Among birders, Kirtland’s warblers are a top priority. They are rare because they have very specific habitat requirements — young jack pine forests five to 20 feet tall and six to 22 years old. Such stands are found in northern Michigan and parts of Wisconsin and Canada.
They nest on the ground; presumably trees these size and age classes provide low branches necessary to hide nests from predators. Younger pines lack such cover, and low branches die off on trees older than 22 years.
Historically, wild fires raced through jack pine stands periodically to create and maintain suitable habitat. Early fire suppression efforts by foresters, however, allowed jack pine forests to grow to maturity until biologists realized that Kirtland’s warblers required younger stands. The warbler was declared endangered in 1967.
In 1974 biologists counted just 167 singing males, all in Michigan. The population remained relatively stable through the 1980s at about 200 singing males. During the 1990s, Kirtland’s warblers increased steadily from 265 singing males in 1990, to 891 in 2000, and 1,826 in 2009.
Until 1995 Kirtland’s warblers had only been known to nest in the northern part of Michigan’s Lower Peninsula. Today, they also nest in the Upper Peninsula, and since 2007, they have nested in Wisconsin and Canada.
Last year biologists reported 12 pairs in Wisconsin and two singing males in Ontario. This year’s census results are not yet available, but Chris Mensing, a biologist with the U.S. Fish & Wildlife Service, reports that no significant increase or decrease is expected.
Despite the Kirtland’s warbler’s resurgence, they remain one of the rarest birds in the world and are a favorite target of birders who keep a life list. Annual spring and early summer Kirtland’s warbler tours attract birders from around the country and the world. Earlier this summer about 800 people took the tour over a six-week period.
I saw my Kirtland’s warbler in 1977 while a graduate student at Michigan State. This large warbler sings boldly from tree tops and is often easy to see. It is dark blue-gray above, yellow below, and pumps its tail almost constantly.
It was named by Spencer Baird to honor Dr. Jared Kirtland (1793-1877), a prominent Ohio physician and founder of the Ohio Academy of Natural Sciences.
Aggressive management to create and maintain suitable habitat for the warblers began in the 1970s and continues today. Mensing says, “Controlled burns are too risky, so we manage the jack pine forest with clear cuts.”
More than 150,000 acres of state and federal forest have been designated as warbler habitat, and it’s managed on a 50-year rotation. This provides at least 38,000 acres of nesting habitat at all times.
Another major problem facing Kirtland’s warbler is nest parasitism by brown-head cowbirds. Since 1972, this has been controlled by trapping and removing an average of 4,000 cowbirds from nesting areas annually.
Consequently nest parasitism has plummeted from 69 percent of nests to less than five percent, and the average number of warblers fledged from nests has increased from less the one to almost three.
Compounding the problem of cowbird nest parasitism is the nesting biology of Kirtland’s warblers. They nest only once each season, so each nest lost to cowbirds is a lost year. They do, however, re-nest if a nest is destroyed by a predator.
Though the Kirtland’s Warbler Recovery Plan called for a minimum of 1,000 pairs, and this has been achieved, it is clear that aggressive habitat management and cowbird control is necessary for the warbler’s recovery to continue.
Furthermore, Kirtland’s warblers occupy their nesting grounds for only about four months; then they migrate to wintering grounds in the Bahamas. There they prefer low, brushy vegetation, and winter populations seem secure.
Migration, however, can be dangerous. Crossing large expanses of open water exposes migrants to storms that can kill many birds quickly, so small populations of migratory endangered species are always at risk.
The Fish & Wildlife Service offers tours to birders looking for this rare bird from May 15 to July 4 each year. For more information, google “Kirtland’s warbler tours.”
(Send questions and comments to Dr. Scott Shalaway, RD 5, Cameron, WV 26033 or via e-mail at his website, http://scottshalaway.googlepages.com.)