Our research aims to simultaneously address fundamental questions in both basic and applied ecology; different projects fulfill the dual objectives to different degrees. Below are projects we are actively working on, followed by topics that we’ve either moved on from or those currently on the back burner.
Too little rain is a serious problem; consequently, the morphology and behavior of animals inhabiting arid regions is often defined by coping mechanism for low precipitation. But does enough rain eventually become too much rain? Read our conceptual synthesis and review on Hygric Niches for Tropical Endotherms or check out this introductory explainer video.
Mounting evidence from the tropics suggests that at the wet end of the spectrum, higher-than-average rainfall may decrease fitness. Using seven years of capture data from a community of birds in the mountains of Costa Rica, we showed that a wetter-than-average year can be negatively associated with apparent survival, but the responses are species-specific… not all birds respond to rainfall variation in the same ways, even in the same communities. We are testing multiple predictions of this hypothesis in our tropical system working in multiple populations of Corapipo altera across precipitation gradients on both the Caribbean and Pacific slope. Elsie Shogren conducted population-intensive studies of social stability and individual condition at Volcán Tenorio National park (in some of the driest Caribbean-slope forests inhabited by White-ruffed Manakins) while past work has also taken place at El Copal, a private reserve in the Reventazón valley and Rara Avis where Alice did her PhD work. We have collected data from five additional populations elsewhere in Costa Rica, adding tests of genomic predictions to this study.
PhD student, Kristen Hobbs is taking up where Elsie left off, expanding both behavioral, physiological, and genetic aspects of this work. We have recently branched out into working with the well-studied manakin genus Manacus because aspects of its behavior make it more conducive to some of the experimental and behavioral predictions we are testing.
Our work with manakins has given rise to several collaborations with other members of the Manakin Genomics Research Collaboration Network.
In 2019, Boyle and Hefley (Statistics) were funded from NSF to study the population biology of grassland birds at the Konza Prairie. This effort integrates our individual-level physiological data, population demographic studies, movement data, and the long-term survey data to understand the relative importance of local factors vs. regional and continental factors in shaping the abundance of this declining guild, and disentangle the direct and indirect effects of precipitation on this systems.
We collected data between 2019 and 2022 on three core grassland-dependent songbirds for this project (Grasshopper Sparrow, Dickcissel, and Eastern Meadowlark), and combined it with related data ongoing since 2013. Graduate students Katy Silber and Emma Smith (Boyle lab) and Meenu Mohankumar (Hefley lab) have been involved in this project, along with post-doc Koley Freeman.
This project is drawing to a close, and all the core products are in or near to being in the publication pathway. Katy’s first dissertation chapter shows that apparent survival of Grasshopper Sparrows is primarily determined by the dispersal of individuals in and out of the population–something we strongly suspected, but are now able to demonstrate quantitatively. Furthermore, lagged rainfall on the breeding grounds is a major determinant of the variation in emigration, while we show that winter precipitation is the strongest of the variables we tested in affecting true survival. This paper will be out at Journal of Wildlife Management shortly!
Meanwhile, in the next few months, we anticipate submitting an Integrated Population Model (IPM) for Grasshopper Sparrows, a manuscript showing how all three species select vegetation structure and composition, how that variation in vegetation is also driven by lagged rainfall (providing a link to emigration results), and the huge impact of incipient woody encroachment on the availability of suitable habitat. We are in the process of revising or awaiting editorial decisions on papers showing short-term effects of storms on adult body composition, the micro-climatic consequences of nest orientation, causes of variation in reproductive investment in sparrows, among others.
We are extremely grateful for support from the National Science Foundation (DEB-1754491) that has made this research possible.
Work elsewhere has shown that songbird development is flexible in the face of predation risk. Another major selective pressure on nestlings and their parents is brood parasitism by Brown-headed Cowbirds. What are the direct and indirect effects of parasitism risk on nestling development and parental allocation?
This is the topic of Sarah Winnicki’s MS research. She studyied the effects of Brown-headed Cowbird (Molothrus ater) brood parasitism on the growth and development of nestlings of three grassland-obligate host species: Grasshopper Sparrow (Ammodramus savannarum), Dickcissel (Spiza americana), and Eastern Meadowlark (Sturnella magna). Using a combination of nest cameras, nestling measurements, and comparative analyses, we related skeletal growth, development (eyes opening, movement capacity), feather growth, and fat/muscle gain of the host nestlings to parasitism, predation, and parental feeding behavior. She also examined how being raised in the nests of different hosts affected cowbird nestling development. Sarah defended her thesis over the summer of 2019 and is preparing two manuscripts for publication. Stay tuned for the punchlines…!
In a current collaboration with Lainy Day, we used high-speed videography to answer these questions! We captured multiple displays of several males of varying ages and attractiveness to females. We think we know how the sounds are made now, and are on a quest to determine what makes a good display. This work builds upon interest in the White-ruffed Manakin breeding system which is characterized by remarkable variability in every aspect we’ve studied.
Grassland birds have a reputation for being less predictable than their relatives living in forested environments. What kinds of movements do they make? Why do they make them? Why do some populations and individuals exhibit high site fidelity while so many others are never seen again?
Mid-continental grasslands are some of the most variable environments on earth. Within and between years, conditions bird experience can vary tremendously because grasslands are maintained by multiple natural disturbance processes. Fire and grazing by large ungulates have shaped this system for millennia, and along with high variability in annual rainfall, vegetation structure and prey communities are patchy mosaics in space and time. It is hardly surprising then that grassland-dependent birds are incredibly mobile, combining annual migrations with flexible settlement decisions that lead to high rates of breeding dispersal within and among years. We focus on Grasshopper Sparrows for this work, a species in which some populations have breeding-season return rates of over 70%, and others of 0%. At the Konza Prairie Biological Station, 16-22% of breeding males return from year to year, allowing us to (a) determine the individual-level correlates of site fidelity, and (b) examine the drivers of inter-annual variation in this behavior.
In our very first year studying these sparrows, we discovered that a remarkable number of them dispersed to new territories mid-way through the breeding season! Emily Williams tackled this pattern during her MS research (read Emily’s MSc thesis!). In her research, she (a) comprehensively described the patterns and spatial scales of within-season dispersal (Williams & Boyle, 2018), (b) tested predation- and parasitism-risk avoidance hypotheses to explain which individuals remained site-faithful and which dispersed (Williams & Boyle, 2019), and (c) tested the role of food availability in shaping the post-dispersal settlement decisions.
Meanwhile, we have also been determining the spatial scales of between-year dispersal movements for Konza-breeding Grasshopper Sparrows using stable isotopes, and discovering some remarkable things about site fidelity, birds skipping years but then returning, and wintering locations. There are many more mysteries to figure out in this system!
In 2017, Alice assumed responsibility for collecting long-term breeding bird transect data. In 2016, she also initiated a new monitoring effort of the winter bird community and spring passage migrants. Unlike the breeding bird data, this new effort uses mark-recapture on permanent plots to assess demographic parameters, and provides an excellent platform for student training, physiological studies, and outreach.
Past (or dormant) Projects
If you are a Grasshopper Sparrow, deciding where to defend a territory in a huge area of beautiful, perfect grassland habitat, why would you decide to stick close to other territorial males? This seemingly simple question still doesn’t have an answer, despite it being one of the first questions we tackled when initiating research at the Konza Prairie. We now have a good idea of what DOESN’T drive patterns of aggregation, however!
In 2013, Steffanie Munguia tackled this as her REU project, experimentally testing the prediction that aggregation functions to minimize the costs of nest predation and brood parasitism. But we saw no difference between birds in aggregated vs. isolated territories. The next summer, Sarah Winnicki followed up in her REU project where Steffanie left off. This project eventually involved testing predictions of every hypothesis we could come up with, yet the data are consistent with none of the alternatives. Read this paper (Winnicki_etal_2020_Ecology) and learn why and how social and genetic mechanisms cannot explain aggregation! Our best guess is that population declines and/or the massive changes the Great Plains have experienced have affected the fitness payoffs of this behavior.
We know that animals select habitat at multiple spatial scales, but typically, we try to understand why we find animals in some but not all locations by only measuring attributes of territories and home ranges. How important is the landscape context? and what features do some of the most threatened birds of tallgrass prairies respond to?
This project was the focus on Mark Herse’s MS thesis. We found that even in the Flint Hills of Eastern Kansas where the last large tracts of prairie remain, Henslow’s Sparrows are extremely rare, move around within breeding seasons, and select habitat at far larger spatial scales than that of their territories. Rather surprisingly, when they arrive in spring, they settle disproportionately frequently in Conservation Reserve Program (CRP) fields, a habitat typically regarded as low-quality compared to native prairie. This is likely due to the prevalence of prescribed spring fires to increase forage quality for cattle. As the season progresses, they move to sites surrounded by large expanses of native prairie (Herse et al. 2017, Landscape Ecology).
Capitalizing on the astounding number of point-count surveys conducted for the Henslow’s Sparrow project, we were able to use abundance data from the much more common Grasshopper Sparrow to test long-standing ideas about the relative importance of habitat area, and the configuration of habitat patches. Mark devised a novel way to distinguish these analytically, and found that indeed, the amount of ‘core’ habitat (grassland area at least 60 m from edges) was a far better predictor of abundance in this species (Herse et al. 2018, JAppliedEcol).
Finally, using data from four declining species, we determined the landscape contexts in which fragmentation effects are strongest for grassland birds. Because most of these species are apparently not at all dispersal-limited, fragmentation effects may be quite different in prairies as compared to forested ecosystems. The final paper in this series was published in Landscape Ecology in 2020.
Alice’s dissertation research and portions of her post-doc tested multiple alternative hypotheses to explain why some, but not all, tropical birds (especially those dependent upon fruits) regularly engage in predictable, seasonal migrations from higher-elevation breeding sites to lower-elevations during the non-breeding season.
This body of research has resulted in numerous publications and one of the most comprehensive set of studies of the ultimate factors shaping migration behavior in wild birds. Much of that work was conducted along a protected gradient of wet forest from La Selva Biological Station in the lowlands, up through Braulio Carrillo National Park and several adjoining private reserves. Alice conducted a great deal of the breeding-season research at Rara Avis reserve, an incredibly beautiful and incredibly wet forest at about 750 m elevation.
A few of the key take-home messages include the following: (a) simple variation in food availability does not drive downhill migration in this system; (b) reductions in foraging time due to heavy rain likely DO drive downhill movements, and which species or individuals migrate depends on traits that influence energetic needs; (c) while migration may increase the chances of surviving during the non-breeding season, it has carry-over effects to the breeding season with migrants having lower chances of reproducing.
There have been many associated papers and side-projects related to this topic, including work that takes the plants’ point-of-view, studying phenological strategies and fruit removal rates in this seed dispersal mutualism.
Tree Swallows nest in boxes. That simple fact makes them ideal subjects for studies examining reproductive behavior and the consequences of individual-level traits on fitness.
Alice collaborated with Dave Winkler at Cornell as a post-doc to address three questions: (1) What are the consequences of individual variation in timing of arrival to breeding sites following spring migration? (2) Why do females lose weight during reproduction? and (3) How, exactly, do chicks die during bouts of bad weather? Two of these projects are, unfortunately, still in the file cabinet (guilt!). The answer to the mass loss question is fascinating… females actively modulate body composition throughout the nesting period in ways that balance risk of starvation if bad weather hits during incubation, and the costs of aerial foraging once chicks hatch.
Birds of high elevations face unique challenges that has shaped their evolutionary ecology, and will influence their persistence in future decades.
Our research on this topic grew out of interest on the selective forces that lead to altitudinal migration behavior in tropical and temperate birds, as well as in bats. In collaboration with Kathy Martin at UBC, we’ve found that high-elevation living has predictable consequences for life histories, with high elevation populations having lower fecundity, but not consistently higher survival than their lower-elevation counterparts. Additionally, around the globe, high-elevation populations have shorter breeding seasons which, in temperate areas in intuitive, but at low latitudes, is intriguing.
High elevation bird communities in British Columbia are also surprisingly diverse, and alpine areas are under-appreciated and potentially critical stop-over areas during fall migration for high-latitude breeding birds of multiple guilds.
Wet tropical forests harbor the highest biodiversity of any ecosystem on earth. They are also some of the most threatened. Which species are most at risk, and why are they declining?
We have used long-term citizen-science data collected at La Selva Biological Station in the Caribbean lowlands of Costa Rica to quantify rates of population change, and associate both declines and successes with species-level traits to test hypotheses regarding why some species are doing well and others are disappearing, even from intact forest. Some of these results are surprising, revealing that not only are understory insectivores at risk (a group commonly identified as being particularly susceptible to fragmentation), but small-bodied birds of all dietary guilds are declining. These results suggest that physiological and energetic constraints along with climatic conditions may be more important than commonly appreciated in driving declines.
Alice has also collaborated with multiple other researchers working in the same region to synthesize evidence in support of alternative explanations for population change in this region.