Ecology and conservation of neotropical-nearctic migratory birds and mixed-species flocks in the Andes
Gabriel J. Colorado, PhD
Advisor: Amanda D. Rodewald
The tropical Andes are widely recognized as one of the world´s great centers of biodiversity. High levels of both species richness and endemism coupled with one of the greatest rates of deforestation among tropical forests have made the Andes a major focal point of international conservation concern. In the face of current and projected rates of deforestation and habitat degradation of Andean forests, persistent large gaps in our understanding of ecological responses to anthropogenic disturbances limit our ability to effectively conserve biodiversity in the region. My dissertation focused on ecology and conservation of two poorly known components of Andean forest bird communities, mixed-species flocks and overwintering Neotropical-Nearctic migratory birds. Specifically, I (1) examined assembly patterns of mixed-species avian flocks, (2) evaluated the sensitivity of mixed-species flocks and Neotropical-Nearctic migratory birds to deforestation and structural changes in habitat, and (3) identified potential physiological consequences of both using shade coffee and flocking to wintering Neotropical-Neartic migratory birds.
To achieve this, I evaluated richness and abundance patterns of the community of wintering Neotropical-Nearctic migratory birds and resident mixed-species flocks across a broad geographical area (approximately 200,000 km2) of Northern and Central Andes, ranging from northwestern Venezuela in the Mérida Cordillera (± 10 o S, 70 o W) to northern Peru's Cóndor Cordillera (± 5 o S, 78 o W), and including the Eastern, Central and Western Colombian Cordilleras. From October-March 2007-2010, I surveyed bird communities and measured habitat characteristics within 84 study sites representing a range of altitudes, from tropical lowlands at 400 m to low-montane tropical forest at 2,600 m. I examined patterns of non-randomness in Andean mixed-species flocks using three assembly models: (a) co-occurrence patterns (b) guild proportionality and (c) constant body-size ratios applied to data on 221 species of resident and Neotropical migrant birds participating in 311 mixed-species flocks in Venezuela, Colombia, Ecuador, and Peru. By applying null model analysis to regional presence-absence matrices of flocking species, I found evidence of assembly patterns for mixed-species flocks in the Andes suggesting that competitive interactions at both interspecific and inter-guild levels played important roles in structuring flock social systems in the Andes. Flock structure seemed less related to morphological (i.e., body size ratios) than to behavioral attributes, such as foraging behavior, as evidenced by the fact that foraging guilds (i.e., insectivore, omnivore, nectivore, frugivore) remained relatively proportional across flocks.
To examine sensitivity of montane forest birds to environmental heterogeneity, I used an information-theoretic approach to study the association of landscape-scale (i.e. percentage of forest cover in 1-km2 pixels) and micro-habitat level (i.e. habitat complexity) on richness and abundance patterns of Neotropical migrants and mixed-species flocks. I conducted systematic avian surveys within five broadly-defined habitat types (shade coffee, pastures with isolated trees, successional, secondary forest and mature forest) at 84 sites distributed from Colombia to Peru based on a stratified-random design. Distance-based line transect surveys (n = 3 per site) were used to quantify patterns of species richness and abundance of Neotropical migrants and mixed-species flocks. I found that patterns in flock and migrant attributes were well explained by environmental heterogeneity at multiple spatial scales, though habitat-specific associations depended upon landscape context. The strength of the association between regional forest cover and Neotropical migrants was habitat-dependent, and forest cover was most strongly positively related to flocks within shade coffee. Increasing levels of habitat complexity had mixed relationships with flock attributes. Whereas complexity was positively associated with abundance and diversity in successional and silvopastoral habitats, the opposite pattern was true in shade coffee and secondary forests. Overall, this research showed that (a) Neotropical migrants and mixed-species flocks were influenced by environmental factors operating across multiple spatial scales, (b) the importance of particular environmental attributes changed with landscape context and habitat type, and (c) intensively managed habitats with overstory trees contributed to avian conservation by supporting both Neotropical migratory birds and mixed-species flocks.
In my final chapter, I explored the suitability of shaded monocultures for overwintering Neotropical-Nearctic migratory birds by examining body condition changes. Because Neotropical-Nearctic migrants frequently join mixed-species flocks during the nonbreeding season, I also evaluated the extent to which body condition changed with flocking behavior. I mist-netted 8 species of Neotropical-Nearctic migrants in shade coffee farms in the Colombian Andes in October-April 2008-2009 and 2009-2010 and identified individuals as either solitary foragers or flock members. Several common migratory species, including Cerulean Warbler, Blackburnian Warbler and Tennessee Warbler, improved their body condition over the course of each day and throughout the nonbreeding season. However, neither body condition nor seasonal change in body condition differed between flocking and solitary individuals for most of the migratory species evaluated. Cerulean and Blackburnian Warblers showed stronger improvements in condition when foraging solitary than in flocks. Because birds improved condition in shade coffee, results also provided additional evidence that agroforestry systems can provide suitable overwinter habitat to several common Neotropical migrants, including species of conservation concern such as Cerulean Warbler.
Overall, my dissertation demonstrates that mixed-species flocks and Neotropical migratory birds are widespread and common components of montane forest avifauna throughout the tropical Andes. Patterns of community assembly suggest that flocks are not random associations of species, but rather are structured at least partly in response to competitive pressures. However, the demonstrated sensitivity of flocks and migratory birds to landscape and local habitat changes suggests that continued patterns and rates of land cover change might disrupt the unique social system of mixed-species flocks as well as suitability of Andean forests for overwintering migratory birds. Fortunately, my research provides evidence that certain management systems, such as shade coffee and silvopasture, have the potential to support abundant and diverse migrants and flocks. Regional conservation efforts should further explore how agroforestry systems can be used to meet both ecological and social needs in human-dominated landscapes of the Andes.