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Species of Concern
Cerulean Warbler Status Assessment: April 2000
View the complete Cerulean Warbler Status Assessment - PDF file; 154 pages.
Cerulean warbler, Dendroica cerulea (Wilson), is a wood warbler in the Subfamily Parulinae of the Family Emberizidae, Order Passeriformes. No controversial or unsettled issues exist in the taxonomy of this bird.
The numbers of cerulean warblers are declining at rates comparable to the most precipitous rates documented among North American birds by the cooperative Breeding Bird Survey. Recent evidence suggests that events on breeding, stopover, and wintering grounds are implicated in this decline. However, no detailed life history study of the species exists. This status assessment is an attempt to assemble what is known of the species into a form that will enable biologists in the U.S. Fish and Wildlife Service to make a decision on whether or not to propose listing of the species under the Endangered Species Act. The report will also help the Service and others establish priorities for monitoring; research; and habitat protection, restoration, and management that will conserve this species.
Cerulean warbler is a small, neotropical migratory bird that weighs approximately 8-10 grams, and has relatively long, pointed wings and a short tail. All plumages have two white wing bars and white tail spots. Males have streaked backs in all plumages; females do not. Males in breeding plumage are blue above, white below, with a blueblack neck ring. Females in breeding plumage are bluish green above, white below washed with yellow, with a white or yellowish line over the eye. Young birds are similar to the adult females but greener.
Cerulean warblers feed primarily on insects throughout the year. Open-cup nests are placed in the canopy of forest trees where the birds raise usually a single brood. Clutch size is usually 3-4 eggs. Adult and juvenile mortality rates are unknown. The longevity record is at least 6 years. Only 1 of 1399 banded individuals has been encountered later away from the original capture locality. Conventional wisdom about habitat for cerulean warblers is that the birds breed in large tracts of deciduous forest having large trees and an open understory. These tracts may be in upland or bottomland situations. Migratory and winter season habitats are poorly known.
Cerulean warblers breed in eastern North America primarily in the Ohio and Mississippi River valleys. The range generally extends from the eastern Great Plains, north to Minnesota; east to Massachusetts; and south to North Carolina and Louisiana. During migration the birds pass through the southern U.S., across the Gulf of Mexico to the highlands of Central America, and on to South America. They winter in the lower elevations of the subtropical zone of the eastern slope of the Andes and other mountains in northern South America.
Historical data on the occurrence and abundance of the species are sparse and do not permit estimation of total numbers. However, it is clear that this species was a conspicuous and abundant bird throughout the Ohio and Mississippi River valleys in the past century. Currently the birds are much less numerous in areas where formerly they were abundant. The North American Breeding Bird Survey suggests that, during the past 30 years, the population has declined at an average annual rate of approximately 4%. Summaries of the Breeding Bird Survey, Breeding Bird Census, Breeding Bird Atlas, conservation status, and other information pertinent to individual nations, states, and provinces are presented for each political division within the range of the species.
Current numbers and distribution of the species are such that an adequate summary of occurrence by land ownership categories cannot be prepared, other than to state that the birds are found on public lands, industrial forest lands, and other private lands. One study found the birds more frequently on public than on other land ownerships. The extent of public lands, both state and Federal, is such that substantial amounts of breeding habitat management for the species could be done there.
Summary of status and threats to the continued persistence and expansion of populations of the species includes several categories of threats, of which destruction of habitat is the most prominent.
Primary threat to the species is the loss of habitat on the breeding and on the winter grounds. Clear documentation of this exists. Arresting declines in habitat will require policy decision making as well as incorporation of information about the species into land-use and land management decisions. Other factors such as predation, nest parasitism, or reduced survivorship during migration, are believed to be directly related to the primary factor which is loss of area of breeding and winter habitat. Neither breeding nor winter habitat is known currently to be the more serious limiting factor. Importance of stopover habitat is not known either. Some losses to population occur as migration catastrophes, and are not directly related to habitat loss.
Successful conservation of cerulean warblers depends upon managing forested landscapes on the breeding and nonbreeding grounds to provide high quality habitat. At the present time, no projects involving reintroductions or other population manipulations are underway, nor are actions designed to manage human interactions with the species contemplated. Identifying those specific silvicultural manipulations and other land management activities that create the appropriate vegetation structure in which cerulean warblers can successfully breed, as well as winter, is a critical step in the process. This step has not yet been taken; it is perhaps likely the full range of actions, involving restoration of abandoned agricultural lands, protection of some existing forests, as well as manipulation of vegetation in other forests will be required. Current suggestions for management involve production of large sawtimber trees on long rotations. Specific management treatments have not been determined.
Currently conservation activities for the species consist of local projects, both of inventory and monitoring nature, in several areas from Ontario to Mississippi, and Minnesota to North Carolina. A rangewide atlas project has been completed, the species forms an important part of planning activities in several areas in the Partners in Flight network, and numerous land managers have become sensitized to the potential of their lands to support the species.
Research into the demography of the species is sorely needed, to determine differences in survivorship and productivity of the species in different landscapes, different parts of the range and under different land management activities, including silvicultural treatments of breeding habitats. Research into the winter survivorship, distribution and relative abundance by habitat in South America is desperately needed. Intensive monitoring of known populations and their responses to management treatments will provide invaluable information about management activities appropriate to the perpetuation of the species. Concern over the distribution of the species in the interior of large tracts of forest suggests that surveys into the occurrence of the species on roadside vs. off-road counts in the same areas will be useful as well. Research into the migratory movements and stopover sites will also be useful.
The species is not in danger of imminent extinction, but it is rare enough to warrant concern, and its future is not assured. Based upon extensive BBS data, cerulean warblers have declined sharply over the past 30 years. Should that trend continue another 30 years, population sizes are predicted to be only 8% of the 1966 levels. It is unclear whether the species could persist with numbers as low as those. Threats to reproductive increase and to survivorship apparently exist in all parts of the annual cycle, necessitating attention to many aspects of the life cycle in recovery or future production efforts. Management programs can be instituted at the present time that do not require major changes in land use practices, but do consider silviculture appropriate to producing habitat for the species. I take full responsibility for them, as well as for all the opinions expressed here.