Research

I use an integrative approach to understand the origin and maintenance of biological (especially behavioural) diversity. My interests include animal migration, life history, and species interactions. While my research questions are rooted in evolutionary and behavioural ecology, I use a variety of physiological and analytical tools that permit a broader understanding of the  causes of variation in traits among and within species than is possible with a single approach. I work at multiple scales: from very broad geographic and phylogenetic scales (e.g., large comparative analyses, meta-analyses), down through landscape and community scales, to detailed studies of populations and individuals. Regardless of the scale, I apply the methods of strong inference, distinguishing among alternative hypotheses via tests of multiple predictions to answer questions of both basic and applied importance.
 
Major questions structuring my research program include the following: What are the causes of variation in migratory and dispersal strategy in diverse animal migration systems? What factors interact with and/or constrain basic life-history trade-offs? How are animal movements and life-history likely to change under anthropogenic climate change? How do frugivorous birds and fruiting plants influence each other's evolution, ecology, and behaviour?  Specific topics can be grouped under the following (related) themes.

Life-history: migration, elevation, climate change

In the manakin system, I found that migration decisions mediate the central trade-off between survival and reproduction, and this has led me to explore the ways that life-history strategies constrain and are constrained by migration and other factors generally. In my tropical system, collaborations with FSU doctoral candidate Megan Jones explore causes of variation in mating behaviour in a White-ruffed Manakins population temporally constrained by migration. Also, with long-term mark-recapture data (~5000 captures of ~3800 individuals) I'm testing comparative predictions regarding the life-history correlates of variation in migratory behaviour. 

Recent work involves investigating the causes of shifts in life history within species along elevational gradients. Interestingly, patterns of life-history variation with elevation seem to be the inverse of latitudinal patterns. In collaboration with Kathy Martin and Brett Sandercock, I conducted a global meta-analysis of life-history variation in  terrestrial vertebrates to both discover how general this pattern is, and test explanations for intra-specific patterns.

With Chris Guglielmo and Dave Winkler, I examined aspects of life-history variation in Tree Swallows. From a mechanistic point of view, we asked if mass loss during incubation could be viewed as a metric of investment in current reproduction, pitting this view against a strategic view of facultative mass loss functioning to reduce foraging costs, and a fitness-neutral view of mass loss as a by-product of gonad regression (Boyle et al. 2012). We integrated this with variation in migration timing, female quality, weather, and condition to explore the life-history consequences of arrival time in this species (Boyle et al., in revision).

In future work, I plan to use intra-population studies to explore how multiple behavioural and ecological factors interact to shape individual- and population-level differences in life history. These studies will be based at the Konza Prairie and other grassland sites in the American SW, focusing on one or more species declining grassland birds.

Altitudinal migration

Why do animals migrate altitudinally? What are the costs and benefits of migrating over short distances up and down mountains compared to remaining resident? What implications do these (often) facultative migratory species tell us about the less-tractable long-distance migrant systems and the evolution of migration?

My dissertation work focused on this question, and I tackled it using phylogenetically-explicit comparative methods (Boyle & Conway 2007; Boyle et al. 2011a), community-level experiments (Boyle 2008a), and detailed population studies (Boyle 2008b; Boyle 2010) to test proposed hypotheses for altitudinal migration in a community of tropical birds living on the Atlantic slope of Costa Rica. Contrary to the widely accepted view of tropical altitudinal migration, I concluded that simple variation in food availability can't explain these movements. Instead, I proposed that elevational differences in climate interact with foraging guild and individual condition in ways that influence the ability to survive storm events.

With Chris Guglielmo (UWO) and Ryan Norris (U Guelph), I tested this new Limited Foraging Opportunities hypothesis in three ways. First, I examined the physiological and behavioural responses of migrant and resident White-ruffed Manakins to the same storm events, finding that residents incur greater costs and likely risk mortality by remaining on the breeding grounds year round (Boyle et al. 2010b). Second, I examined whether putative survival costs of residency are balanced by reproductive gains in White-ruffed Manakins, finding that resident males are better able to maintain high social status or increase their status at leks, and better able to attract females to mate with them (Boyle et al. 2011b). [Watch videos of manakins displaying and mating!] Third, I tested a community-level prediction using multi-year datasets, finding that in years with more severe high-elevation storms, more altitudinal migrants reach low-elevation forests (Boyle 2011).

Ongoing and future work on altitudinal migrants includes both tropical and temperate systems, and includes bats in addition to birds. In collaboration with Liam McGuire, we recently reviewed the entire body of literature pertaining to altitudinal migration of bats. In this review, we document what is known of these poorly-described movements, articulate hypotheses that could explain them, and outline research endeavours needed to better understand and conserve them (McGuire & Boyle 2013).

Climate, migration, and dispersal

The migratory decisions of both White-ruffed Manakins and Tree Swallows are strongly influenced by climatic variation. This finding has major implications for the conservation of migratory animals in the face of changing climatic regimes. Major questions My research plans include projects to determine if proximate cues and ultimate causes reflect the same or different axis of environmental variation, how physiological tolerances constrain migration decisions, and the extent to which genetic and developmental processes shape responses to environmental cues.

In my tropical system, I'm combining the detailed behavioural and ecological dataset of manakin migration with forecasts of climate change in a modelling context. In such facultative systems, climatic effects would likely quickly lead to dramatic changes in the incidence and extent of altitudinal migration in tropical forests with important implications for seed dispersal interactions and the ability of plant distributions to shift as climates change.

Recently, 
I've become fascinated by the high rates of breeding dispersal in grassland birds, and the processes that drive variation in dispersal among and within species. In new work, I am using Grasshopper Sparrows to address questions not only pertaining to migration, but to dispersal, exploiting both broad geographic variation in this behavior, as well as individual-level variation within local populations.

Frugivores & fruit: avian seed dispersal interactions

While much of my work has been focused on animals (especially birds), I am also interested in how avian frugivores interact with the plants whose seeds they disperse. In work with one of my PhD supervisors, we examined the phenological diversity of two groups of bird-dispersed plants, relating that variation to biotic interactions with dispersers and pathogens (Boyle & Bronstein 2012). With REU student Heather Lumpkin, we examined variation in sugar concentrations and fruit removal in fruits in forests of different ages in an experimental context (Lumpkin & Boyle 2009). I've created and maintained a web-based image gallery and database of seeds of tropical plants, and continue ongoing work on the ways that plants manipulate their avian dispersers through various phenological and signalling strategies. 

Tropical conservation

Tropical forests are among the most fascinating and exciting places to study biology, but sadly, are severely threatened. As ecologists in the 21st century, we have an obligation to provide basic knowledge that can inform conservation initiatives in the systems we study. Research not falling naturally into any of the previous sections pertaining to applied questions in tropical avian community ecology includes a collaboration with Bryan Sigel examining species-specific patterns of population change of Costa Rican lowland birds (Boyle & Sigel 2015), a multi-authored synthesis of the causes of tropical understory bird declines (Visco et al 2015), and a study of potential cavity limitation in cavity-nesting birds at La Selva Biological Station in collaboration with two REU students (Boyle et al. 2008).

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