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Kentucky Warbler Life History

Habitat

Habitat ForestsIn breeding season, the Kentucky Warbler uses lowland hardwood forests, often near streams, with dense understory. Mayapple, white avens, spicebush, and many other understory plants are associated with Kentucky Warbler nesting. They need large tracts of forest habitat (over 1,200 acres) for nesting, although gaps such as treefall gaps, trails, or small roads are important for creating a patchwork of shaded and well-lit areas. They inhabit similar gaps in their wintering range, such as tropical lowland rainforest and shade-coffee or cacao plantations. Back to top

Food

Food InsectsKentucky Warblers eat insects and their larvae, spiders, bugs, ants, beetles, grasshoppers, locusts, plant-lice, caterpillars, and fruit. When foraging, they spend most of their time hopping on the forest floor, turning over leaf litter, scratching with feet, and probing with the bill to find prey. They also feed in the understory and lower parts of trees, gleaning insects and occasionally grabbing insects from the undersides of leaves. On their tropical wintering grounds in primary and secondary lowland forests, Kentucky Warblers frequently follow army-ant swarms, capturing prey displaced by the ants. Back to top

Nesting

Nest Placement

Nest GroundFemales choose the nest site, usually at the base of a plant such as a fern or shrub that is well concealed by understory plants. Occasionally females build nests a few feet up in a shrub.

Nest Description

The cup-shaped nest is built on top of large oak leaves and fashioned of coarse grasses lined with fine grasses and rootlets. On rare occasions, females bend living herbs or other small plants over the nest to create a partial dome. Dimensions of the nest average 3.2 inches across by 2.5 inches tall, with the interior of the cup 2 inches across and 1.5 inches deep.

Nesting Facts

Clutch Size:3-6 eggs
Number of Broods:1-2 broods
Egg Length:0.7-0.8 in (1.77-2.05 cm)
Egg Width:0.6-0.6 in (1.42-1.55 cm)
Incubation Period:11-13 days
Nestling Period:8-9 days
Egg Description:Eggs gray to cream with brown blotches and/or dots concentrated at the large end, sometimes wreathed.
Condition at Hatching:Eyes closed and mostly naked with sparse down.
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Behavior

Behavior Ground ForagerMales arrive on breeding grounds before females to sing and establish territory, which they defend from rivals with threat displays (raising the crown, chipping) and chases. Both sexes defend territories and forage mostly within their own territory. During pair-bonding, as many as 5 males and females chase each other around the territory, an activity that attracts others. Pairs are socially monogamous, even though half of nests usually contain a nestling from an “extra-pair fertilization.” Males guard females throughout incubation. After the young fledge, the parents often go their separate ways and split up caring for fledglings. Wintering birds in the tropics maintain and defend territories as well.Back to top

Conservation

Conservation Declining

According to the North American Breeding Bird Survey, Kentucky Warbler populations declined by an estimated 0.69% per year between 1966 and 2019. Partners In Flight estimates a global breeding population of 2.6 million. The species ranks 14 out of 20 on the Continental Concern Score and is on the Yellow Watch List for species with declining populations. Kentucky Warblers avoid forests without understory (such as stands that have been over-browsed by deer) and managed, even-aged stands of timber. They may fare well in large forests that have been logged but retain small thickets here and there. Storm-damaged forests, in which patches of regenerating understory grow rapidly, also attract this species. This species winters in mature lowland forests of Middle America, which are being deforested at a rapid rate. Kentucky Warblers, like most songbirds that migrate nocturnally, often strike buildings and other structures and are killed. Feral and domestic cats probably kill Kentucky Warblers year-round, as the species forages close to the ground where most cats hunt.

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Credits

Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.

McDonald, Mary Victoria. (2013). Kentucky Warbler (Geothlypis formosa), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

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

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2019). The North American Breeding Bird Survey, Results and Analysis 1966–2019. Version 2.07.2019. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

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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