The Cerulean Warbler is a Neo- tropical migrant that breeds in the midwestern and eastern United States and winters in South American countries such as Colombia, Venezuela, and Peru. Thirty years of census data from the North American Breeding Bird Survey indicate that Cerulean populations declined at an average annual rate of 3.5 percent per year from 1966 to 1996. Although the destruction or degradation of habitat on the birds’ wintering grounds in Latin America may be partly responsible for these declines, the loss of large tracts of bottomland hardwood forests in the United States, the warbler’s primary breeding habitat, has been extensive and is well verified. Partners in Flight (PIF) ranks this species as having a high conservation priority throughout its range and is developing bird conservation plans that include the cerulean as well as many other species of conservation concern. As the PIF planning process got underway, it became clear that much about the ecology of the Cerulean Warbler was still unknown. To help correct that, the Cornell Lab of Ornithology, with funding from the U.S. Fish and Wildlife Service, organized the Cerulean Warbler Atlas Project to determine the current status and specific whereabouts of existing Cerulean Warbler populations.

When I was asked in May of 1998 to help find people to survey for ceruleans in Missouri, I thought that canoe surveys of various river drainages in the Ozarks would probably be the most efficient way to detect concentrations of the bottomland-dwelling warbler. I was hoping we could map locations of birds on topographic maps of the rivers. We needed people who could identify birds by song, while carefully following the course of the river on the map, so that the locations of birds could be plotted with a minimum of error. I found these qualities in a combination of old friends, Tom Hall (a St. Louis musician with an ear for birds) and Tim Kippenberger (a fine-quality cabinetmaker who knows just how and when to be meticulous). When I explained the project, it didn’t take much effort to convince them they should canoe Ozark rivers to look for rare and exciting birds. Team Cerulean was soon off and floating.

Tom and Tim did not find Cerulean Warblers on the first river they surveyed, but subsequent canoe trips to rivers deep in the heart of the Ozarks revealed densities of more than 4.5 singing males per river mile. Nevertheless, there were still significant stretches of river where warblers were not present, or where they were present either in high or low densities. We began to wonder what was associated with the patterns we were seeing and whether it had anything to do with habitat factors at either the local or landscape scale.

Meanwhile, Mark Robbins (an ornithologist from the University of Kansas) had been canoeing for Cerulean Warblers on yet another Ozark River, where he found densities of approximately 3.5 males per river mile. Mark had data on where good populations were; equally exciting, he knew from past canoe surveys of other Missouri rivers where the birds were not found.

Stretches of river that were surveyed by Tom, Tim, and Mark in 1998 were entered into a Geographic Information System (GIS) last fall at the Missouri Department of Conservation. Information on warbler distributions was then superimposed upon a map of landcover (in other words, the amount and distribution of cover types such as forest, pasture, and urban areas) within a 10-kilometer distance on either side of the area of river in question. The maps will be updated following the 1999 season. Frank Thompson (U.S. Forest Service) and David Diamond (Missouri Resource Assessment Program) have agreed to contribute their skills to the statistical analyses needed to determine the significance of relationships among landscape variables (for example, percentage of forest in the landscape and patterns of forest fragmentation) and warbler distributions. Results of the analyses will help us better understand what geographic scale we need to consider as we attempt to conserve this high-priority species.