Species Spotlight: The Kirtland's Warbler

Identification

Kirtland’s warblers are best identified by their larger size, gray and black feathering on top, and bright yellow underside. These birds have a white crescent around their eyes that stands out well. There are also slight differences between males and females. Breeding males are usually a brighter yellow with dark marks in front of their eyes. Females are a more brownish gray color with a paler yellow underside. Juvenile birds appear similar to females with a brownish coloring and streaks on their belly (Cornell University Lab of Ornithology, Michigan Natural Features Inventory).

Habitat

The Kirtland’s warbler exclusively occupies young jack pine forests in their northern range. They are very picky when it comes to breeding habitat and will only nest in trees that are 5-15 years old, and around 5-15 feet tall. This is due to the tree’s size. When a jack pine is around this age and height, the Kirtland’s warbler is able to build its nest on the ground at the base of the tree underneath its low branches. These branches provide important cover and shelter for their young. As jack pines grow older, their upper branches absorb all the sunlight and shade out their lower branches, causing them to die. Thus, the Kirtland’s warbler has no cover for its nest and moves on to a younger tree. (Cornell University Lab of Ornithology). 

On their wintering grounds in the Bahamas, this species likes to occupy scrubby areas with low shrubs. They prefer vegetation up to around 20 feet off the ground. 



Diet

The diet of a Kirtland’s warbler consists of primarily insects. Common examples include spittlebugs, ants, aphids, wasps, and beetles. These birds are also known to feed on a variety of other foods like pine needles, grasses, and even blueberries. During the winter months, they feed on insects and fruit. Kirtland’s warblers have also been known to probe flowers for nectar (Cornell University Lab of Ornithology, University of Michigan Museum of Zoology).



Behavior

Kirtland’s warblers are skillful, moving quickly and swiftly through the dense underbrush that makes up their habitat. Territories created by the males tend to be larger when vegetation is more sparse. Males arrive first in the spring and begin singing immediately, with the females soon following. Oftentimes males and females will return to the same territory each year. The males are also very aggressive when defending territories, even to the point of engaging in fierce physical interactions with one another. Kirtland’s warbler pairs are seasonally monogamous, meaning males and females form pair bonds with each other during the breeding season, but they may not be those same pair bonds year after year (USDA Forest Service, Cornell University Lab of Ornithology, University of Michigan Museum of Zoology).



Both males and females share nesting duties when caring for young. Kirtland’s warblers are also highly susceptible to parasitism from brown-headed cowbirds. These birds lay their eggs inside of a Kirtland’s warbler nest, therefore ensuring that the warbler will take care of the cowbird’s young and not its own. A brown-headed cowbird can lay as many as 3 eggs inside of a warbler nest. 

Conservation

The Kirtland’s warbler has a widely known conservation story. They were one of the first species listed with the passage of the Endangered Species Act in 1973. Populations declined due to habitat destruction and cowbird parasitism. With statewide conservation efforts focused on harvesting mature jack pines and planting jack pine seedlings in logged areas, the Kirtland's warbler was officially delisted in 2019. However, this species still remains threatened in the state of Michigan. A recent survey by the U.S. Fish and Wildlife Service in 2021 shows global population estimates to be 2,245 pairs, more than double the 1,000-pair recovery goal.