Kirtland's Warbler, Dendroica kirtlandii, is a small songbird of the New World warbler family (Parulidae), named after Jared P. Kirtland, an Ohio doctor and amateur naturalist. Nearly extinct just 50 years ago, it is well on its way to recovery. It requires habitat destruction to survive and has been intensely studied by scientists for this peculiar trait.
These birds have bluish-grey upper parts with dark streaks on the back and yellow underparts with streaked flanks. They have thin wing bars, dark legs and a broken white eye ring. Females and juveniles are browner on the back. Like the Palm Warbler and Prairie Warbler, they wag their tails frequently. Their song is a loud chip-chip-chip-too-too-weet-weet often given from the top of a pine.
Kirtland's Warblers forage in the lower parts of trees, sometimes hovering or searching on the ground. These birds eat insects and some berries, also eating fruit in winter. For breeding they require stands of young (4 to 20 year old, 2-4 meters high) Jack Pine trees. However, their open cup nest is not built in these but a short distance away on sandy soil.
Ecology of Jack Pine
Jack Pines are somewhat smallish pine widespread from the Canadian tundra and taiga to the Great Lakes region and the Atlantic Ocean; they are a boreal species, only occurring in a certain climate. Their cones open only after trees have been cleared away by forest fires or, after logging, in the summer sun. About all of its present-day range was covered by solid ice as late as 10.000 to 15.000 years ago; the range of the pine (and as it seems the warbler also) was probably a contiguous swath between the Appalachian Mountains and the Great Plains. The pine's peculiar reproductive strategy fits well with a dry taiga or cool temperate habitat as would have predominated there, probably with a higher incidence of forest fires than today, as the ice age climate was somewhat drier overall.
Decline to near-extinction
As global climate changed out of the ice age through the last 10 millennia or so, Jack Pine, and consequently also Kirtland's Warbler, shifted their habitat north. As the Kirtland's Warbler - and Parulidae in general - is not able to expand into subarctic climate well, most Jack Pine woods are too far north for the species. Moreover, the Great Lakes, which formed before the receding ice, were an obstacle for its spread. Kirtland's Warbler found itself blocked by the expanse of water, while the cold-hardy Jack Pine expanded its range as far as the Northwest Territories.
With European settlement of North America progressing in earnest, much of the forest in the southern Great Lakes region was cut away, never to be restocked. Kirtland's Warbler became trapped on the Northern Peninsula. It may or may not have occurred in Dr. Kirtland's home state of Ohio in recent times, but if it did it would seem to have been extirpated from the state around the time when its namesake himself died in 1877. What habitat there might have been was cleared away in the latter half of the 19th century, and certainly the bird was not breeding there anymore in 1906. Kirtland's Warblers used to breed in Ontario but have not done so since the 1940s. By the mid twentieth century its numbers had crashed to near-extinction. The Kirtland's Warbler population reached lows of probably less than 500 individuals around the 1970s, and in 1994 only 18 km² of suitable breeding habitat was available.
Today this bird's habitat is being preserved by controlled burns and staggered timber harvests in its limited breeding range. Since this habitat management regime was begun in the 1970s, the birds' numbers have steadily risen, though they are still at dangerously low levels. People have also intervened to protect this bird against nest parasitism by Brown-headed Cowbirds, to which these birds are highly susceptible.
They have still been observed in Ontario and the Upper Peninsula of Michigan, and though it is still only rarely recorded in NW Ohio (where there is hardly any significant woodland left), the numbers of recorded birds are increasing. Beginning in 2005, a small number have been observed in Wisconsin. In 2007, three Kirtland's Warbler nests were discovered in central Wisconsin and one at CFB Petawawa, providing an auspicious sign that they are recovering and expanding their range once again.
Although there seem to be no more than 5,000 Kirtland's Warblers as of late 2007, four years earlier they had numbered just 2,500-3,000. Classified as Vulnerable to extinction since 1994, Kirtland's Warbler was downlisted to Near Threatened in 2005 due to its encouraging recovery. It is not clear to what extent the birds depend on Bahamas pine during winter; deforestation on the wintering grounds may eventually become a bigger threat to the birds' recovery than the situation in its breeding range.
The Michigan Department of Natural Resources has recently made optimistic reports about the populations of Kirtland's Warblers.
Recent reports indicate that the population is growing, but also suggest that the Kirtland Warbler may continue to be dependent upon human protection and intervention forever.
There is a Kirtland's Warbler Festival, which is sponsored in part by Kirtland Community College.