During the breeding season, the highly specialized Kirtland's Warbler is confined to the north-central region of Michigan in about 13 counties. Only 6 or so of these counties have suitable breeding habitat at any one time. The species' wintering range is known only to be in the Bahamas, where it occupies interior scrub and young forest.
In 2007, 2 singing males were observed in Ontario and 8 in Wisconsin. In Spring 2007, a pair nested in Wisconsin, and another pair nested in Ontario. The species had not bred outside Michigan since the 1940's, when this warbler bred in Ontario.
This songbird nests only in dense stands of young jack pines. When trees reach about 22 feet, the birds no longer use them. This means that 2 or 3 years after a fire, the forest is occupied for about 20 years. Small clearings with maples, oaks, and aspen are also important ingredients. The wintering habitat in the Bahamas includes permanent scrub, scrub transitioning to forest, pinelands, and the border between mangrove forests and salt marshes.
Hopping through the thick foliage of jack pines and young deciduous trees, the Kirtland's Warbler picks insects and their larvae from pine needles, twigs, and leaves. This summer diet includes flies, immature grasshoppers, moths, and sawflies. Occasionally, it will hover in front of a plant to pluck an insect from its surface. Ripe berries are also an important food source for adults and young. The winter diet includes many insects and small fruits.
In late spring, the male Kirtland's Warbler arrives before the female and establishes a territory of 8-20 acres, in a loose colony. When territories are being formed, males will attack each other, often in the air, grasping bills. Pairs are typically monogamous and produce one brood, but a second nesting is possible, if the first is lost or fledges early in the season. The female chooses a site under thick cover and uses her breast to depress a small bowl in the soft, sandy ground. She then pushes plant fibers and strips into the bowl, which is lined with grass, moss, and hair.
For about 2 weeks, the female incubates 3-6 whitish eggs, delicately marked with brownish spots. The male delivers food to her and the nestlings, which she keeps warm. In only 9 days, the young leave the nest and can fly weakly. The brood is split between the adults, and juveniles are fed until they are about 6 weeks old.
The Kirtland's Warbler migrates from the Bahamas to central Michigan to breed and probably makes the journey in one, long flight. Few of these warblers have ever been observed in migration. Spring migrants appear to reach Michigan in early May, and their route probably crosses over the Carolinas. Fall migration begins in late August and some adults linger on the breeding grounds into October.