Raptor and large soaring bird migration across the Isthmus of Tehuantepec, Mexico: distribution, seasonality, and phenology

• Published in: Bird conservation international 2017-03, Vol.27 (1), p.111-126
• Main Authors: CABRERA-CRUZ, SERGIO A., CERVANTES-PASQUALLI, JUAN A., RUELAS INZUNZA, ERNESTO, HERNÁNDEZ-MORALES, TERESO, VILLEGAS-PATRACA, RAFAEL
• Format: Article
• Language: English
• Subjects: Annual variations; Aquatic birds; Autumn; Bird migration; Buteo platypterus; Buteo swainsoni; Cathartes aura; Falco sparverius; Mycteria americana; Pelecanus; Phenology; Seasonal variations; Spring
• ISSN: 0959-2709; 1474-0001
• DOI: 10.1017/S0959270916000423

We present the first study of the spatial and temporal dynamics of raptors and large soaring birds from the Isthmus of Tehuantepec, Mexico. Using systematic migration counts from multiple localities in the southern states of Oaxaca and Chiapas, as well as observations of their flight trajectories during eight consecutive years (2007–2014), we describe the magnitude of these movements, their geographic extent, and the phenology of the most abundant species in both spring and fall seasons. The most abundant species were Turkey Vulture Cathartes aura , Swainson’s Hawk Buteo swainsoni , Broad-winged Hawk Buteo platypterus , Wood Stork Mycteria americana , American White Pelican Pelecanus erythrothynchos , Franklin’s Gull Leucophaeus pipixcan , and American Kestrel Falco sparverius . In spring, the seasonal average magnitude of migration was over 28,000 birds, while in autumn the average was over one million. The aggregated seasonal phenologies recorded illustrate a variety of migration patterns. The inter-annual variation is lower in autumn than in spring. Migrating raptors and other soaring birds did not seem to use any topographical feature as a leading line for their movements in spring, while in autumn they did. We estimated the main axis of spring flights to run along a SE–NW vector, while autumn migration follows a WNW–ESE general trajectory. Our results place the isthmus as one of the five most important sites in the world for raptors and soaring migrants. Sustaining annual migration counts at these sites is of high importance to track substantial portions (> 90%) of the global population of Turkey Vulture and Swainson’s Hawk, as well as over 10% of the global population of Broad-winged Hawk. Autumn migration counts have the potential for long-term population monitoring. Presentamos resultados de la primer investigación sobre la dinámica espacial y temporal de aves rapaces y otras aves planeadoras grandes en el Istmo de Tehuantepec, México. Utilizando conteos sistemáticos de la migración en múltiples localidades en los estados sureños de Oaxaca y Chiapas, así como observaciones de sus trayectorias de vuelo durante ocho años consecutivos (2007–2014), describimos la magnitud de estos movimientos, su distribución geográfica y la fenología de las especies más abundantes registradas en primavera y otoño. Las especies más abundantes fueron Cathartes aura , Buteo swainsoni , Buteo platypterus , Mycteria americana , Pelecanus erythrothynchos , Leucophaeus pipix