Keeping Populations of the Cerulean Warbler Healthy: An International Effort

United States Department of Agriculture

The USDA Forest Service (FS) is part of an international effort to maintain viable populations of the cerulean warbler—a forest songbird once common in the eastern United States, now rarely seen. In an article in the January issue of The Auk , Paul Hamel, research wildlife biologist with the FS Southern Research Station, provides an overview of the status of the cerulean warbler, including current research findings and future needs. The article also details the formation of the Cerulean Warbler Technical Group, an international effort to focus research and use the results to conserve the species.

Migrating to the lower slopes of the Andes in August, cerulean warblers return in April or May to build nests in the upper canopy of forests in the southeastern United States. Since 1966, populations of the species have declined an estimated 70 percent, the decline tied to the fragmentation and destruction of habitat in both breeding and winter ranges. For its breeding range, the bird needs large areas of mature deciduous forest, often along streams: in the southeastern United States, much of this habitat has been lost to agriculture or development. In its winter range in South America, forests are also being lost to agriculture.

Hamel summarizes what is currently known about the biology of the cerulean warbler and the research questions that remain to be answered. “Cerulean warblers are very difficult to study in the field because they nest and forage in the high canopy,” says Hamel. “Fortunately, we have learned a few tricks, such as surveying from canoes and using carved wooden decoys to attract the males. We have also developed genetic tools to help us track the movement of specific populations. We are seeing a dramatic shift in range. Land use change is certainly one cause, but climate change—either short- or long-term—may also be a factor.”

Surprisingly little is known about the behavior and population ecology of the cerulean warbler, mostly due to the difficulty of catching females, locating nests, and observing the young. “Most glaring is our ignorance of the cerulean warbler during the non-breeding season,” says Hamel. “So far, we only have two published studies from South America.”

Hamel has been instrumental in the Cerulean Warbler Technical Group (CWTG), an international collaboration formed to develop a proactive, broad-based strategy to conserve the cerulean warbler. “We followed the example of other ad hoc conservation groups such as the Louisiana Black Bear Conservation Committee in our basic philosophy of including as many partners as possible,” says Hamel. “We agree to leave agendas at the door, and to keep the focus on identifying meaningful solutions through sound science, clear communication, and trust. Our hope is that this group can serve as a model for other efforts to conserve forest bird species.”

The partnership includes industry, State and national government, nongovernmental organizations, and universities in North and South America. CWTG is organized approximately around the breeding and non-breeding seasons. The breeding season group is developing a research design to document the effect of land use change and to determine which forest management methods benefit the species. The non-breeding season group, El Grupo Ceruleo, is gathering information on the winter range of the cerulean warbler by developing a network of observers and conducting an analysis of habitat in South America.

In March 2003, El Grupo Ceruleo, which includes scientists from both the breeding and non-breeding ranges, met with in Ecuador to discuss the conservation of the cerulean warbler and other migratory and resident neotropical birds and to outline research needs. The USDA Forest Service and the Nature Conservancy are providing the funding for South American biologists to conduct new research on cerulean warblers in winter 2003 and 2004.

Research findings in North America confirm habitat loss as the main reason for the decline of cerulean warbler populations. Studies have also found a growth in populations of cerulean warblers in areas where forests are regenerating. “We have some evidence that we can regenerate and manage forests to create or improve habitat for the cerulean warbler,” says Hamel, “but we need to act quickly and throughout the bird's range.”

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