Jill L. Deppe

Fat, weather, and date affect migratory songbirds' departure decisions, routes, and time it takes to cross the Gulf of Mexico.

Significance Bird migration has captivated the attention of scientists and lay people for centuries, but many unanswered questions remain about how birds negotiate large geographic features during migration. We tracked songbirds across the Gulf of Mexico to investigate the factors associated with birds’ departure decisions, arrival at the Yucatan Peninsula (YP), and crossing times. Our findings suggest that a bird’s fat reserves and low humidity, indicative of favorable synoptic weather patterns, shape departure decisions. Fat, date, and wind conditions predict birds’ detection in the YP. This study highlights the complex decision-making process involved in crossing the Gulf and its effects on migratory routes and speeds. A better understanding of the factors influencing migration across these features will inform conservation of migratory animals. Approximately two thirds of migratory songbirds in eastern North America negotiate the Gulf of Mexico (GOM), where inclement weather coupled with no refueling or resting opportunities can be lethal. However, decisions made when navigating such features and their consequences remain largely unknown due to technological limitations of tracking small animals over large areas. We used automated radio telemetry to track three songbird species (Red-eyed Vireo, Swainson’s Thrush, Wood Thrush) from coastal Alabama to the northern Yucatan Peninsula (YP) during fall migration. Detecting songbirds after crossing ∼1,000 km of open water allowed us to examine intrinsic (age, wing length, fat) and extrinsic (weather, date) variables shaping departure decisions, arrival at the YP, and crossing times. Large fat reserves and low humidity, indicative of beneficial synoptic weather patterns, favored southward departure across the Gulf. Individuals detected in the YP departed with large fat reserves and later in the fall with profitable winds, and flight durations (mean = 22.4 h) were positively related to wind profit. Age was not related to departure behavior, arrival, or travel time. However, vireos negotiated the GOM differently than thrushes, including different departure decisions, lower probability of detection in the YP, and longer crossing times. Defense of winter territories by thrushes but not vireos and species-specific foraging habits may explain the divergent migratory behaviors. Fat reserves appear extremely important to departure decisions and arrival in the YP. As habitat along the GOM is degraded, birds may be limited in their ability to acquire fat to cross the Gulf.

TEMPORAL PATTERNS IN FALL MIGRANT COMMUNITIES IN YUCATAN, MEXICO

Abstract 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.

How do en route events around the Gulf of Mexico influence migratory landbird populations

ABSTRACT Habitats around the Gulf of Mexico (GOM) provide critical resources for Nearctic–Neotropical migratory landbirds, the majority of which travel across or around the GOM every spring and fall as they migrate between temperate breeding grounds in North America and tropical wintering grounds in the Caribbean and Central and South America. At the same time, ecosystems in the GOM are changing rapidly, with unknown consequences for migratory landbird populations, many of which are experiencing population declines. In general, the extent to which events encountered en route limit migratory bird populations is not well understood. At the same time, information from weather surveillance radar, stable isotopes, tracking, eBird, and genetic datasets is increasingly available to address many of the unanswered questions about bird populations that migrate through stopover and airspace habitats in the GOM. We review the state of the science and identify key research needs to understand the impacts of en route events around the GOM region on populations of intercontinental landbird migrants that breed in North America, including: (1) distribution, timing, and habitat associations; (2) habitat characteristics and quality; (3) migratory connectivity; and (4) threats to and current conservation status of airspace and stopover habitats. Finally, we also call for the development of unified and comprehensive long-term monitoring guidelines and international partnerships to advance our understanding of the role of habitats around the GOM in supporting migratory landbird populations moving between temperate breeding grounds and wintering grounds in Mexico, Central and South America, and the Caribbean.

Stopover ecology of American Golden-Plovers (Pluvialis dominica) in Midwestern agricultural fields

ABSTRACT Stopover locations represent critical habitat in the life cycle of migratory birds and the alteration of this habitat can profoundly influence a population. American Golden-Plovers (Pluvialis dominica) migrate though the Midwestern United States each spring, where most natural habitat has been converted to row crop agriculture. We investigated the stopover ecology of the golden-plover in the agricultural matrix of east-central Illinois and west-central Indiana between 2008 and 2012. We found that golden-plovers remained in the region for ∼45 days and individuals spent on average 24 days in the area before departing to the northwest. During a period of peak migration, golden-plovers preferred fields with standing water and, to a lesser extent, soybean fields. Over the 45-day stopover duration, golden-plovers moved extensively (shown by a dynamic occupancy model and area used estimation), with some evidence for tilled fields becoming unoccupied at greater rates than untilled fields. The tendency to use fields with standing water and the movement of individuals from tilled fields suggests that food accessibility, rather than food abundance, is likely a critical factor associated with the prolonged stay, movement, and field type selection of golden-plovers. Food accessibility is important to the golden-plover because they undergo molt into breeding plumage in the region and must refuel for the next leg of their migration. The Midwest is a key stopover location for American Golden-Plovers and promoting foraging conditions by manipulating the drainage of agricultural fields, via the temporary blockage of drain tiles, should be a management focus.

Swainson's Thrushes do not show strong wind selectivity prior to crossing the Gulf of Mexico

During long-distance fall migrations, nocturnally migrating Swainson’s Thrushes often stop on the northern Gulf of Mexico coast before flying across the Gulf. To minimize energetic costs, trans-Gulf migrants should stop over when they encounter crosswinds or headwinds, and depart with supportive tailwinds. However, time constrained migrants should be less selective, balancing costs of headwinds with benefits of continuing their migrations. To test the hypotheses that birds select supportive winds and that selectivity is mediated by seasonal time constraints, we examined whether local winds affected Swainson’s Thrushes’ arrival and departure at Ft. Morgan, Alabama, USA at annual, seasonal, and nightly time scales. Additionally, migrants could benefit from forecasting future wind conditions, crossing on nights when winds are consistently supportive across the Gulf, thereby avoiding the potentially lethal consequences of depleting their energetic reserves over water. To test whether birds forecast, we developed a movement model, calculated to what extent departure winds were predictive of future Gulf winds, and tested whether birds responded to predictability. Swainson’s Thrushes were only slightly selective and did not appear to forecast. By following the simple rule of avoiding only the strongest headwinds at departure, Swainson’s Thrushes could survive the 1500 km flight between Alabama and Veracruz, Mexico.

Space-based ornithology: studying bird migration and environmental change in North America

Natural fluctuations in the availability of critical stopover sites coupled with anthropogenic destruction of wetlands, land-use change, and anticipated losses due to climate change present migratory birds with a formidable challenge. Space based technology in concert with bird migration modeling and geographical information analysis yields new opportunities to shed light on the distribution and movement of organisms on the planet and their sensitivity to human disturbances and environmental changes. At the NASA Goddard Space Flight Center, we are creating ecological forecasting tools for science and application users to address the consequences of loss of wetlands, flooding, drought or other natural disasters such as hurricanes on avian biodiversity and bird migration. We use an individual-based bird biophysical migration model, driven by remotely sensed land surface data, climate and hydrologic data, and biological field observations to study migratory bird responses to environmental change in North America. Simulation allows us to study bird migration across multiple scales and can be linked to mechanistic processes describing the time and energy budget states of migrating birds. We illustrate our approach by simulating the spring migration of pectoral sandpipers from the Gulf of Mexico to Alaska. Mean stopover length and trajectory patterns are consistent with field observations.

Estimating apparent survival of songbirds crossing the Gulf of Mexico during autumn migration

Many migratory bird species are declining, and the migratory period may limit populations because of the risk in traversing large geographical features during passage. Using automated radio-telemetry, we tracked 139 Swainsons thrushes (Catharus ustulatus) departing coastal Alabama, USA and crossing the Gulf of Mexico to arrive in the Yucatan Peninsula, Mexico during autumn. We estimated apparent survival and examined how extrinsic (weather variables and day of year) and intrinsic (fat load, sex and age) factors influenced survival using a mark-recapture approach. We also examined how favourability of winds for crossing the Gulf varied over the past 25 years. Fat load, day of year and wind profit were important factors in predicting which individuals survived crossing the Gulf. Survival estimates varied with wind profit and fat, but generally, fat birds departing on days with favourable wind profits had an apparent survival probability of greater than 0.90, while lean individuals with no or negative wind profits had less than 0.33. The proportion of favourable nights varied within and among years, but has increased over the last 25 years. While conservation strategies cannot improve extrinsic factors, they can provide opportunities for birds to refuel before crossing large geographical features through protecting and creating high-quality stopover sites.

Track Annotation: Determining the Environmental Context of Movement Through the Air

Volant organisms are adapted to atmospheric patterns and processes. Understanding the lives of animals that inhabit this aerial environment requires a detailed investigation of both the animal’s behavior and its environmental context—i.e., the environment that it encounters at a range of spatial and temporal scales. For aerofauna, it has been relatively difficult to observe the environment they encounter while they move. Large international efforts using satellite and weather model reanalysis now provide some of the environmental data on atmospheric environments throughout the globe. Track annotation—the approach of merging the environmental data with the movement track measured via telemetry—can be conducted automatically using online tools such as Movebank-Env-DATA or RNCEP. New parameterization approaches can use environmentally annotated tracks to approximate specific atmospheric conditions, such as uplift and tail wind, which are not typically observed at the exact locations of the movement, but are critical to movement. Reducing the complexity of movement to single-dimensional characteristic (such as flight speed, elevation, etc.) and defining the temporal scope of the movement phenomenon in the focus of the analysis (seasonal, daily, minutely, etc.) makes it possible to construct empirical models that explain the movement characteristic as driven by the environmental conditions during flight, despite the highly dynamic, complex, and scale-dependent structures of both the flight path and atmospheric variables. This chapter will provide several examples for such empirical movement models from different species of birds and using several resources for atmospheric data.

An experimental evaluation of the performance of acoustic recording systems for estimating avian species richness and abundance

Comparisons between field observers and acoustic recording systems have shown great promise for sampling birds using acoustics methods. Comparisons provide information about the performance of recording systems and field observers but do not provide a robust validation of their true sampling performance—i.e., precision and accuracy relative to known population size and richness. We used a 35-speaker bird song simulation system to experimentally test the accuracy and precision of two stereo (Telinga and SS1) and one quadraphonic recording system (SRS) for estimating species richness, abundance, and total abundance (across all species) of vocalizing birds. We simulated 25 bird communities under natural field conditions by placing speakers in a wooded area at 4–119 m from the center of the survey at differing heights and orientations. We assigned recordings randomly to one of eight skilled observers. We found a significant difference among microphones in their ability to accurately estimate richness (p = 0.0...