant, revealed a mosaic of species composition across habitats varying i...

MIGRATION OF THE SKYLARK ALAUDA ARVENSIS ALONG THE SOUTHERN FRENCH ATLANTIC COAST

Mist-nettings of Eurasian skylarks ( Alauda arvensis ) were conducted throughout the day near the coastline in south-western France during the fall migratory period. The main objective of this work was to document some aspects of body condition (mass and fat stores) in skylarks. Between 1997 and 2004, a total of 8903 birds were captured in October-November. The median capture dates of fall migrations for females and males were October 24 and October 26, respectively. About 58.4% and 34.0% of birds were females and males respectively, the remaining 7.6% corresponding to unsexed birds. Skylarks arrived to our study site with low fat stores, 42% of the individuals showing no visible fat deposits. Rates of mass changes were found to be positively related to the time of day: individuals captured during the night before 07:00 were 3-4% heavier than individuals captured during daylight between 07:00 and 20:00. On the daytime, the average rate of body mass gain was about 5% for both sexes, suggesting that migratory birds were not only able to gain energy for their regular metabolism, but they were also able to store fat for migration. According to allometric equations to estimate the energetic cost of flight, flight ranges for the majority of birds were estimated to be less than 180 - 240 km for females and lower than 160 - 220 for males. As migratory stopover sites are used for refuelling by many migrant land birds, our data suggest that most of the skylarks might exhibit a « feed-by-day, fly-by night » strategy in our region, highlighting the need for preserving stopover sites, such as those encountered in south-western France, for successful migration. Considering that many migrant birds such as skylarks feed heavily on seeds, changes in agricultural practices might affect survival capabilities of skylarks during their fall migration and in winter.

Stopover Habitat use by Spring Migrant Landbirds: The Roles of Habitat Structure, Leaf Development, and Food Availability

AbstractStopover habitat quality may influence the abundance of migrant landbirds, yet little is known about how spatial and temporal changes in stopover habitat quality influence the use of habitats by migrant landbirds. From late April to late May, 1997–1999, we surveyed migrant landbirds within five habitats (early successional shrub-sapling-stage forest, midsuccessional pole-stage forest, mature forest interior, mature forest-agricultural edge, and mature suburban forest) in central Pennsylvania. To assess relative quality of stopover habitats, we measured species abundance, species richness, and foraging behavior of Yellow-rumped Warblers (Dendroica coronata). We measured vegetation structure and phenology to examine proximate cues of potential importance in habitat selection. Of nine transient (nonbreeding) species analyzed, seven-Least Flycatcher (Empidonax minimus), Nashville Warbler (Vermivora ruficapilla), Northern Parula (Parula americana), Magnolia Warbler (D. magnolia), Black-throated Blue Warbler (D. caerulescens), Yellow-rumped Warbler, and Blackpoll Warbler (D. striata)-were most abundant in mature forests, and especially edge-dominated mature forests (forest-agricultural edge and suburban forest). Habitats used by mature-forest-breeding transients changed between years, but edge-dominated forest types were used most frequently in all three years. Although canopy leaf development at mature forest sites showed little relationship to site use by mature-forest-breeding transients within weekly intervals, year-to-year differences in leaf phenology were consistent with year-to-year changes in habitat use by migrant landbirds. Foraging behavior of Yellow-rumped Warblers suggested higher food availability and quality in forest-agricultural edges, compared with suburban forest and forest interior. Mature-forest-breeding transients were positively associated with sites that had large trees (>38 cm diameter at breast height) and denser understory vegetation (stems 0–2.5 cm in diameter). Extensive use of mature edge-dominated forests by migrating forest birds during spring suggests that these habitats may be important for the conservation of migrant landbirds.Uso de Hábitat durante las Escalas Migratorias por Aves Migratorias Terrestres: el Papel de la Estructura del Hábitat, el Desarrollo de las Hojas y la Disponibilidad de Alimento

Abundance, Density, and Diversity of Neotropical Migrants at the Lula Lake Land Trust, GA

I used point counts and mist netting at the Lula Lake Land Trust, GA, a reclaimed mining area, to assess its suitability for migrating and breeding long-distance migratory landbirds. The results suggest that the property is suitable for migrants. The proportion of migrants is similar to other areas of the southeastern United States, as well as other reclaimed forestland. In addition, migrant populations were either stable or increased over the three years of monitoring, and 4% of migrants banded returned in subsequent years. Management of the Trust properties should continue to emphasize maintaining large areas of mature forest, and Trust managers should take measures to ensure that increased human recreational activity on the property does not degrade habitat quality.

STOPOVER HABITAT USE BY SPRING MIGRANT LANDBIRDS: THE ROLES OF HABITAT STRUCTURE, LEAF DEVELOPMENT, AND FOOD AVAILABILITY

Abstract Stopover habitat quality may influence the abundance of migrant landbirds, yet little is known about how spatial and temporal changes in stopover habitat quality influence the use of habitats by migrant landbirds. From late April to late May, 1997–1999, we surveyed migrant landbirds within five habitats (early successional shrub-sapling-stage forest, midsuccessional pole-stage forest, mature forest interior, mature forest-agricultural edge, and mature suburban forest) in central Pennsylvania. To assess relative quality of stopover habitats, we measured species abundance, species richness, and foraging behavior of Yellow-rumped Warblers (Dendroica coronata). We measured vegetation structure and phenology to examine proximate cues of potential importance in habitat selection. Of nine transient (nonbreeding) species analyzed, seven-Least Flycatcher (Empidonax minimus), Nashville Warbler (Vermivora ruficapilla), Northern Parula (Parula americana), Magnolia Warbler (D. magnolia), Black-throated Blue W...

Stopover Habitat Along the Shoreline of Northern Lake Huron, Michigan: Emergent Aquatic Insects as a Food Resource for Spring Migrating Landbirds

AbstractRecent work in Michigan's eastern Upper Peninsula suggests that terrestrial areas bordering northern Lake Huron provide important stopover habitat for spring migrating landbirds, principally because of the presence of emergent aquatic midges (Diptera: Chironomidae). Migrants were concentrated in lakeshore habitats abundant with midges during spring migration. American Redstarts (Setophaga ruticilla) and Black-throated Green Warblers (Dendroica virens) foraged and used habitat differently, depending on their distance from the lakeshore. Here, we describe results of an integrative study in which we sampled resources, quantified American Redstart foraging behavior, and estimated mass change in American Redstarts and five other common migrant landbird species to evaluate the importance of adult midges as an early season resource for spring migrants. Resource sampling and American Redstart foraging behavior suggested that more food was available in shoreline habitats than inland during spring migration. Furthermore, migrants gained mass in shoreline habitat during stopover, which supports the argument that nearshore areas provide important stopover habitat for spring migrants. Finally, resource sampling, mass change estimates, and American Redstart foraging behavior suggested that midges and spiders (Araneae: Arachnidae) provided an important early season resource for migrating landbirds. Evidence suggests that midges were responsible for elevated spider abundance at the shoreline and that birds foraged on both midges and spiders. Midges appear to play an important role in providing high-quality stopover habitat for landbirds migrating through Michigan's eastern Upper Peninsula.Hábitat de las Paradas Migratorias a lo Largo de la Costa Norte del Lago Huron, Michigan: Insectos Acuáticos Emergentes como Recurso Alimenticio para las Aves Terrestres Migratorias Durante la Primavera

STOPOVER HABITAT ALONG THE SHORELINE OF NORTHERN LAKE HURON, MICHIGAN: EMERGENT AQUATIC INSECTS AS A FOOD RESOURCE FOR SPRING MIGRATING LANDBIRDS

Abstract Recent work in Michigan's eastern Upper Peninsula suggests that terrestrial areas bordering northern Lake Huron provide important stopover habitat for spring migrating landbirds, principally because of the presence of emergent aquatic midges (Diptera: Chironomidae). Migrants were concentrated in lakeshore habitats abundant with midges during spring migration. American Redstarts (Setophaga ruticilla) and Black-throated Green Warblers (Dendroica virens) foraged and used habitat differently, depending on their distance from the lakeshore. Here, we describe results of an integrative study in which we sampled resources, quantified American Redstart foraging behavior, and estimated mass change in American Redstarts and five other common migrant landbird species to evaluate the importance of adult midges as an early season resource for spring migrants. Resource sampling and American Redstart foraging behavior suggested that more food was available in shoreline habitats than inland during spring migratio...

Distribution‐wide effects of climate on population densities of a declining migratory landbird

1. Increases in global temperatures have created concern about effects of climatic variability on populations, and climate has been shown to affect population dynamics in an increasing number of species. Testing for effects of climate on population densities across a species' distribution allows for elucidation of effects of climate that would not be apparent at smaller spatial scales. 2. Using autoregressive population models, we tested for effects of the North Atlantic Oscillation (NAO) and the El Niño Southern Oscillation (ENSO) on annual population densities of a North American migratory landbird, the yellow-billed cuckoo Coccyzus americanus, across the species' breeding distribution over a 37-year period (1966-2002). 3. Our results indicate that both the NAO and ENSO have affected population densities of C. americanus across much of the species' breeding range, with the strongest effects of climate in regions in which these climate systems have the strongest effects on local temperatures. Analyses also indicate that the strength of the effect of local temperatures on C. americanus populations was predictive of long-term population decline, with populations that were more negatively affected by warm temperatures experiencing steeper declines. 4. Results of this study highlight the importance of distribution-wide analyses of climatic effects and demonstrate that increases in global temperatures have the potential to lead to additional population declines.

An Annotated Checklist of the Birds of Camiguin Island, Philippines

Fifty-five species of resident breeding and two species of migratory land birds have been documented on Camiguin Island, including one bittern, one eagle, one junglefowl, two rails, eight doves and pigeons, one parrot, three cuckoos, one owl, three swiftlets, one dollarbird, two kingfishers, one bee-eater, one hornbill, one pitta, one triller, two bulbuls, one crow, four thrushes, three warblers, six flycatchers, one pipit, one wood swallow, two starlings, three sunbirds, two flowerpeckers, two white-eyes, and two munias. At least seven species reported here are first records for Camiguin. Ten species are widespread Philippine endemics, two are near-endemics, and one (Loriculus camiguinensis, described in this volume) is endemic to Camiguin. Additionally, four endemic subspecies are recognized from Camiguin, Ixos everetti catarmanensis, Hypothymis azurea catarmanensis, Diceum trigonostigma isidroi, and Zosterops nigrorum catarmanensis. While this list is not comprehensive, the presence of 57 species demonstrates that many species were able to cross a narrow but permanent sea channel, and the presence of four endemic subspecies and one endemic species indicates that some genetic differentiation has resulted.

INSULAR AND MIGRANT SPECIES, LONGEVITY RECORDS, AND NEW SPECIES RECORDS ON GUANA ISLAND, BRITISH VIRGIN ISLANDS

Abstract We conducted mist netting each October from 1994 to 2004 on Guana Island, British Virgin Islands, and recorded bird sightings to develop a more complete inventory of the island's resident and migrant species. During our study, we recorded four new species for the British Virgin Islands: Magnolia Warbler (Dendroica magnolia; 1996), Golden-winged Warbler (Vermivora chrysoptera; 1997), Swainson's Thrush (Catharus ustulatus; 2000), and Red-necked Phalarope (Phalaropus lobatus; 2004). Blackpoll Warbler (Dendroica striata) was the most frequently captured Neotropical migrant landbird, despite only being first detected in the region in 1989. Captures and detections of other Neotropical migrant landbirds suggest that many species may be more common in the region than previously believed, or, as speculated by other researchers, that migrant routes may be shifting eastward due to habitat degradation on western Caribbean islands. We also used recapture data to establish longevity records of resident species...

Molt Strategies and Age Differences in Migration Timing Among Autumn Landbird Migrants in Southwestern Idaho

AbstractIntraspecific patterns of autumn migration timing are not well known, particularly in the western United States. Here, we (1) describe autumn migration timing and age ratios of landbird migrants in southwestern Idaho, (2) examine differences in timing among age and sex classes, and (3) demonstrate how prebasic molt strategies affect migration timing differences between age classes. As a group, Neotropical migrants were most common from late July through early September, whereas temperate migrants were most common from mid-September into early October. Proportion of hatch-year birds was 74.5% for all migrants combined and ranged from 33.3% to 100% for individual species. Timing differences between sex classes were detected in only a few species and no general patterns emerged. In 22 of 31 Neotropical and temperate migrants examined, there were significant differences in timing between adults and hatch-year birds. In species in which adults begin fall migration before replacing flight feathers, adults migrated earlier than hatch-year birds. Conversely, in species in which adults molt flight feathers on or near the breeding grounds before departing on fall migration, hatch-year birds migrated earlier than adults in all but one case. Therefore, it appears that molt strategy is a powerful determinant of intraspecific migration timing differences and, to our knowledge, this is the first study to document this pattern among migrant passerines of North America.Estrategias de Muda y Diferencias en el Momento de Migración Otoñal en Migrantes Terrestres en el Suroeste de Idaho

COMPOSITION, ABUNDANCE, AND TIMING OF POST-BREEDING MIGRANT LANDBIRDS AT YAKUTAT, ALASKA

The eastern Gulf of Alaska coastline is suspected of providing an important pathway for birds migrating to and from Alaska. Because no intensive study of landbird migration has been conducted in this region, we used mist nets to study the post-breeding migration of landbirds along the coast from 1994 through 1999. Over six post-breeding periods, we netted for a total of 316 days (23,538 net-hr) and captured 13,490 individuals of 46 species (57.3 birds/100 net-hr). Six species constituted >65% of all captures (ordered by abundance): Orange-crowned Warbler (Vermivora celata), Hermit Thrush (Catharus guttatus), Lincoln's Sparrow (Melospiza lincolnii), Ruby-crowned Kinglet (Regulus calendula), Fox Sparrow (Passerella iliaca), and Yellow Warbler (Dendroica petechia). Most birds captured (71%) were Nearctic-Neotropical migrants, and percentages of hatching-year (HY) birds varied from 51 to 90% among common species. Daily capture rates of all species were highest between mid-August and mid-September. Migration of HY individuals preceded that of after-hatching-year (AHY) birds in 70% of the Nearctic-Neotropical species. Masses of HY Nearctic-Neotropical migrants were significantly less than those of AHY individuals. High capture rates and consistent annual use indicate that the eastern Gulf of Alaska coast is an important pathway for many small landbird migrants, particularly Nearctic-Neotropical species, departing breeding grounds in southern Alaska.

Nocturnal activity and energetic condition of spring landbird migrants at Braddock Bay, Lake Ontario

Abstract We investigated the relationship between migratory restlessness and stored energy reserves in two species of landbird migrants at Braddock Bay, Lake Ontario during spring 1999 and spring 2000. There was no significant difference in the amount of nighttime locomotor activity between lean and fat Swainson's Thrushes (Catharus ustulatus) or White-throated Sparrows (Zonotrichia albicollis) held overnight in activity cages. We found no significant relationships between nocturnal activity level and energetic condition (mass/wing chord) or date of capture. In addition, there were no significant differences in nocturnal activity levels between age groups in Swainson's Thrushes. These results suggest that the migratory restlessness of spring migrants stopping at this site is not strongly influenced by their energetic condition upon arrival, regardless of age or date of passage. Birds nearing the end of their spring migratory route that are very close to their breeding grounds may show decreased levels of ...

Diversity of Neotropical migratory landbird species assemblages in forest fragments and man-made vegetation in Los Tuxtlas, Mexico

We investigated the presence of Neotropical migratory landbirds in a 90-km2 landscape in the region of Los Tuxtlas, Veracruz, Mexico. Using the fixed-radius count point procedure, migratory landbirds were surveyed in 21 forest fragments and in four replicates of shaded (coffee, cacao and mixed) and unshaded (citrus and allspice) plantations, live fences, non-arboreal crops (corn and jalapeno chili pepper) and pastures. The surveys resulted in the count of 4732 birds representing 72 species. While forest fragments accounted for 65% of the total species count, 73% of the birds were counted in the arboreal man-made habitats. Pastures contributed to 10% of the species and to 1% of the individuals counted. Live fences were particularly rich in individuals, accounting for 28% of the birds counted. Rarefaction analysis showed that forest fragments were the sites richest in species, followed by shaded and unshaded plantations and by live fences. Pastures were the habitats poorest in species, followed by non-arboreal crops. Species richness of Neotropical migratory landbirds was associated to vertical and horizontal diversity of vegetation in the habitats investigated. Shaded and unshaded plantations as well as live fences were more similar to forest fragments in species assemblages than non-arboreal crops and pastures. We discuss the conservation value of arboreal agricultural habitat and of live fences in conjunction with forest fragments as temporary habitats for Neotropical migratory landbirds that stop over or winter in Los Tuxtlas.

Stopover Ecology of Autumn Landbird Migrants in the Boise Foothills of Southwestern Idaho

Abstract The topography of western North America provides a complex landscape for landbird migrants, and stopover patterns in this region are poorly understood. We examined seven years of stopover data (1997–2003) from a montane area in southwestern Idaho to determine whether this area provides suitable stopover habitat. We compared the proportion of birds recaptured, stopover duration, and changes in energetic condition within and among species and between two mist-netting sites located in different habitats. The proportion of birds recaptured ranged from zero to over 20%, and fewer than 5% of individuals were recaptured in most species. Mean minimum stopover durations from recapture data ranged from 1 to 10 days; most species averaged less than 6 days. Stopover duration estimates from open-population models were comparable but generally greater than estimates from recapture data. As found in stopover studies from other regions, stopover metrics varied within and among species in Idaho. However, most migrants in this study exhibited an ability to gain mass, evidenced both by recapture data and by regression of energetic condition against time since sunrise. These data imply that montane habitats in Idaho are suitable stopover sites. It follows that these habitats might serve an important role for many landbird migrants during the period of late summer molt and autumn migration, a time when many lowland areas of the West, including some riparian systems, are especially arid. We suggest that including montane nonriparian habitats in future stopover ecology studies will allow for a more complete understanding of migrant habitat needs in the West. Ecología de Aves Migrantes de Otoño Durante Períodos de Escala en el Piedemonte de Boise, Suroeste de Idaho Resumen. La topografía del oeste de Norte América representa un paisaje complejo para las aves terrestres migratorias, y los patrones de escala migratoria en esta región son poco conocidos. En este estudio examinamos datos de escalas migratorias colectados a través de siete años (1997–2003) en un área montana del suroeste de Idaho para determinar si esta área provee hábitats de escala adecuados. Comparamos la proporción de aves recapturadas, la duración del período de escala y los cambios en la condición energética por especie y entre especies y entre dos sitios de captura con redes de niebla ubicados en hábitats diferentes. La proporción de aves recapturadas varió entre cero y más del 20%, y en la mayoría de las especies menos del 5% de los individuos fueron recapturados. La duración mínima promedio de los períodos de escala estimada a partir de datos de recapturas estuvo entre uno y 10 días, y la estancia promedio de la mayoría de las especies fue menor de seis días. Los estimados de la duración de los períodos de escala calculados con modelos de población abierta fueron comparables (pero generalmente mayores) a los estimados basados en datos de recaptura. Tal como se ha encontrado en estudios sobre escalas migratorias realizados en otras regiones, en Idaho las mediciones tomadas durante el período de escala variaron dentro de cada especie y entre especies. Sin embargo, la mayoría de las especies migrantes que estudiamos tuvieron la habilidad de incrementar su peso, lo que se evidenció por medio de los datos de recaptura y de análisis de regresión entre la condición energética y el tiempo transcurrido desde la salida del sol. Estos datos implican que los ambientes montanos de Idaho son lugares de escala migratoria adecuados. Por lo tanto, estos hábitats pueden ser importantes para muchas aves terrestres durante el período de la muda del final del verano y la migración de otoño, un momento durante el cual muchas áreas de tierras bajas del oeste, incluyendo sistemas riparios, son especialmente secas. Sugerimos que incluir ambientes montanos no riparios en estudios futuros de la ecología de los períodos de escala migratoria permitirá tener un entendimiento más completo de los requerimientos de hábitat de las aves migratorias en el oeste.

STOPOVER ECOLOGY OF AUTUMN LANDBIRD MIGRANTS IN THE BOISE FOOTHILLS OF SOUTHWESTERN IDAHO

The topography of western North America provides a complex landscape for landbird migrants, and stopover patterns in this region are poorly understood. We examined seven years of stopover data (1997-2003) from a montane area in southwestern Idaho to determine whether this area provides suitable stopover habitat. We compared the proportion of birds recaptured, stopover duration, and changes in energetic condition within and among species and between two mist-netting sites located in different habitats. The proportion of birds recaptured ranged from zero to over 20%, and fewer than 5% of individuals were recaptured in most species. Mean minimum stopover durations from recapture data ranged from 1 to 10 days; most species averaged less than 6 days. Stopover duration estimates from open-population models were comparable but generally greater than estimates from recapture data. As found in stopover studies from other regions, stopover metrics varied within and among species in Idaho. However, most migrants in this study exhibited an ability to gain mass, evidenced both by recapture data and by regression of energetic condition against time since sunrise. These data imply that montane habitats in Idaho are suitable stopover sites. It follows that these habitats might serve an important role for many landbird migrants during the period of late summer molt and autumn migration, a time when many lowland areas of the West, including some riparian systems, are especially arid. We suggest that including montane nonriparian habitats in future stopover ecology studies will allow for a more complete understanding of migrant habitat needs in the West.

TEMPORAL PATTERNS IN FALL MIGRANT COMMUNITIES IN YUCATAN, MEXICO

We quantified temporal turnover in the composition of fall migrant landbird communities along the northern coast of the Yucatan Peninsula using Detrended Correspondence Analysis. The presence of winter residents of many migrant species at the site prevented turnover from being complete. However, early and late season transient communities shared few, if any, species in common. Point-count surveys showed greater compositional change than net surveys that included winter residents. The rate of species turnover was generally slow until the middle of the season, when it reached a maximum, and decreased again toward the end of the season as species composition began to converge on that characteristic of the winter community. Within-season patterns of species turnover were similar between net- and point-count surveys and were consistent across years. Species that winter farther south arrived at the site significantly earlier than those wintering at more northern latitudes; however, there was no association with breeding latitude. Additionally, insectivores arrived significantly earlier than omnivores and granivores. The amount of turnover among foliage-gleaning insectivores was less than that for all species in the community, which is inconsistent with the hypothesis that competition during migration has played a role in shaping temporal patterns in species co-occurrence and turnover. For species that co-occur in time during migration, spatial segregation at various scales, as well as differences in foraging behavior and diet, may act to reduce competition at stopover sites. Patrones Temporales en Comunidades Migratorias de Otoño en Yucatán, México Resumen. Cuantificamos el recambio temporal en la composición de comunidades de aves terrestres migratorias de otoño a lo largo de la costa norte de la península de Yucatán usando análisis de correspondencia con efecto de arco corregido. La presencia de residentes invernales de muchas especies migratorias en el sitio evitó el recambio completo de especies. Sin embargo, las comunidades transitorias a principios y fines de la estación compartieron pocas o ninguna especie. Los muestreos de conteos por punto mostraron cambios en la composición mayores que los muestreos de redes de niebla que incluyeron residentes invernales. La tasa de recambio de especies fue generalmente lenta hasta mediados de la estación, cuando alcanzó un máximo, y disminuyó de nuevo hacia finales de la estación a medida que la composición de especies comenzó a parecerse a la de la comunidad invernal. Los patrones de recambio de especies de cada estación fueron similares entre los muestreos de redes de niebla y los conteos por punto, y fueron consistentes entre años. Las especies que invernaron más al sur llegaron al sitio significativamente más temprano que aquellas que invernaron en latitudes más septentrionales; sin embargo, no hubo una asociación con la latitud en la que criaron las especies. Además, los insectívoros llegaron significativamente más temprano que los omnívoros y los granívoros. La cantidad de recambio entre insectí voros que toman sus presas en el follaje fue menor que el recambio de todas las especies de la comunidad consideradas juntas, lo cual no es consistente con la hipótesis de que la competencia durante la migración ha jugado un papel en modelar los patrones temporales en la presencia conjunta y el recambio de especies. Para las especies cuya presencia coincide en el tiempo durante la migración, la segregación espacial a varias escalas, como también las diferencias en el comportamiento de forrajeo y en la dieta, pueden operar para reducir la competencia en los sitios de escala.

MOLT STRATEGIES AND AGE DIFFERENCES IN MIGRATION TIMING AMONG AUTUMN LANDBIRD MIGRANTS IN SOUTHWESTERN IDAHO

Abstract Intraspecific patterns of autumn migration timing are not well known, particularly in the western United States. Here, we (1) describe autumn migration timing and age ratios of landbird migrants in southwestern Idaho, (2) examine differences in timing among age and sex classes, and (3) demonstrate how prebasic molt strategies affect migration timing differences between age classes. As a group, Neotropical migrants were most common from late July through early September, whereas temperate migrants were most common from mid-September into early October. Proportion of hatch-year birds was 74.5% for all migrants combined and ranged from 33.3% to 100% for individual species. Timing differences between sex classes were detected in only a few species and no general patterns emerged. In 22 of 31 Neotropical and temperate migrants examined, there were significant differences in timing between adults and hatch-year birds. In species in which adults begin fall migration before replacing flight feathers, adu...