Scott Sillett Profile
I study how events throughout the annual cycle of migratory birds are interconnected and how multiple mechanisms, both natural and human-related, operate to limit and regulate these bird populations. My research combines field observations and experiments with statistical and demographic modeling to address three general questions:
What determines the demographic rates of migratory populations?
Understanding the processes that determine abundance of migratory birds has been difficult because these species move between diverse and distant locations, often on different continents. Furthermore, events during one stage of the annual cycle are likely to influence demographic rates in subsequent stages.
In collaboration with Richard Holmes (Dartmouth College) and Nicholas Rodenhouse (Wellesley College), I am engaged in long-term studies of a black-throated blue warbler (Dendroica caerulescens) population breeding at the Hubbard Brook Experimental Forest, New Hampshire.
We have found that warbler demographic rates in winter and summer vary with fluctuations in a global climate cycle, the El Niño Southern Oscillation (ENSO). Adult survival and fecundity are lower in dry, El Niño years and higher in wet, La Niña years, due in part to ENSO's impact on food availability.
Fecundity, in turn, is positively correlated with the subsequent recruitment of new individuals into both breeding and overwintering populations. We have also shown empirically and experimentally that warbler fecundity is significantly, negatively correlated with warbler density. The negative feedback on fecundity generated by warbler density is sufficient to regulate population size within the levels observed over the past 35 years.
In 2002, we extended our studies at Hubbard Brook to determine how weather, and ultimately, climate, in combination with biotic factors, affects the spatial distribution, abundance, and demography of bird species that breed in temperate forests. We are comparing songbird populations breeding along an elevational gradient at Hubbard Brook with those breeding in a fragmented landscape in the Champlain Valley of Vermont (with Therese Donovan, University of Vermont).
We are quantifying annual shifts in the spatial distribution and abundance of birds at a landscape scale. Shifts are being compared to landscape structure, within- and between-year differences in local weather conditions and biotic factors (e.g., food supply, nest predator abundance), the presence of conspecifics, and, for black-throated blue warblers, variation in the birds' reproductive success.
This work will contribute to an understanding of how weather, in combination with habitat structure and other biotic factors, influences forest birds. Such information is key to assessing the potential local effects of climate change on bird populations, and ultimately to predicting how these populations and the biological communities of which they are a part will respond in the future to environmental change.
How does migration interact with climate variation to shape avian life histories?
Recent shifts in a fundamental component of avian life history, the timing of spring migration and arrival on breeding grounds, indicate that bird populations are responding to climate changes. Given that migration is costly and associated with high mortality, trade-offs between fecundity and survival should exist among individuals with different migratory behaviors. Therefore, predicting the effects of future climate change on migratory bird populations will require information about how environmental variation determines the nature of these trade-offs.
For the past two years, I have been developing a large-scale investigation of how migration shapes avian life histories along an environmental gradient, in collaboration with Cameron Ghalambor (Colorado State University), Geoff Geupel (Point Reyes Bird Observatory), Susana Peluc and John Rotenberry (both at the University of California - Riverside). We are using the orange-crowned warbler (Vermivora celata) as a model species.
This warbler is one of the few North American passerines that exhibits strong, latitudinal variation in both migration behavior and timing of molt, and is therefore well-suited for studying the interactive effects of migration, physiology, and environment on life history strategies. Three of the four V. celata subspecies (celata, lutescens, orestera) are medium- to long-distance migrants, breeding in the northern and western U. S., and wintering in the southern U. S. and in Mexico.
In contrast, the threatened and declining fourth subspecies, V. c. sordida, is endemic to the California Channel Islands. Some sordida are year-round island residents, but most overwinter in southern California. Orange-crowned warbler life history covaries with latitude.
At one extreme, birds breeding in the north (V. c. celata) have relatively large clutch sizes (4 - 6 eggs) and low annual adult survivorship (40 - 50% survive from year1 - year2), and complete breeding and molt between June and August. At the other extreme, V. c. sordida begin breeding in late February, lay 2 - 3 egg clutches, have high annual survivorship (60 - 70%), and molt from the end of breeding in July into September. Since 2003, we have worked with the Catalina Island Conservancy to study the population biology and behavior of V. c. sordida nesting on Santa Catalina Island, CA.
We intend to expand our research to explain the ecological basis of V. celata life history variation by: 1) comparing clutch size, annual fecundity, molt, survival, circulating hormones, and food availability between populations in Alaska (celata), northern California (lutescens), and southern California (sordida); and 2) using both correlative and experimental approaches to examine how food limitation, age-specific mortality, and breeding season length (via trade-offs between reproduction, molt, and migration) drive differences in life history strategies.
How does environmental variation affect trophic interactions?
Numerous studies have shown responses of plants and animals to increased temperature regimes associated with global warming. Little attention, however, has been given to whether the effects of climate change are affecting species at similar rates, particularly across trophic levels.
Allan Strong, Scott Schwenk (both at the University of Vermont), and I began a study in spring 2004 to examine how climatic variation influences a tritrophic interaction in New England forests.
Using whole tree exclosures (which eliminate the effects of bird predation), we will take advantage of a climatic gradient at Hubbard Brook to address whether bird predation on leaf-chewing insects has effects that covary with local climatic conditions. These data will enable us to assess how direct (bird predation on leaf chewing insects, leaf chewing insect effects on plant productivity) and indirect (bird predation on plant productivity) effects in a tritrophic interaction could be altered in response to predicted climate changes.
Nicholas Rodenhouse, Matthew Ayres (Dartmouth College), and I recently received funding through the renewal of the Hubbard Brook NSF LTER grant. Starting in summer 2005, we will test the hypothesis that changes in nutrient cycling affects the dominant herbivore in this forest, lepidoptera larvae, and their predators through changes in foliage quality. This project will take advantage of a whole-watershed calcium addition experiment that occurred at Hubbard Brook in 1998.
Observed changes in leaf greenness and protein content in this watershed suggest that the quality of food resource for caterpillars is changing. We will measure the growth and abundance of lepidoptera larvae on multiple plant species across an elevational gradient within the experimental watershed and a control (where most of our long-term bird research is conducted).
We will also intensively monitor reproductive success, parental feeding rates, and nestling growth of black-throated blue warblers in the two treatments.
