Spring migration from its wintering grounds in Colombia and Venezuela started back in early April, and by now the Cerulean Warbler has flown across the Gulf of Mexico, passed through Mississippi, Alabama, Louisiana, and Georgia, and is continuing north and northeast.
During breeding season, this warbler builds its nest and forages high in the canopy of older and mature deciduous forests (up to 3,500 feet). The species prefers large tracts of forest consisting of a variety of hardwood tree species and relatively little undergrowth.
Click here to listen to the song of the Cerulean Warbler.
Until the middle of the 20th century ‘Dendroica cerulea’ was common throughout much of eastern North America, and was most abundant in the central Appalachian Mountains. But today the Cerulean is America’s fastest declining migratory songbird.
Faced with habitat loss in both their wintering and breeding grounds, populations of the species have been steadily declining for decades; showing as much as a 70% drop since the 1960s, and the trend continues downward. This beautiful blue denizen of mature deciduous forests has suffered following widespread deforestation for agricultural and energy development.
Within the Cerulean Warbler’s historical breeding range, over 50% of forests have been cleared, and 40 to 50% of South American shade coffee plantations — a highly preferred wintering ground — have been converted to monocultures of sun coffee, devoid of the large trees that the species needs to survive.
Prime Cerulean breeding habitat in North America happens to correspond with prime coal producing regions of Appalachia where mountaintop removal is practiced. Researchers with the USGS Biological Resources Division completed a study in 2002 that indicated Ceruleans have an unexpected preference for ridgetops. They found that “92% of [breeding] territories occurred only in fragments with ridgetop habitat remaining.” This is precisely the habitat destroyed by mountaintop removal mining.
The USGS study also found that Cerulean breeding density is lower in forest habitats that are fragmented or closer to mine edges. The bird is now increasingly found in marginal secondary forest habitat that has regenerated following the abandonment of farms, growth of trees following timber harvests, and other reforestation efforts.
Mountaintop mines are reclaimed primarily with grasses. The compacted nature of the soil slows or even prohibits the natural succession of forest in these areas, making fragmentation effects long-lasting.
Cowbirds feed in grasslands, so this is another factor that may be hurting Cerulean populations. Like many of its warbler cousins, Ceruleans may receive the unwelcome attention of parasitic cowbirds. These cowbirds attempt to foist their young onto unsuspecting adoptive parents by pushing Cerulean eggs out of the nest, then laying their own replacements. While the cowbird adults shirk their parenting duties, Ceruleans will energetically raise the changeling youngsters because they do not recognize cowbird eggs or young.
The Cerulean Warbler is on the Audubon Watch List, and is also recognized as a species of conservation concern through Audubon’s Important Bird Areas program. Attempts to categorize the bird as ‘threatened’ under the United States Endangered Species Act had not succeeded as of November 2008. It is, however, listed as a species of special concern in Canada, where it is protected. Additionally, the Cerulean Warbler is considered “Vulnerable” by the International Union for Conservation of Nature.