Ever
since its discovery in the middle of the last century,
Kirtland's Warbler has been considered a rarity. The first
individual known to science was a migrant shot near
Cleveland, Ohio, in May 1851. The breeding range in Michigan
and the wintering range in the Bahamas were discovered in
the 1870s, but relatively few birds were seen in either
place. A total of 71 specimens were taken in the Bahamas in
the two decades after Kirtland's Warbler was discovered
there, and only 6 in Michigan in roughly the same period. In
1903 the bird's nesting grounds in central Michigan were
located, and in the 1980s its wintering grounds were found
to extend farther south (to the Dominican Republic) and into
drier habitat than previously thought. A survey of Kirtland's
Warblers in 1951 turned up 432 singing males. In 1961 the
number had increased to 502, but by 1971 it had declined
sharply to 201. However, by 1995 the number of singing males
had recovered to 765. The species is now restricted to just
six Michigan counties. It is an extreme habitat specialist,
nesting only in areas of 5- to 6-year-old jack pines -- if
the stand is extensive enough and contains grassy clearings.
Such areas are produced when a fire of appropriate intensity
sweeps through a mature stand of pines permitting the
germination of their seeds. The birds will start to nest in
an area when the pines are young. Wildfires once provided
abundant habitat, but now improved forest fire-fighting
capability has greatly reduced burned areas. To compensate,
jack pine stands specifically designed to meet the needs of
Kirtland's Warblers rather than the needs of forestry have
been planted. In addition to habitat
shrinkage in this century, population increases of
Brown-headed Cowbirds threaten the warblers. Cowbirds have
been abundant in Kirtland's habitat only recently as forests
in the area were cleared for agriculture. The warblers are
not adapted to defend against them. The percentage of
parasitized nests increased from about 55 percent in the
1940s and 1950s to over 70 percent in the 1960s. The cowbird
is an especially dangerous enemy because it parasitizes
numerous species and is thus not dependent on Kirtland's
Warbler. As it reduces the warbler population, its own
population size will not necessarily decline -- its assault
can be unrelenting. Thus when the decline in warbler
populations was observed, the U.S. Fish and Wildlife Service
(with the aid of several state agencies and the Detroit and
Pontiac Audubon Societies) began a program of trapping and
removing cowbirds. By about 1980 over 40,000 cowbirds had
been removed, and as a result the level of parasitism of
warbler nests became negligible. Kirtland's fledging rates
have tripled, so that their nesting success is now higher
than that known for any other warbler. By 1977 the decrease
in warbler numbers had clearly been halted, and 219 singing
males were counted, a slight increase over 1971. There were
210 pairs censused in 1986, but a sharp decline to 167 in
1987. That rate of recovery seems
quite low, however, considering the great increase in
reproductive success. Survival through the first year of
life seems to be less than 20 percent. This could be due to
high fledgling mortality before migration or perhaps unusual
mortality during migration in recent years. Other
possibilities are that the now limited breeding habitat
makes too small a "target" for inexperienced birds returning
from the Bahamas, that there is simply not enough of that
habitat to support a larger population (although it seems
uncrowded), or that some change unrecognized by
ornithologists has made the northern West Indies a less
satisfactory wintering ground. Ecologists John Terborgh and
David Wilcove have speculated, however, that the main reason
that Kirtland's has not gone extinct already is its
semicolonial breeding behavior which keeps individuals
returning to the small target area. Were it not for that,
failure to find mates might have pushed it down the path
taken by Bachman's Warbler, where returning migrants
scattered over the entire southeastern United States, and
many of the last sightings were of apparently unmated
males. The future of Kirtland's
Warbler remains in doubt. Some Blue jays, important nest
predators, are removed in the course of the cowbird control
program, but more intensive predator control may be
required, along with increased production of suitable pine
stands, if the species is to be saved. The only long-range
solution will be to maintain enough habitat in Michigan for
Kirtland's Warblers to persist without constant human
interference to control the warblers' parasites and
predators, and good luck (or constant vigilance) to keep the
West Indian wintering grounds in suitable
condition. SEE: Brood
Parasitism;
Cowbirds. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.