Summary1. The relative contribution of deterministic and stochastic processes in the assembly of biotic communities is a central issue of controversy in community ecology. However, several studies have shown patterns of species segregation that are consistent with the hypothesis that deterministic factors such as competition and niche-partitioning structure species assemblages in animal communities. 2. Community assembly provides a theoretical framework for understanding these processes, but it has been seldom applied to social aggregations within communities. 3. In this research, we assessed patterns of non-randomness in Andean mixed-species flocks using three assembly models: (i) co-occurrence patterns; (ii) guild proportionality; and (iii) constant body-size ratios using data from 221 species of resident and Neotropical migrant birds participating in 311 mixed-species flocks at 13 regions distributed in Venezuela, Colombia, Ecuador and Peru. 4. Significant assembly patterns for mixed-species flocks based on co-occurrence models and guild proportionality models suggest that competitive interactions play an important role in structuring this social system in the Andes. Distribution of species among foraging guilds (i.e. insectivore, frugivore, omnivore, nectivore) was generally similar among flocks, though with some regional variation. 5. In contrast, we found little evidence that structuring of mixed-species flocks in the Andes was mediated by body size. Rather, we found greater than expected variance of body-size ratios within flocks, indicating that birds did not segregate morphologically. 6. Overall, our findings suggest that deterministic factors associated to competitive interactions are important contributors to mixed-species flock assemblages across the Andes.
Aim In birds, patterns of migratory connectivity are influenced by landscape structure experienced throughout the annual cycle. Assessing how landscape patterns drive the movements and distributions of migratory species is important for understanding the evolution of migration and determining conservation priorities. We determined migratory connectivity in the Canada Warbler (Cardellina canadensis), a declining Neotropical migratory bird, and evaluated the degree to which the Andean mountains influence winter population distribution in this montane forest specialist. Location Seven study regions across the Western, Central, and Eastern Andes of Colombia. Methods We inferred breeding and natal origins of Canada Warblers overwintering in Colombia by analysing stable hydrogen isotope values (δ2H) in feathers and assigning wintering populations to breeding origins using an established feather δ2H isoscape for North America. Results We found strong evidence for parallel migration between breeding and wintering sites, giving rise to moderately strong migratory connectivity between breeding and wintering regions. The model best explaining population distribution in the Colombian Andes implied that populations were most similar within basins or valleys. Such segregation is notable, given that basins are separated by less than 300 km on the wintering grounds, while breeding populations may be separated by more than 3000 km from east to west. Main conclusions Our results suggest that the high spine of each Andean cordillera acts to shape the winter distribution of individuals from different breeding populations. To our knowledge, this is the first demonstration that the topography of the Andean mountains can act to shape the winter distribution of populations of a migratory bird. The strength of migratory connectivity found in Canada Warblers may inhibit evolutionary responses to environmental change, and the more rapid population declines recorded in some eastern breeding populations may be related to habitat loss and local environmental conditions in the Eastern Andes, where individuals from these populations were more likely to overwinter.
South of Mexico, little is known about the fall migration patterns of most Neotropical migrants. I studied the migration of Empidonax flycatchers using mist-net surveys in northwestern Colombia from late September to mid-October in 4 yr (2003)(2004)(2005) 2008). Empidonax species were identified using linear measurements and color patterns. About 62% of captured individuals were reliably identified to species, with 86% identified as Willow Flycatchers (Empidonax traillii) and 14% as Alder Flycatchers (E. alnorum). No Acadian Flycatchers (E. virescens) were identified. Most birds captured were adults (84.9%) and, due to overlap in measurements, I was only able to determine the sex of 16.3% of the birds. Most Empidonax flycatchers migrated through northwestern Colombia during September and October, with individuals migrating through my study area over a period of at least 1 mo. Willow Flycatchers tended to migrate earlier than Alder Flycatchers, a pattern consistent with the fall movements of these two species at other locations. No captured flycatchers were molting either remiges or rectrices, and most (89%) had either no or slight traces of subcutaneous fat. No Empidonax flycatchers were recaptured, suggesting that stopover duration at my study site was brief. My results show that many Empidonax flycatchers can be identified as Willow and Alder flycatchers during the nonbreeding period, and such identification will enhance our knowledge of their geographical distribution and improve our understanding of possible patterns of segregation on their wintering grounds. RESUMEN. Migración otoñal de Empidonax en el noroeste de ColombiaSe sabe muy poco sobre los patrones de migración de la mayoría de las aves que se mueven al sur de México. Estudie la migración de papamoscas (Empidonax) utilizando redes de niebla para hacer censos en el noroeste de Colombia. El trabajo se llevó a cabo desde tarde en septiembre hasta mediados de octubre durante los años [2003][2004][2005] 2008. Se utilizaron medidas lineales y patrones de coloración para identificar las especies de Empidonax. Un 62% de las aves capturadas se pudieron identificar adecuadamente hasta el nivel de especie. De estos, un 86% resultaron ser individuos de E. traillii, y 14% de E. alnorum. El común E. virescens, no fue identificado. La mayoría de las aves fueron adultos (84.9%) y debido a un solapamiento en las medidas solo pude determinar el sexo del 16.3% de los individuos. La mayoría de los. Empidonax migran a través del noroeste de Colombia durante septiembre y octubre, con individuos migrando sobre elárea de estudio durante al menos un mes. E. traillii tiende a migrar más temprano que E. alnorum, lo que es un patrón consistente con los movimientos otoñales de esta especie a través de otras localidades. Ninguna de las aves capturadas estuvo mudando y la mayoría (89%) no tenía grasa subcutánea. Ninguna ave fue recapturada, sugiriendo que el tiempo de estadía en el lugar de estudio es breve. Mis resultados muestran que muchos Empidonax pueden ser identificados co...
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