Kirtland's Warbler Life History

Habitat

Kirtland’s Warblers are very choosy when it comes to their breeding habitat. They use young jack pine forests exclusively, typically between 5 and 15 years old with trees about 5–15 feet tall. Jack pine is the dominant tree species, with only a few scattered openings or clumps of oak and low shrubs. The habitat is often extremely dense, especially in the lower branches. As the trees grow and these lower branches die and break off, Kirtland’s Warblers gradually abandon the habitat. On their wintering grounds in the Bahamas, Kirtland’s Warblers are less specific, though they still primarily use young, scrubby habitat. Kirtland’s Warblers are only rarely seen in migration, but the birds tend to use low vegetation, up to about 20 feet off the ground.

Food

Kirtland’s Warblers eat mostly insects (spittlebugs, aphids, ants, wasps, beetles, and caterpillars) and some fruit (blueberries). In the winter they eat insects and fruit, sometimes hunting on the ground, and have been known to probe flowers for nectar.

Nesting

Nest Placement

Kirtland’s Warbler nests are placed on the ground, near the edge of a jack pine thicket and concealed in grass and low vegetation.

Nest Description

The female builds a simple cup of grasses, sedges, and long pine needles, lined with soft rootlets, deer hair, and grassy fibers.

Nesting Facts

Behavior

Kirtland’s Warbler is a bird of dense underbrush, moving deftly through shrubby thickets while continually pumping its tail. Because of the very specific nature of their breeding habitat, Kirtland’s Warbler nests can be concentrated, with as many as 100 males per square mile in some densely populated core areas. Territories tend to be larger when the vegetation is more sparse. A male Kirtland’s Warbler aggressively defends his territory from his neighbors, to the point of fierce physical interactions at the beginning of the mating season when boundaries are not yet established. The male sings from an exposed perch in the tallest trees—in prime habitat with short jack pine trees, this means the birds often sing from oaks. Once a female Kirtland’s Warbler chooses a mate, she searches his territory for an appropriate nest site where she builds the nest. Pairs are seasonally monogamous, and both parents share nestling feeding duties. Kirtland’s Warblers are especially susceptible to nest parasitism from Brown-headed Cowbirds, which sometimes lay as many as 3 eggs in a Kirtland’s Warbler nest. The warblers seem unable to differentiate these eggs from their own.

Conservation

Kirtland’s Warblers are rare and were federally listed as Endangered from 1973 to 2019. Partners in Flight estimates the global breeding population at 4,800 individuals and rates the species a 16 out of 20 on the Continental Concern Score. The group lists Kirtland’s Warbler on the Yellow Watch List for species with restricted ranges. The primary conservation concerns are habitat loss/degradation and parasitism by Brown-headed Cowbirds. Kirtland’s Warblers evolved to use very young stands of jack pine created by wildfires. Fire suppression in the twentieth century led to a decline in suitable habitat as jack pine stands quickly grew too old for the warblers to use. Efforts to restore breeding habitat began in 1957 with attempts to set aside forest in central Michigan and rotate timber harvests so that there would always be some habitat of the appropriate age for the warblers. Intensive nest parasitism by Brown-headed Cowbirds also caused population decline, but in 1971 land managers began trapping cowbirds. Those efforts were very successful, and nest parasitism eventually dropped to 3%, virtually eliminating the problem. When the Endangered Species Act became law in 1973, national and state agencies worked with private interests to establish a recovery plan for Kirtland’s Warblers. This plan has been largely successful in stabilizing the Michigan population, and similar initiatives have been put into place in neighboring Wisconsin and Ontario. It is likely, however, that intensive management will need to continue indefinitely, requiring a private-public partnership for long-term conservation now that the species has been removed from the Endangered Species List.

Credits

Bocetti, Carol I., Deahn M. Donner and Harold F. Mayfield. (2014). Kirtland's Warbler (Setophaga kirtlandii), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Lutmerding, J. A. and A. S. Love. Longevity records of North American birds. Version 2021.1. Patuxent Wildlife Research Center, Bird Banding Laboratory 2021.

Partners in Flight. (2020). Avian Conservation Assessment Database, version 2020.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

Stephenson, T. and S. Whittle (2013). The Warbler Guide. Princeton University Press, New Jersey, USA.

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