Winter Distribution of Willow Flycatcher Subspecies

Abstract

Documenting how different regions across a species' breeding and nonbreeding range are linked via migratory movements is the first step in understanding how events in one region can influence events in others and is critical to identifying conservation threats throughout a migratory animal's annual cycle. We combined two studies that evaluated migratory connectivity in the Willow Flycatcher (Empidonax traillii), one using mitochondrial DNA sequences from 172 flycatchers sampled throughout their winter range, and another which examined morphological characteristics of 68 museum specimens collected in the winter range. Our results indicate that the four subspecies occupy distinct but overlapping regions of the winter range. Connectivity between specific breeding and winter grounds appears to be moderate to strong, with distributions that suggest migration patterns of both the chain and leap-frog types connecting the breeding and nonbreeding grounds. The Pacific lowlands of Costa Rica appear to be a key winter location for the endangered Southwestern Willow Flycatcher (E. t. extimus), although other countries in Central America may also be important for the subspecies. Distribución Invernal de las Subespecies de Empidonax trailliiResumen. Documentar cómo diferentes regiones que hacen parte de la distribución reproductiva y no reproductiva de una especie están ligadas mediante movimientos migratorios es el primer paso para entender cómo los eventos sucedidos en una región pueden afectar eventos en otras. Además, es critico para identificar amenazas de conservación a lo largo del ciclo anual de los animales migratorios. Combinamos dos estudios que evaluaron la conectividad migratoria en Empidonax traillii, uno que empleó secuencias de ADN mitocondrial de 172 individuos muestrados a lo largo de su distribución invernal y otro que examinó rasgos morfológicos de 68 especimenes de museo coleccionados en la distribución invernal. Nuestros resultados indican que las cuatro subespecies ocupan regiones diferentes, pero superpuestas, de la distribución invernal. La conectividad entre áreas de cria y de invernada especificas parece ser moderada a fuerte, y las distribuciones sugieren patrones de migración entre las áreas reproductivas y no reproductivas de tipo cadena y de tipo salto de rana. Las tierras bajas del Pacifico de Costa Rica parecen ser una localidad de invernada clave para la subespecie amenazada E. t. extimus, aunque otros paises centroamericanos también podrían ser importantes para esta subespecie.

Migration of Alder Flycatchers (Empidonax alnorum) and Willow Flycatchers (Empidonax traillii) through the Tuxtla Mountains, Veracruz, Mexico, and the Identification of Migrant Flycatchers in Collections

“…However, to explain our toe-pad sequence data would require an extremely high level of unidirectional hybridization. One would expect similar levels of sequence mismatch in samples of E. traillii, which Paxton et al (2011) did not report. Surveys of nuclear loci could determine the frequency of hybridization, but it is not likely high enough to explain why so many E. alnorum were identified as E. traillii.…”

“…Because field collectors had not searched consistently over the migration periods for Empidonax, we do not list extreme or median dates of passage. Paxton et al (2011) found E. traillii to be common migrants along the west coast of Mexico and Central America, suggesting that these two sibling species have different principal migration routes through the Isthmus of Tehuantepec in spring and fall.…”

“…Sedgwick (2000) wrote that Willow Flycatchers mostly migrated along the Pacific slope of Mexico and that eastern breeding birds likely migrated across the Gulf without making landfall in eastern Mexico. Paxton et al (2011) documented a large percentage of Willow Flycatchers along the Pacific coast.…”

Survivorship across the annual cycle of a migratory passerine, the willow flycatcher

“…In fact, a number of migratory species are experiencing higher population declines than resident birds (Sanderson et al 2006, Sauer andLink 2011), suggesting for some species that the cost of migration may be overwhelming potential benefits. However, to determine the ecological drivers that shape mortality throughout the annual cycle more studies ground study was within the core range of the southwestern subspecies, at two of the largest known breeding sites, and analysis of molecular genetics and morphology both indicate strong connectivity between the southwestern subspecies and the Pacific lowlands of Costa Rica where our wintering study was conducted (Paxton et al 2011). Despite the fact that the southwestern subspecies only constitutes approximately 0.2% of the estimated 3.2 million willow flycatchers (Rich et al 2004), and the species winters over a large area from central Mexico to South America, we documented direct linkage between our breeding and wintering study sites via 2 individuals banded in the southwestern subspecies' range that were recaptured in Costa Rica .…”

Current Topics in Avian Conservation Genetics with Special Reference to the Southwestern Willow Flycatcher

“…Zink [11] performed but did not show a phylogenetic analysis of 93 mtDNA Cytb haplotypes deposited in Genbank, but not analyzed by, E. Paxton. According to Paxton et al [27], these haplotypes came from breeding birds sampled in New York, Tennessee, Illinois, North Carolina, Virginia, North Dakota, South Dakota, Minnesota, and Maryland, Washington, Oregon, and California. The 93 Genbank haplotypes include those used by Paxton et al [26].…”