Demography and habitat use of Cerulean Warblers on breeding and wintering grounds
Because their annual movements span continents, Nearctic-Neotropical migratory birds represent one of the most challenging groups for which effective conservation strategies can be developed. Knowledge of the ecology and management of migratory bird communities comes primarily from studies conducted on the breeding grounds. However, recent work demonstrates that events that occur throughout the annual cycle may also contribute to population declines. The Cerulean Warbler (Dendroica cerulea), a Neotropical migrant exhibiting precipitous population declines, is an excellent example of a species that may be impacted by events on both the breeding and nonbreeding grounds. My dissertation research examined habitat use and population demography of Cerulean Warblers on breeding (southern Ohio) and wintering (Venezuelan Andes) grounds to evaluate potential factors that contribute to declines in Cerulean Warblers.
During the breeding season, we surveyed Cerulean Warblers across 12 mature forest sites in southeast Ohio, 2004-2006. Research on the breeding grounds identified (1) how clearcutting impacted spatial distribution, density, and nesting success of Cerulean Warblers at multiple spatial scales (i.e., from local/edge to landscape), and (2) specific microhabitat and nest-patch characteristics selected by Cerulean Warblers. At each site, Ceruleans were intensively spot-mapped 8 times each year from May to July, adult behavior was used to locate and monitor nesting attempts, and nest, local, and landscape habitat characteristics were quantified. Results suggest that the presence of regenerating clearcuts did not influence density or nesting success of Cerulean Warblers in adjacent mature forest. Instead, local habitat features explained variation in warbler density and daily nest survival better than landscape-scale characteristics. Density and nesting success were positively associated with features typical of heterogeneous steady-state phase forests. In particular, nest plots had 14%, 24%, and 94% greater canopy openness, understory stems, and number of grapevines, respectively.
On the nonbreeding grounds, my research examined (1) the suitability of shade coffee plantations and (2) foraging and habitat use by wintering migrant birds, with emphasis on Cerulean Warblers. This portion of the study was conducted in 3 primary forest sites and 3 shade coffee plantations on the western slope of the Cordillera de Mérida of the Andes Mountains. At each site, migrants were surveyed using distance-based line transects, mist-netted and banded, and observed to characterize habitat use and flocking behavior during November–February 2005/06 and 2006/07. During these two seasons, 29 individual Cerulean Warblers were color-banded and resighted to estimate apparent monthly survival, annual return rates, and apparent annual survival. Densities of migrants were 3-14x higher in shade coffee plantations than primary forest sites, even after accounting for differences in detectability. Apparent monthly survival of Cerulean Warblers was estimated at 97% and overwinter persistence was similarly high. Banding data also suggest that migrants using shade coffee improve their body condition over the winter. Adult Cerulean Warblers had 62% higher apparent annual survival than juvenile birds (0.73 versus 0.45). Apparent monthly migration survival for adults (0.97) was similar to values throughout the remainder of the annual cycle, though juveniles experienced up to 6x higher mortality during migratory periods. Abundance of Neotropical migrants in shade coffee plantations was significantly related to both structural and floristic characteristics where upper canopy foragers and lower canopy/ground foragers were positively associated with (1) number of large trees (>38 cm dbh), tree height, and understory vegetation density, and (2) numbers of small (8–23 cm dbh) and medium (23–38 cm dbh) trees and increased canopy cover, respectively.
Most species of Neotropical migratory birds have been well-studied on their breeding grounds in North America, while considerably less is known about the wintering and migratory ecology of these same species. My research efforts stand out as a relatively unique body of research given that I simultaneously examined population ecology during two phases of the annual cycle. Collectively, my research efforts have allowed me to generate specific management recommendations on both the breeding and nonbreeding grounds that can benefit Cerulean Warblers.