Blue-gray Gnatcatcher Life History

Habitat

Blue-gray Gnatcatchers nest and forage in a broad range of wooded habitats across their extensive breeding range. Although they shun coniferous forests lacking rich understory growth, they use a range of broadleaf and mixed woodlands from chaparral to mature forests. They prefer moist areas, often near habitat edges. In the northernmost parts of their range, they nest along rivers and streams or near lakes, particularly in ash, maple, and oak forests. In the Southwest, oak and pinon-juniper woodlands, chaparral, and willow and cottonwood woodlands near water all provide habitat. Throughout the south-central and mid-Altantic states, they claim territories in upland broadleaf and mixed forests as well as along streams and rivers. Farther south, they add swamp forests and pine flatwoods with oak understory. They favor the edges of forest gaps, so extensive patch-cut logging can provide abundant habitat. Their winter habitats are similarly various from region to region, and may include cypress swamps, citrus orchards, mangroves, savannah with scattered groves, and a range of woodlands from sea level swamp forests to highland oaks. Back to top

Food

Blue-gray Gnatcatchers eat small insects, spiders, and other invertebrates. Prey species vary across their extensive range. In the Southwest, for example, prey include treehoppers, froghoppers, leaf hoppers, plant bugs, tree bugs, leaf beetles, weevils, wolf spiders, caterpillars, and grasshoppers. In the Northeast, adult and larval moths can provide up to half of prey taken. The smallest prey are swallowed alive. The wings are torn off larger prey and their bodies beaten on a perch prior to being eaten. Parents generally feed the young these same foods, offering progressively larger whole prey as the chicks mature. Back to top

Nesting

Nest Placement

Male and female jointly choose a nest site, usually in a live broadleaf tree in a less dense bit of their territory. Nests are built well out on side limbs, often saddled against a side branch or around a twig or knot for support. Nests tend to be higher than the midpoint of the tree.

Nest Description

Both sexes cooperate in building the neat, open, cuplike nest. They take up to two weeks to build the 2–3-inch wide nest, which is held together and attached to its branch with spider webbing and decorated with lichen. The nest's high walls are built in flexible layers. The main structural layer is built of fibrous materials like plant stems, bark strips, and grasses, all held together by spiderweb or caterpillar silk. Inner layers become progressively finer, and the roughly 1.5-inch-wide cup is lined with plant down, paper, cocoons, hair, or feathers. The outside is covered with webbing or silk decorated with bits of lichen or bark flakes. They often build a series of nests during a summer to counteract the effects of predation, mite infestations, or cowbird parasitism. Materials from earlier nests are frequently recycled to build later nests, which may be why they are usually completed more quickly than first nests. The male often builds second nests nearly solo, with the female finishing the inside of the first nest with softer materials.

Nesting Facts

Behavior

The Blue-gray Gnatcatcher is a busy forager. It flits through dense outer foliage, hops and sidles along branches, peering with quick head movements to glean small insects and spiders. Flashing the white edges of their long tail may help them flush prey, which they then snap up while the gnatcatcher is perched, hovering, or sallying into the air after them. Pairs bond quickly on the breeding ground, with the male's initial aggression toward intruders softening as he leads the female around the territory. They explore possible nesting sites together, with one or the other occasionally mimicking nest-building. These gnatcatchers are pugnacious defenders of their territories, attacking much larger birds as well as intruding neighbors. Adults mob potential predators with other small birds. The male and female of a pair often cooperate in challenging an interloper. The male's soft territorial songs and the female's aggressive calls quickly turning to extended chase, usually by the male. More intense aggression involves soaring aerial confrontation and bill snapping, and even prolonged grappling on the ground. Although the female does most of the brooding, the male shares nest-building, incubation, and the feeding of nestlings and fledglings. While there seems to be little predation on adults, nestlings and eggs are taken by jays, magpies, and woodpeckers, and probably by snakes, crows, grackles, raccoons, squirrels, and chipmunks. Back to top

Conservation

Blue-gray Gnatcatchers are numerous, and their overall populations have increased by approximately 0.3% per year between 1966 and 2019, according to the North American Breeding Bird Survey. Growth appears to have been particularly strong in the West. Partners in Flight estimates a global breeding population of 160 million and rates them 7 out of 10 on the Continental Concern Score, indicating a species of low conservation concern. The largest populations are in the southeastern U.S. Despite their high numbers, nests are very vulnerable to cowbird parasitism. Adult gnatcatchers attack intruding cowbirds, but once a cowbird has laid an egg in their nest, these tiny birds have no ability to eject or puncture it and will raise the chick.

Credits

Kershner, Eric L. and Walter G. Ellison. (2012). Blue-gray Gnatcatcher (Polioptila caerulea), 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. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.

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

Root, R. B. (1969b). The behavior and reproductive success of the Blue-gray Gnatcatcher. Condor 71:16-31.

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.

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