For many Indiana bird watchers, the cerulean warbler is one of the most sought-after species during birding trips. Yet, the species is surrounded by great uncertainty and even greater controversy. That’s pretty impressive for an animal that only weighs 8 to 10 grams. The cerulean warbler is named after the brilliant blue coloring of the male. An adult male is bright blue above and white below with a narrow blue-black band across the throat. The female is bluish-green above and whitewashed with yellow below. She has a distinct white or yellowish line over the eye. Both sexes have two white wing bars and white tail spots in all plumages.

Cerulean warblers spend more than one-half of the year in the tropics, and very little is known about their habits on the wintering grounds. The species winters in broad-leaved, evergreen forests, and their range mainly includes the east slopes of the Andes Mountains in northern South America. The winter range of the cerulean warbler is limited to one of the most extensively logged and cultivated regions of the Neotropics. These forests are being converted to pastures and farms for the production of coffee, tea and cocoa, making them unsuitable for ceruleans. Ironically, while timber harvest may be detrimental to the species’ wintering habitat, it may be an important tool for the conservation of this species on the breeding grounds.

elimination of some populations in the center of the species breeding range in states such as Ohio, Kentucky and Tennessee.  The North American Breeding Bird Survey (BBS) is a standardized roadside survey conducted across North America. Started in 1966, the primary objective of the BBS has been to estimate population changes in songbirds. BBS trends should be interpreted with caution because of possible biases; however, they are often the best source of data available. According to data from the BBS, populations of ceruleans have been declining rangewide since the 1960s with annual declines of -4.3 percent. This is the steepest decline of any warbler in the country. In Indiana, BBS data indicated an even sharper decline of -12.5 percent for the cerulean.

Concerns for this species over the last decade resulted in a number of conservation groups petitioning the U.S. Fish and Wildlife Service to list the species as “threatened” under the Endangered Species Act. After six years, the U.S. Fish and Wildlife Service  announced on Dec. 6, 2006, that while cerulean warbler populations are declining and will continue to decline, the listing of the species as “threatened” is not warranted. They determined that there was effectively no chance for the species to become extinct in the next 100 years unless conditions change beyond what they can anticipate. Several organizations including the National Audubon Society have expressed grave concerns over the future of the cerulean warbler following this announcement. Although the cerulean warbler may not be in imminent danger of extinction, it is rare enough to warrant concern.

The species tops many conservation lists and is considered a high priority species by Partners-In-Flight and a Watchlist species by the National Audubon Society. While Butler (1898) considered the species a common migrant and summer resident in Indiana, the cerulean is now listed as a species of special concern by the state (Bruner 1998). So, what is causing the declines in this species? Ceruleans are considered “interior” forest species because they tend to be found in large, mature blocks of forest. Population declines have been linked to the loss of mature deciduous forests, fragmentation and changes in forest management practices including shorter rotation periods and even-aged management (Hamel 2000).

Fragmentation can be described as the disruption of extensive habitats into small, isolated patches, and the overall loss of total habitat area. Most of the deciduous forests within the breeding range of the cerulean warbler have been cut during the last 200 years in favor of agricultural and urban/suburban land uses (Hamel 2000). Cerulean warblers, like many migratory song birds are area-sensitive. They are more common in larger forested tracts and absent from smaller forests. Such area-sensitive species show greater declines in numbers and range distribution than would be expected from the proportion of habitat loss. Fragmentation not only results in a reduction of the amount of forest, but it also isolates the remaining forested tracts from one another because of intervening land use, such as agricultural or urban uses. Mountaintop mining may have a greater effect on cerulean warblers than other sources of forest fragmentation, given that ridges are removed during mining. Several studies have indicated that cerulean warbler territories occur more often than expected along ridgetops (Buehler et al. 2006, Weakland and Wood 2005). The effects of forest fragmentation may be especially strong on ceruleans because males often settle where others are already present and form “loose colonies.” This tendency may keep ceruleans from occupying forest tracts that are too small to contain a core group of territory holders. The effects of fragmentation on the cerulean warbler may be related to factors associated with fragment size such as brood parasitism by the brown-headed cowbird and high rates of nest predation by generalist predators such as blue jays and raccoons.

Like most birds, ceruleans spend a lot of their energy on parental care, patiently incubating their eggs and then bringing food to their young until they are old enough to care for themselves. On the other hand, brown-headed cowbirds have evolved a different strategy for “parenting.” Like any good parasite, cowbirds benefit greatly from their selected host (much to the detriment of many bird species). Female cowbirds will remove eggs from another bird’s nest and then lay their own eggs. Cowbirds hatch more quickly than many species, and nestlings are usually much larger and more aggressive than the nestlings of other birds. This allows baby cowbirds to out-compete other nestlings for food and nest space. Since ceruleans nest high in the canopy of forests and their nests are difficult to find, we have very little information regarding the rates of cowbird parasitism in Indiana. The first documented cerulean warbler nest in Indiana was found on the Hoosier National Forest just south of the Charles C. Deam Wilderness. The nest was located over 70 feet from the ground and successfully fledged one brownheaded cowbird and one cerulean warbler. Nevertheless, we have learned some fascinating behaviors that cerulean warblers exhibit during the breeding season. Many males actively help females choose a nest site. Birders can detect the presence of a nest by listening to the males. Once a nest has been established, males can be heard “whisper” singing (singing very softly) to the female. But, the key to finding the nest is observing the female. While incubating eggs, females regularly depart from the nest to forage. Females often exit their nests with a behavior known as “bungee jumping.” Females will often drop, wings collapsed, right out of the nest and fall towards the ground. Just above the forest floor, she will untuck her wings and swiftly fly off parallel to the ground.