Kirtland’s warbler

Kirtland’s warbler, (Setophaga kirtlandii), energetic migratory songbird of the wood warbler family whose summer range is limited to Michigan, Wisconsin, and Ontario, and whose winter range includes the Turks and Caicos Islands, the Bahamas, and Cuba. The Kirtland’s warbler is one of North America’s rarest songbirds. It was hailed as a 21st-century conservation success following its population rebound after nearly dying out decades earlier because of its extremely limited geographic range and close association with young jack pine trees (Pinus banksiana) for reproduction.

The bird is named for naturalist Jared Potter Kirtland, on whose Ohio farmland it was collected in 1851, when it was first recorded as a distinct species. The population of the Kirtland’s warbler is suspected to have peaked in the late 1800s or early 1900s following the lumber boom in Michigan. As swaths of pines were cleared by lumberjacks, forest fires burned thousands of sparse, dry acres. These wildfires cleared the land and created an environment favorable for young jack pines to grow. As towns and cities were built, however, roads and firebreaks were put in place. This reduced the number of wildfires, which decreased the Kirtland’s warbler’s habitat, leading to a substantial decline in the species’ population starting in the 1940s.

Natural history

The Kirtland’s warbler is fairly large and round for a warbler, approximately 14–15 cm (5.5–5.9 inches), with an average weight of 14 grams (about 0.5 ounce). Males have a distinctive bright yellow breast, with stone gray upperparts flecked with black. The head features a black mask and a bright eye ring. Females and immature males lack some of the bright coloration on the upper feathers, with brown being more apparent. The bird is often mistaken for the related yellow-rumped warbler or the prairie warbler. The distinctive three-phase song of a breeding male Kirtland’s warbler can help differentiate it. The call is a brief chipping sound.

Kirtland’s warblers are primarily insectivores, feeding on ants, wasps, caterpillars, beetles, and other arthropods. They also supplement their diet with pine needles, grasses, and fruits such as blueberries. Adults are vulnerable to snakes and rodents, which can climb into their nests, whereas eggs and nestlings are sometimes prey for foxes and feral cats. During the breeding season, brown-headed cowbirds (Molothrus ater) often lay their eggs in warbler nests. If the eggs are allowed to hatch, the chicks can compete with warbler nestlings for food. Adult Kirtland’s warblers have developed the ability to recognize cowbird eggs, however; they eliminate this threat by building a new nest on top of the parasitized nest (see also cuculiform: brood parasitism).

Kirtland’s warblers breed almost exclusively in acres of jack pine trees, conifers that grow in sandy soils and have tightly closed seed cones that open only when exposed to the high heat of fire or intense sunlight. As jack pine populations declined in the Midwest during the middle of the 20th century, some birds switched to stands of red pine (P. resinosa) for breeding. Kirtland’s warblers select jack pines that stand between 1.5 and 4.6 meters (5 and 15 feet) tall, making the trees roughly 5–20 years old, for breeding and nesting. The ground-nesting birds may have evolved such particular habitat requirements because of the safety provided by the dense, scrubby lower tree cover that young jack pines usually provide. As the trees age, their upper branches fill in to form a dense canopy, which blocks sunlight from reaching lower branches. These branches atrophy over time, creating open spaces near the ground and making Kirtland’s warbler nests vulnerable to predation. Because of their specific site requirements, the birds generally are in loose colonies in stands of young jack pines, though they are fairly territorial. Males usually return to the same breeding site.

During the bird’s breeding season in late spring, it makes nests made of grasses, pine needles, and leaves. Egg-laying first occurs in May and continues into June. Females deposit a clutch of 3–6 eggs that hatch after a 14-day incubation period. Both males and females provide care, and their young grow quickly and leave the nest about 10 days after hatching. They become sexually mature after one year, and most do not live beyond nine years in the wild.

Conservation status

The International Union for Conservation of Nature and Natural Resources (IUCN) has listed the Kirtland’s warbler as a near threatened species since 2005. The loss of the bird’s breeding habitat during the middle of the 20th century has been the single greatest threat to its survival, resulting in dramatic population decline from 530 singing male birds in the 1950s to 502 in 1961 to perhaps 334 adult birds (based on call surveys that assumed the existence of 167 pairs) in 1987. The Kirtland’s warbler population is also threatened by the reduction of the scrub habitat in the species’ wintering grounds, attributed to climate change and landscape modifications. The birds also die from collisions with wind turbines located along their migration routes. The Kirtland’s warbler was added to the list of endangered species in 1973 (see Endangered Species Act).

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A number of organizations developed a plan to ensure the survival of the Kirtland’s warbler. Conservation efforts are shared by Michigan Audubon, the U.S. Forest Service, the Michigan Department of Natural Resources, and the U.S. Fish and Wildlife Service. In 1980 the wildlife service established the Kirtland’s Warbler Management Area, nearly 6,700 acres (2,700 hectares) that have been planted with jack pines. Controlled burns of forested land are staged to encourage the jack pines to reproduce. Population-control strategies for the brown-headed cowbird, which threatens the survival of young Kirtland’s warblers through nest parasitism, were also put in place. In addition, the Forest Service restricts public access to nesting sites in the birds’ habitat during breeding season.

Taken together, these measures have contributed to the species’ recovery. Surveys report that the population rose to 1,000 birds by 2001 and between 4,500 and 5,000 adult birds by 2016. The Kirtland’s warbler was removed from the endangered species list in 2019.