Birds of the Mid-Atlantic #9: Cerulean Warbler
The Latin word caeruleus, from which the species's English and scientific names are derived, is related to the Latin caelum, or "sky." It means sky-blue - not a light blue, but deep blue like the midday sky. It is an appropriate name for this little bird, which is brilliant blue on its head and back and white underneath, with a blue "necklace" and blue stripes on its sides. (Females are more of an aqua-green color.) Despite its sharp colors, this bird defies any observer who tries to see it. Cerulean warblers forage and nest near the very tops of trees; searching them out will induce the malady known as "warbler neck."
Fortunately for us, cerulean warblers have a distinctive song: 3-5 notes on a single pitch followed by a rising buzzy trill. It can be distinguished from the northern parula because the parula's song rises in a continuous trill and then falls over the top, without introductory notes on an even pitch. The blackburnian warbler's song has a similar rhythm but is much higher pitched without the buzziness. Its ending note is almost at the limit of human hearing. The song of the black-throated blue-warbler has a lazier pace and is a thinner-sounding buzz than the cerulean's.
Cerulean warblers have undergone two demographic shifts since the species was first discovered. Prior to the twentieth century, the species was rare as a breeder east of the Appalachians. Over the last century, the species has expanded its range steadily into the Mid-Atlantic and Northeast. In Maryland, the breeding population is concentrated in the mountainous western counties and somewhat less common in the central Piedmont region.
The other shift has been a steep decline in numbers. Cerulean warblers have endured one of the steepest declines among North American birds, a loss of close to 80% of the population between 1966 and 2003. It has landed on the Audubon Watchlist as a result. So far, attempts to have it listed as an endangered or threatened species have been unsuccessful because of federal inaction.
As with many other species, the cause of the cerulean warbler's decline is habitat loss and degradation. Cerulean warblers require large tracts of mature forests in order to breed. Such tracts are becoming harder to find due to logging and development. In the Appalachians, cerulean warblers face a threat from mountaintop removal mining. The species also faces pressures on its wintering grounds in northern South America, where deforestation for agricultural uses has become common. (A Colombian reserve was recently founded to address this problem.)
Even in some places where otherwise suitable habitat exists, cerulean warblers have disappeared because of the lack of an understory due to overbrowsing by white-tailed deer. A study by the USFWS in Pennsylvania found that ceruleans disappeared when the deer population rose about 20 per square mile. In many areas of the eastern United States, the population is far higher than that.
It is hard to be optimistic about this species's outlook. While cerulean warblers are not about to go extinct in the near future, decline is likely to continue without serious conservation measures. Such measures face many political and financial roadblocks and may be difficult to achieve in the current climate.
While we seek for the causes and solutions for the decline of the cerulean warbler and other eastern songbirds, we need to enjoy the birds while we have them. When I heard and saw cerulean warblers last Sunday, I was not thinking about their problems. I was thinking about what a beautiful bird it was and how happy I was to be seeing it, finally.
Labels: Birds of the Mid-Atlantic