- Anthropogenic climate change is threatening biodiversity as I currently understand it. There is now a large body of work highlighting species responses, globally, to this threat. Importantly, responses at the species level emerge from responses at lower levels of biological organization (individuals and populations) across a species’ geographic range. For widespread species in particular, perturbations due to climate warming or other environmental changes in one part of a species’ range may not have the same effect as in another part of that species’ range. In fact, different populations within a species may show opposing trends in response to the same environmental stressor. Species level responses also have consequences for higher levels of biological organization (communities and ecosystems), depending on the rate and duration of the response. Integrating across biological, spatial, and temporal scales is therefore critical to a deeper understanding of biotic responses to anthropogenic climate change.
My dissertation takes an integrative, multi-scale approach to better understand the role climate has played in shaping the distribution and dynamics of an iconic falcon in North America, the American kestrel. This species, a widespread generalist that is common across North America, is experiencing one of the most dramatic declines among all North American raptor species. Determining the degree to which anthropogenic climate change has played a role in this decline, or not, is an urgent area of inquiry. My second chapter addresses gaps in knowledge by presenting an analysis of whether the interactive effects of climate, the structure and composition of the landscape, and primary productivity of the breeding grounds have impacted nest success in a population of American kestrels in the high desert region of Central Oregon over a 7-year period and examining how the spatial and temporal grain of the data impacts results. I find that primary productivity has greater explanatory power than either climate or landscape variables. However, seasonal variables (nest initiation date, nest age and day-of-year) emerge as the most important predictors of variation in nest success regardless of primary production. These models also suggest that longer-term annual averages of environmental variables may not be as informative with respect to nest success as variables that capture within-season variation in the environment. In addition, I find no evidence of advancing phenology as has been found in other populations in similar habitats and at similar latitudes as this population.
Chapters 3 and 4 of my dissertation I turn to the relationship between body size and climate in the American kestrel. Evidence from bird species worldwide has shown reductions in mean body sizes over the past 50-100 years consistent with the notion of a response to climate warming. Body size has been shown to scale allometrically with many fundamental physiological, ecological, and evolutionary processes, raising concerns that asynchronous shifts in body size among species could lead to unexpected shifts in community and ecosystem dynamics. Yet previous studies of body size in birds vary with respect to how body size is measured. Measuring size is not a trivial matter, and the methods used can impact interpretation. In Chapter 3, I quantify size of kestrel sternums, a structural size element, via 3D geometric morphometrics to assess which standard non-structural (linear) morphological measurements that are typically applied to living birds or historical museum specimens (study skins) best capture the true structural size of a specimen. I find that single, commonly applied proxy measures of size in birds, such as mass, wing cord, or tarsus length, are poor representatives of structural size in the American kestrel, and that a combination of measurements from the wing, tail, leg and bill may be more appropriate for studies relating size to environmental parameters in this species. In Chapter 4, I then use a combination of measurements from a data set of museum study skins to evaluate the degree to which American kestrels across North and Central America conform to Bergmann’s Rule and Allen’s rule, common ecogeographic rules relating body size and appendage length, respectively, to latitude. I use a spatial regression framework to examine patterns in size and appendage length across 60° of latitude and over a 112-year period. I then integrate climate models with satellite-derived data on primary productivity and a dataset of competitor richness and body sizes compiled from the literature to test common hypotheses about the mechanisms underlying these patterns. I find no evidence for a change in the average body size of kestrels through time when examined at the continental scale, although, at smaller spatial scales, many regions do show significant trends in morphology through time. I do find support for Bergmann’s rule in American kestrels across the Northern hemisphere. In addition, the mechanism predicting size variation in American kestrels are found to be congruent across different spatial scales. I also find that bill size in American kestrels aligns with predictions from Allen’s rule.
Taken as a whole, the studies presented in this dissertation add to the body of research currently underway to address the long-term declines in an iconic species of the American landscape. The research integrates insights across scales, from a single population in Central Oregon experiencing variability in climate and microhabitats over a sub-decadal period to multiple populations and subspecies arrayed across the entire North American continent that have been shaped by anthropogenic climate change over more than a century. In doing so, this work also addresses logistical concerns in how size is measured in birds and addresses a long-standing debate over the multiple potential drivers of Bergmann’s Rule, a fundamental ecogeographical rule found in every ecology textbook.