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Bay-breasted Warbler Life History



Bay-breasted Warblers breed mostly in mature boreal forests of spruce and fir, less frequently in other coniferous trees such as hemlock or pine, or in mixed forests with birch or maple. During large outbreaks of spruce budworm, they may nest in many different forest types, including very young mixed forests, but typically they use older forests, often sites near water. To reduce competition with other spruce budworm specialists, such as Cape May Warblers, Bay-breasted Warblers forage and nest in the trees’ lower strata. During migration, however, they use many kinds of forests and edges and often forage at even lower levels than during breeding season. Migrants in the Caribbean frequent all habitats, from coastal lowlands to montane forests, and sometimes forage in gardens and parks. Bay-breasted Warblers wintering from Costa Rica to Ecuador and Venezuela tend to remain in wet lowland forests, both primary and second-growth habitats.

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Like most warblers, Bay-breasted Warblers feed chiefly on insects and small spiders during the breeding season. Although spruce budworm—the pupae and larvae of Choristoneura fumiferana—form the most important part of the diet, Bay-breasted also eats larvae of many beetles, flies, midges, moths, and butterflies, along with locusts, grasshoppers, and dragonflies. Breeding Bay-breasted Warblers feed in the middle levels of the tree, usually away from the outer edges of the tree, on larger limbs, where they hop or fly among the vegetation searching out insects methodically and slowly on the upper side of the foliage or in lichen. They usually glean prey while they are perched but sometimes hover-glean or make short sallies to pluck prey. Migrants forage in a similar manner but in many different habitats and at many different levels in the vegetation (even occasionally on the ground), often in company with mixed-species flocks. Their diet includes much more fruit in fall and winter, including berries of dogwood, Virginia creeper, and mulberry during migration and small tropical fruits such as gumbo-limbo in Central and South America, especially during the dry season, in late winter.

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Nest Placement


Nests are most often noted in dense spruce or balsam fir, usually on a branch in the lower third of the tree.

Nest Description

The female, with some assistance from the male, constructs a crude cup of coniferous bark and twigs, dried grass, lichen, spider silk, and plant down, lining it with rootlets, hair, moss, pine needles, and grasses. Nests average about 4.25 inches across and 2.25 inches tall, with interior cup 2.15 inches across and 1.3 inches deep.

Nesting Facts

Clutch Size:4-7 eggs
Egg Description:White or creamy with bold dark spots.
Condition at Hatching:Helpless with sparse brown down.
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Foliage Gleaner

Male Bay-breasted Warblers arrive earlier on the nesting grounds than females and quickly set up territories, usually in areas with abundant food. Males sing from outer branches and also sing as they forage, frequently chasing other males in flight, raising the crown feathers in warning, and at close quarters sometimes snapping the bill loudly. They are also sometimes aggressive against Blackburnian Warblers, which nest higher in the trees but also consume budworms. Courtship and nest site selection are not yet described in this species. Both parents incubate and feed the young. Once young have fledged, Bay-breasted Warblers often join mixed-species flocks, and this behavior continues during migration and on the wintering grounds. Observations on the wintering grounds indicate that Bay-breasted dominates other, smaller warbler species by chasing them, especially when food is scarce. Wintering birds typically follow mixed flocks and forage on fruit, nectar, and insects, usually in the canopy.

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Low Concern

Bay-breasted Warbler is an uncommon species with a remote breeding range that makes it difficult to track population trends. According to the North American Breeding Bird Survey its numbers were roughly stable between 1966 and 2015; Partners in Flight estimates a 9% decline since 1970. Partners in Flight estimates a global breeding population of 9.9 million birds and rates the species an 11 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. Population declines may result from pesticide spraying for spruce budworms, as well as from the reduction of mature forest habitats in favor of managed forests with short harvest cycles. In the case of this species, forest fragmentation appears to decrease populations. The loss of wintering habitat in the tropics, as for many Neotropical migrant birds, probably also diminishes populations of this species. Collisions with communications towers and other structures during migration are common and may also contribute to population declines.

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Venier, Lisa, Steve Holmes and Janet Mci. Williams. (2011). Bay-breasted Warbler (Setophaga castanea), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

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