A Matter of Timing: Climate Change Impacts on Bird Migration
Tiny Microbes Have Big Effects

Research Interests and Current Projects

My research focuses on the interplay between the physiology and ecology of wild vertebrates, with an emphasis on wild species of conservation interest. Using a combination of field and laboratory approaches, I have studied a diversity of vertebrates including carnivorous salamanders, herbivorous waterfowl and grouse, as well as insectivorous, frugivorous, and granivorous passerine birds. Two common themes are evident in my research: (1) I regularly use comparisons between species to reveal broadscale patterns in nutritional and physiological ecology; and (2) I study a given organism from many perspectives and levels of organization. For example, I combine work on membrane transport of nutrients, digestive physiology, nutritional requirements, feeding behavior, constraints (e.g., morphological, developmental, physiological) on prey and predator behavior, habitat and diet selection, and plant responses to grazing. To accomplish such an integrative research program requires successful collaboration. Below I provide examples of some current research projects that demonstrate these common themes.

Avian Herbivores and the Importance of Body Size
Geese provide an interesting model for studying how avian herbivores circumvent the problem of combining the high energy demands of flight with the ecological limitations associated with eating leaves which are low in energy and nutrients and typically are high in fiber. Much theory focuses on the implications of body size for the physiological ecology of mammalian herbivores, but it has yet to be adequately applied to avian herbivores. Geese are an excellent group in which to study such issues because species such as the Canada goose vary 7-fold in body size.

My previous work on avian herbivores has shown that highly selective feeding is one way geese escape some of the constraints associated with being a small avian herbivore. However, geese also show remarkable abilities to modulate digestive features (e.g., hydrolytic activity of digestive enzymes, nutrient uptake rates across gut membrane, size of the absorptive regions of the gut) in response to changes in diet quality and, as a result, they are able to maintain relatively high digestive efficiency on a wide range of diets.   In addition, my past work has shown that geese are able to utilize dietary fiber by using microbial fermentation in their hindgut. I have been invited to present these results at a special symposium on avian gastrointestinal physiology (proceedings published in Journal of Experimental Zoology) and at an invited symposia on digestion in avian ecology at the International Ornithological Congress in Durban, South Africa (proceedings published in Ostrich).

My current studies of geese focus on the allometrics of metabolic rate, gut capacity, digestive physiology, and foraging strategy within and between species. I am also conducting work in subarctic Canada on the nutritional and physiological ecology of gosling growth. The goal of the gosling growth study is to identify the effects of protein limitation and dietary fiber on growth rates of sympatric Canada and Snow geese. Information on growth rates and nitrogen requirements of Canada and Snow geese is particularly pertinent, yet inadequate. Increased numbers of Snow geese have caused widespread destruction of their preferred salt-marsh plants. In response to this habitat destruction, snow geese are now nesting or raising broods in areas traditionally used primarily by Canada geese. The consequences of this habitat shift include increased competition between goslings of Canada and Snow geese for limited nutrients. This study is a collaborative project with Dr. Robert Rockwell (American Museum of Natural History), Dr. Evan Cooch (Cornell Univ.), and graduate student Kris Winiarski and is part of the Hudson Bay Cooperative Research Project. Recently I have been invited to present these results at a special symposium on the effects of elevated carbon dioxide on ecosystems including those in the arctic (chapters were published in an edited book in 2004) and as a plenary speaker at the North American Arctic Goose meeting (publications in J. Animal Ecology in 2014).

Physiological Ecology of Songbirds
Goals of my current research with songbirds include: (a) evaluate new nondestructive methods for measuring body composition dynamics in songbirds; (b) study the nutritional ecology and physiology of songbirds during migration; and (c) use stable isotopes to test contemporary hypotheses about the effect of diet quality and fasting on the metabolic routing of dietary nutrients in songbirds.

Dynamics of body composition in small migratory songbirds
Understanding the dynamics of body composition in small migratory songbirds has fundamental and practical importance. The dynamics of body composition influences nutrient requirements which then interact with resource availability to determine length of stopover at sites along the migration route, the pace of migration, and ultimately the success and survival of individuals. The increase in body mass in birds during migration has been commonly assumed to be composed of fat and no protein. More recent studies, however, suggest that the reserves in birds may be composed of appreciable amounts of protein as well as fat. However, these more recent results come almost entirely from studies of large-bodied shorebirds and waterfowl that migrate long distances.

Of the few studies reporting utilization of fat and protein reserves in migratory passerines, most use regressions of fat mass and body mass from birds sampled during migration to quantify protein reserves. Unfortunately, this method has a poor theoretical rationale and methodological shortcomings. The fundamental problem with such an approach is that changes in body composition across individuals in a population rarely provides accurate estimates of body composition changes within individuals. The solution to this problem is to repeatedly measure the protein and fat content of individual birds using nondestructive techniques. Megan Whitman (an M.Sc. student in NRS) and I completed the first cross-validation study that simultaneously uses two different techniques (TOtal Body Electrical Conductivity [TOBEC] and isotope dilution) to independently estimate lean and fat mass of small songbirds during their migration. Such independent estimates of body components are a prerequisite for determining the lean-mass proportion of body-mass gains and losses in field-caught wild passerines.

We are now extending this work to studies of songbirds during spring migration as they cross the Negev Desert in Israel (supported by US-Israel Binational Science Foundation). This research explores the thermoregulatory physiology of small migrating passerine birds during the process of refueling at stopovers. We are testing the hypothesis that birds enter rest-phase hypothermia to reduce nighttime energy expenditure and increase fuel accumulation rate. As many small passerines switch from a normally insectivorous to a frugivorous diet while migrating, we are also testing the hypothesis that the fatty acid composition of the birds’ diets during stopover influences their use of rest-phase hypothermia, and hence their daily energy expenditure.

Nutritional ecology of birds during migration
Despite the obvious importance of stopover sites for recovery of stored energy and nutrients, the ecology and physiology of birds at stopover sites during migration is one of the least studied topics in bird migration. I am particularly interested in the interaction between diet, body composition, and digestive features in small migratory birds during recovery of body mass because ecological studies at stopover sites suggest such an interaction is relevant and important.

Ecological field studies at stopover sites have revealed two interesting patterns related to replacement of used energy and nutrients. First, many insectivorous songbirds switch to feeding primarily on fruits during migration. This dietary switch from insects to fruits has been proposed as an energy-conservation strategy in that abundant fruits are less energetically expensive to obtain compared to insects. Fruits may be nutritionally adequate if only fat reserves must be replenished. However, fruits may be inadequate if birds must replenish both fat and protein reserves during migration because, in general, fruits contain relatively little protein. A basic understanding of the nutritional adequacy of particular diets requires understanding which endogenous stores are used and how exogenous nutrients in particular diets are routed during recovery of endogenous stores. Graduate students Megan Skrip, Kristen DeMoranville, and Clara Cooper-Mullin (PhDs) and Wales Carter (MSc) are conducting research along these lines.

In addition, Barbara Pierce (former Ph.D. student in NRS) and I are currently studying songbird preferences for specific dietary fatty acids and how fatty acid composition of birds affects performance during migration. Theoretically, selectively feeding on long-chain unsaturated fatty acids may be advantageous because such fatty acids may be absorbed and/or metabolized more efficiently than saturated fats into a bird’s fat depots. We are testing this hypothesis by offering songbirds in different physiological states choices between diets that vary only in their fatty acid composition. In addition, we are studying how dietary fatty acids influence fatty acid composition of fat depots in birds, the energetics of activity, and its ecological implications. During my sabbatical at the Max Planck Institute for Ornithology (Germany) in 2005, I was able to demonstrate for the first time that birds with more polyunsaturated fat stores expended less energy during a simulated migratory flight (6-hrs continuously flying in a windtunnel) than birds with more monounsaturated fat stores. This research has led to successive NSF grants for the last 10+ years that focus on how fat quality matters for migrating songbirds.

A second interesting pattern apparent in field studies at stopover sites is that recovery of body condition after arrival at stopover sites is typically slow for 1 to 2 days and then much more rapid despite apparently abundant food resources. Although ecological conditions influence rate of recovery, birds exhibit the two-step recovery after fasting even when provided food ad libitum in the laboratory. Physiological mechanisms to explain the initial mass loss after arrival at a stopover site are largely unexplored, but two hypotheses have some support. The nutrient-limitation hypothesis suggests that the initially slow rate of mass gain occurs because birds utilize protein reserves during migration, and recovery of these protein reserves must occur first and is slow. Only after recovery of protein reserves can lipid reserves be repleted and, once initiated, this recovery of lipid reserves is faster. This hypothesis is supported by studies of migrating hummingbirds, domestic geese, and a few other birds.

Alternatively, the gut-limitation hypothesis suggests that the initially slow rate of mass gain at stopover sites occurs because birds lose digestive tract tissue and hence function during fasting, and rebuilding of the gut takes time and resources and itself restricts the supply of energy and nutrients from food. This hypothesis is supported by studies that show reduced gut size after fasting in mammals and chickens and by a few studies on migratory songbirds that show food intake and gut function are reduced after fasting. Barbara Pierce and I tested these two hypotheses (gut- or nutrient-limitation) by measuring food intake, digestive features, and body composition of birds before and after a fast, and during recovery of body mass in birds after a fast.

Use of stable isotopes to investigate the metabolic routing of dietary nutrients
Plant physiological ecologists have used natural variation in stable isotope ratios to study photosynthesis, water balance, and nitrogen metabolism in plants. In contrast, animal physiological ecologists have made much less use of naturally occurring stable isotopes in their research despite its great potential. Recently, animal ecologists interested in migratory birds have used stable isotope ratios to reconstruct diets and to trace movements between breeding and wintering areas. However, all such work relies on important untested assumptions about the effect of physiological processes on the stable isotope ratios. David Podlesak (a recent Ph.D. student in NRS), Kris Winiarski (a M.Sc. student in NRS), and I tested some of these key assumptions by using diets that have their protein and carbohydrate components uniquely labelled with stable isotopes to determine the effect of diet quality on the metabolic routing of dietary nutrients in small songbirds. We are also conducting field studies of (a) diet switching in free-living songbirds during migration by comparing patterns of stable isotopes in selected wild fruits and insects, and in the breath, blood, and feathers of birds; and (b) habitat use of goslings in western Hudson Bay (Canada) by comparing stable isotope signatures of forage plants with that of certain tissues in goslings collected throughout growth. This study is a collaborative project with Dr. Robert Rockwell (American Museum of Natural History), Dr. Evan Cooch (Cornell Univ.), and former graduate student Kris Winiarski and is part of the Hudson Bay Cooperative Research Project.

Forest Management and Wildlife
Conservation of early successional forest within the eastern United States is an important management concern because these forests and their associated wildlife species are relatively rare and require active management. Conservation agencies in RI are actively promoting forest management activities to create early successional habitat on government and private land. This research increases understanding of critical issues and directly strengthens outreach programs to promote more effective forest management in southern New England. The research focuses on two primary objectives: (A) Conduct research on three key issues related to the promotion of early successional habitat: (1) Current status and recent trends of early successional habitat; (2) Experience to date in promoting early successional habitat in RI; (3) Impact of forest management interventions on populations of key wildlife species; and (B) Strengthen cooperative extension programs involved in forest management and wildlife conservation by disseminating the results of research related to the first project objective. This is a collaborative project with Brian Tefft (RI DEM), Dr. Pete August, Dr. Thomas Husband, Dr. Bill Buffum, and colleagues at NRCS.

Effects of Offshore Wind Energy Development on Seabirds and Seaducks
The State of Rhode Island has recently invested > $10 million dollars in baseline monitoring of natural resources including seaducks and in the development of the RI Ocean SAMP (Special Area Management Plan) that will guide offshore development including wind farms. Since 2008, Dr. Peter Paton and I have conductive collaborative research documenting the distribution and abundance of birds for the RI Ocean SAMP project. This collaborative research has recently expanded to include an assessment of potential effects of land-based wind energy development on birds and bats, as well as the following objectives related to offshore wind energy development: 1) use existing data from the RI Ocean SAMP to develop spatially explicit models that identify the key biotic and abiotic factors that affect the abundance and distribution of seaducks in Rhode Island nearshore and offshore waters, (2) apply these spatially explicit models to predict affects of global climate change on seaduck abundance and distribution, (3) estimate seasonal changes in population size of seaduck species and apply these population models to harvest management plans, and (4) collect additional data on the movement patterns of selected seaducks in relation to biotic and abiotic factors to test the spatially explicit models developed from existing data and to better inform decisions about extent and placement of offshore wind power as well as inform existing survey data (e.g., Atlantic flyway seaduck survey, USFWS). This is a collaborative project with Jay Osenkowski (RI DEM), and colleagues at USGS, BOEMRE, and RI CRC.