Kirtland’s warbler

 

First identified less than two centuries ago, North America’s rarest warbler makes an impressive comeback. 

By Tim Tiner

 

Among species at risk, North America’s rarest warbler provides one of the few good-news stories. Brought back from the brink of extinction on its tiny Michigan breeding range, the tame, effusive, tail-bobbing Kirtland’s warbler has multiplied in recent years and now even seems to be making inroads in Ontario.

Kirtland’s warblers are among the most particular and demanding of birds. They spend about eight months of the year solely in the open Caribbean pine woods of the Bahamas, and then fly in early May to the northern half of Michigan’s Lower Peninsula. There, they congregate in large, almost pure stands of dense, young jack pines, 1.5 to 6 metres tall, mixed with small clearings containing blueberry, sweet fern, lichens, moss, grasses and sedge, where these birds nest.

Ornithologists identified the blue-grey-backed, yellow-breasted birds, much valued for their beauty, resonant singing and rarity, only in 1851, and their breeding grounds were not discovered until 1903. The large jack pine stands on which Kirtland’s warblers depend, however, relied primarily on fire to become established, because the trees’ resin-sealed cones remain tightly closed until subjected to intense heat. Effective forest-fire control in the 20th century resulted in a steep decline in this bird species by the early 1970s.

In 1979, the Kirtland’s warbler was declared endangered in Canada on the basis of the belief that it might nest in Ontario, or at least once did. In 1916, an army dentist at Canadian Forces Base (CFB) Petawawa reported singing males there throughout the summer. When they are present, the birds’ loud, clear, persistent song, similar to that of a northern waterthrush, makes them easy to find. The same man went back to the base in 1939 and reported seeing another male. In the subsequent 65 years, only about half a dozen reports of Kirtland’s warblers on breeding habitat in Ontario have been made, though scores of sightings of migrants, most heading north in spring, were reported.

Recently, however, that pattern changed. Surveys conducted over the past decade, part of a recovery effort led by the Canadian Wildlife Service (CWS), hit pay dirt in May and June 2006, finding three singing Kirtland’s warblers at CFB Petawawa. The following June, an active nest was discovered, the first ever in Canada, hidden amid grass and blueberry beneath a small jack pine in the middle of a clearing on the base. Two young fledged from the nest. Small numbers of singing males have been found on the base every year since. Another nest was reported in 2008, two in 2009 and one early this year.

Profile

Scientific name: Dendroica kirtlandii (from the Greek words dendron oikeo, “tree dweller,” and the surname of Jared P. Kirtland, on whose farm the bird was first identified)
Length: 14–15 cm
Wingspan: 22–23 cm
Weight: 12–16 grams
Breeding territory: 3–8 hectares on average
Clutch: 3–6 eggs
Incubation period: 13–15 days
Fledging age: 9–11 days
Food: Aphids, spittlebugs, ants, beetles, caterpillars, wasps and blueberries

Ontario’s good fortune likely stems from the success of rescue measures in Michigan that increased the state’s annual count of singing males from 167 in 1974 to more than 1,800 last year. Growing densities have pushed first-time breeders farther afield, and scattered nestings are now being reported in Michigan’s Upper Peninsula and Wisconsin. The U.S. Forest Service and the Michigan Department of Natural Resources manage some 76,000 hectares on the Lower Peninsula for Kirtland’s warblers, cutting and replanting to maintain at least 15,000 hectares of young jack pine forest, six to 20 years old. Prescribed burns were discontinued when they proved too difficult to control.

Michigan’s recovery program also includes annual trapping and euthanizing of thousands of brown-headed cowbirds, nest parasites that severely reduce Kirtland’s warbler reproduction. Cowbirds, which spread into the area from the prairies in the 1880s, lay eggs in nests of other birds and leave them to be hatched and raised by the hosts. Cowbird mothers destroy any warbler eggs that they replace with their own. Their burly offspring hatch two days earlier than the remaining warbler young and then elbow them out of the way at feeding time, often trampling them. Cowbird fledglings can weigh more than twice as much as the warbler parents feeding them.

Though cowbirds are not a problem at CFB Petawawa, Canada’s Kirtland’s warbler recovery strategy will probably replicate many of the practices used in Michigan. For now, though, much of the focus remains on looking for the birds. Paul Aird, professor emeritus with the University of Toronto’s Faculty of Forestry, has been at it for more than 30 years and was responsible for most of the breeding-habitat sightings during much of that period. One of three members of Kirtland’s Warbler Recovery Team, he is convinced that many more birds will be found, given Ontario’s vast jack pine woodlands and the consistent reports of northbound spring migrants from Point Pelee to Cabot Head, on the Bruce Peninsula.

“We hope to find a lost Kirtland’s warbler village somewhere,” says Aird of the birds, whose nesting territories tend to cluster together in colonies. “Somebody should find it.”

Since 2002, surveys by Ontario’s Ministry of Natural Resources (MNR), CWS, private consultants and volunteers have perused promising habitat from Chapleau and North Bay to Bancroft and the Bruce Peninsula. But few of the sites checked so far are optimal, says Daryl Coulson, MNR’s Pembroke district ecologist and another member of the Kirtland’s recovery team. Most of central Ontario’s jack pine forests, he notes, have been replaced or converted with stands that are not dense or large enough for the birds – which usually nest in woods covering more than 80 hectares – or have been mixed with or replaced by red and white pine. In his own district, covering mainly Renfrew County and including CFB Petawawa, only about four suitable sites have been identified so far.

Ontario has begun some forest management with Kirtland’s warblers in mind, removing poplar saplings in a couple of square kilometres in Algonquin Provincial Park several years ago to favour jack pine. According to Coulson, the proposed new forest management plan for his district, which takes effect in 2011, includes specific prescriptions to create habitat for the warblers. He has also provided technical information to other forest management units that he hopes will follow suit in the future.

“One of the objectives of these plans is to maintain forest diversity,” says Coulson. “I think there is enough information and evidence to say with reasonable confidence that this bird has been here historically. It’s part of the biodiversity of this province. And because of its rarity globally and provincially, it warrants that effort.”

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contribs_tinerTim Tiner is the co-author of a series of best-selling Ontario nature guide books.

Comments

One Comment on "Kirtland’s warbler"

  1. Ken McIlwrick on Thu, 25th Oct 2012 5:55 pm 

    I would like to contact Tim Tiner regarding the Kirtland’s Warbler to find out if he knows who in Ontario would be an authority on the bird and who would be an authority on its songs and calls.

    Thanks Ken McIlwrick

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