Merlin

Swainson's Warbler Life History

Habitat

Habitat Forests

Swainson's Warblers nest in moist but not flooded woodlands in the southeastern United States. Over most of the breeding range, they favor mature bottomland hardwood forests with well-developed canopy, dense understory, and extensive leaf litter. In Appalachia, they breed in forested mountain ravines that have abundant understory plants such as rhododendron. More recently, researchers have discovered Swainson’s Warblers nesting in loblolly pine plantations, usually those less than 15 years old (where trees are about 20–40 feet tall). In all cases, their breeding areas contain plenty of understory plants such as Viburnum (arrowwood), holly, palmetto, pawpaw, pepperbush, spicebush, privet, wax myrtle, huckleberry, and giant cane, along with vines such as grape and greenbrier. Wintering Swainson’s Warblers in the Caribbean and southern Mexico also select forests with abundant understory and leaf litter, from lowland mangrove forests to montane forests of many types, including dry limestone forests in central Jamaica.

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Food

Food Insects

Swainson’s Warblers eat a wide variety of insects that live on the ground, including ants, bees, wasps, crickets, grasshoppers, katydids, bugs, flies, beetles, caterpillars, as well as centipedes, spiders, and eggs of many insects and spiders. Wintering birds also consume tiny lizards. They forage almost entirely on the ground, walking slowly through leaf litter and flipping over leaves to search for food, sometimes pivoting or swiveling as they excavate in the leaf litter. Only occasionally do they glean or hawk insects from understory plants.

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Nesting

Nest Placement

Nest Shrub

Swainson’s Warblers place the nest above ground, suspended in vine tangles, cane, or crotches of understory plants.

Nest Description

Females build the nest. An untidy exterior of dried leaves and sticks, perhaps built to appear like a loose collection of leaf litter, conceals a cup of leaves and twigs lined with pine needles, Spanish moss, rootlets, hair, grass, and other fine plant matter. Nests average 5.5 inches across, 3.4 inches tall, with interior cup 2.3 inches across and 1.7 inches deep.

Nesting Facts
Clutch Size:1-7 eggs
Number of Broods:1 brood
Egg Length:0.7-0.9 in (1.8-2.18 cm)
Egg Width:0.6-0.6 in (1.41-1.56 cm)
Incubation Period:11-15 days
Nestling Period:8-12 days
Egg Description:White, usually unmarked, but may have faint reddish brown spotting.
Condition at Hatching:Helpless and naked.
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Behavior

Behavior Ground Forager

Upon arrival in breeding areas in spring, males immediately set up large (up to 45-acre) territories, which they defend vigorously against rivals. Reminiscent of thrushes and waterthrushes, Swainson’s Warblers fly warily and rapidly when investigating intruders in their territories, rarely perching in the open for long. Males chip loudly and chase intruders in flight, sometimes singing, but direct attacks are not uncommon. Threat displays by males, seldom observed, may include spreading and vibrating the wing and tail feathers and raising the crown. A similar display to the female is probably associated with courtship. Females do not appear to be strongly territorial. Swainson’s Warblers seem to have a monogamous mating system, but there are a few suspected instances of polygyny, in which one male mates with multiple females. During the breeding season, members of a pair forage very near each other. Males often bring food to females that are incubating, and both adults feed the young at the nest and after fledging. Migrants select habitats very similar in structure to the breeding and wintering habitats. Wintering birds establish territories and drive away others of their species.

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Conservation

Conservation Low Concern

Swainson's Warbler populations are stable or growing—the North American Breeding Bird Survey estimates a stable trend or slight increase from 1966–2015, while a 2016 estimate by Partners in Flight suggests as much as a 67% increase between 1970 and 2015. Partners in Flight estimates a global breeding population of 160,000 and rates the species 13 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. Their particular habitat requirements make them especially vulnerable to habitat destruction and degradation, including hurricane damage. Lighthouses, communication towers, and other large structures also pose threats to migrants, which often collide with such structures during nocturnal migration, particularly during inclement weather. One study estimated that Swainson’s Warblers were among the most vulnerable of all species to communication tower collisions.

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Credits

Anich, Nicholas M., Thomas J. Benson, Jeremy D. Brown, Carolina Roa, James C. Bednarz, Raymond E. Brown and J. G. Dickson. (2010). Swainson's Warbler (Limnothlypis swainsonii), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Graves, G. R. (2002). Habitat characteristics in the core breeding range of the Swainson's Warbler. Wilson Bulletin 114:210-220.

Lutmerding, J. A., and A. S. Love (2017). Longevity records of North American birds. Version 2017.1. Patuxent Wildlife Research Center, Bird Banding Laboratory, Laurel, MD, USA.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Rosenberg, K. V., J. A. Kennedy, R. Dettmers, R. P. Ford, D. Reynolds, J. D. Alexander, C. J. Beardmore, P. J. Blancher, R. E. Bogart, G. S. Butcher, A. F. Camfield, A. Couturier, D. W. Demarest, W. E. Easton, J. J. Giocomo, R. H. Keller, A. E. Mini, A. O. Panjabi, D. N. Pashley, T. D. Rich, J. M. Ruth, H. Stabins, J. Stanton, and T. Will (2016). Partners in Flight Landbird Conservation Plan: 2016 Revision for Canada and Continental United States. Partners in Flight Science Committee.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2017). The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 2.07.2017. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

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

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

Strong, A. M., and T. W. Sherry. (2001). Body condition of Swainson's Warblers wintering in Jamaica and the conservation value of Caribbean dry forests. Wilson Bulletin 113:410-418.

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