The Kirtland's warbler spends part of the year in northern central Michigan. The area it lives in is about 100 miles long and 60 miles wide. It migrates south and spends the winter in the Bahamas.
A bird that spends the breeding season in North America and winters in the tropics is called a neotropical migrant. Does the Kirtland's warbler fit that definition?
The Kirtland's warbler nests in groves of young Jack pines (Pinus banksiana). They prefer trees that range in height from 5 to 18 feet. They also seek out areas with ground cover composed of blueberries, bearberry, or sweetfern. These warblers also require a very specific soil type, the Grayling Sands. They nest on the ground, and their nests would be flooded if rain water did not drain away quickly. The Grayling Sands drain quickly enough for the birds to nest there. But the Grayling Sands are only found in certain places. That is why nearly 90% of these birds breed in the drainage area of a single stream.
Their winter habitat in the the Bahamas consists of low scrub. During the night they retreat to higher shrubs to roost.
Nests are constructed in late May, followed by the laying of eggs in late May to mid-June. The number of eggs per clutch ranges from 3 to 6. In the rare event that a pair has a second clutch, fewer eggs will be laid than in the first. The female incubates the eggs for approximately 14 days before hatching occurs. During the incubation period, defense and care of the nest are predominantly the responsibilty of the female; however, the male will bring food to her. After the eggs hatch, both parents tend to the needs of the altricial young. The young gain weight rapidly during the first five days after hatching, doubling their weight every two days. During the last three days in the nest, their growth rate decreases and their energy is directed towards providing their own body warmth, plumage development,and heightened physical activity. The young leave the nest 9-10 days after hatching. During the fledging period, the brood is divided in half, each parent taking care of select offspring. The post juvenile molt occurs approximately one month after fledging.
Males are the first to arrive at the nesting grounds in the spring, and they begin singing immediately. Females soon follow with late arriving males. Each male usually returns to the territory that he occupied the previous year; however, this true of females. Males aggressively defend their territories against other males, but territories are usually in close proximity to those of neighboring males, indicating a tendency towards loose colonialism.
This species feeds primarily on insects; however, it is known to sample an array of other food materials including pine needles, grasses, and bluberries. Food is gathered by gleaning and flycatching on the wing.
The warbler's endangered species status has affected the ability of private landowners to develop property containing warbler habitat.
The Kirtland's Warbler has been the focus of much attention over the last 25 years because of its rarity and need for a very specific habitat. Natural forest fires were the original providers of such habitat, but the advance of white settlers resulted in the clearing of much of Michigan's natural forests. At first, the warbler benefited from such clearing; however, so did the Brown-Headed Cowbird (Molothrus ater). The Cowbird had a major impact on the warbler as a nest parasite. Cowbirds lay their eggs in the nests of other birds and leave the rearing of their young to these hosts. Cowbird young usually develop much faster than the young of the host species and are thus able to out-compete the hosts' young for food resources. Many of the hosts' young die as a result. Recognition of the effects of this phenomena on the Kirtland's Warbler in the early 1970's lead to a program of killing Cowbirds in the warbler's range. This program, coupled with the management of breeding grounds by way of controlled burns, has significantly aided the warbler. However, the Kirtland's Warbler seems to be faced with other problems that effect it during migration or during its time in the Bahamas. As a result, the breeding population in Michigan has not changed significantly recently from spring to spring.
Ethan Kane (author), University of Michigan-Ann Arbor.
Harrison, Hal H. 1984. Wood Warbler's World. Simon and Schuster, New York. pp. 172-178.
Mayfield, Harold. 1960. The Kirland's warbler. Cranbrook Institute of Science, Bloomfield Hills, MI. pp. 18, 19, 44, 45, 53, 54, 80, 84, 87, 89, 91, 100, 106, 107, 110, 123, 124.
Peterson, Roger Tory. 1980. A Field Guide to the Birds East of the Rockies. Houghton Mifflin, Boston. pp. 234-235.
Schreiber, Rudolf L., Anthony W. Diamond, Roger Tory Peterson, and Walter Cronkite. 1989.
Save the Birds. Houghton Mifflin, Boston. pp. 190- 191.
Mayfield, H. F. 1992. Kirtland's Warbler (Dendroica kirtlandi). Birds of North America, 19:1-16.