Merlin

MacGillivray's Warbler Life History

Habitat

Habitat Open Woodlands

MacGillivray’s Warbler, like its close relative the Mourning Warbler, breeds in disturbed areas of second growth within coniferous and mixed forests. In Canada and the United States, species of spruce, pine, fir, Douglas-fir, cedar, redwood, hemlock, birch, aspen, poplar, cottonwood, and maple are key trees in the vicinity of many breeding areas. MacGillivray’s often nests in places without any canopy, such as following a forest fire or after a forest has been clearcut, when dense thickets often form. Over most of the breeding range, they favor streamside thickets (especially willow, alder, dogwood, rhododendron, and various berry bushes). Forest type seems less important that the presence of a rich understory or scrub community for this species, and the species uses many understory plants including wild plum, salmonberry, snowberry, gooseberry, thimbleberry, chokecherry, huckleberry, cranberry, serviceberry, currant, bitterberry, buckthorn, wild rose, skunkbrush, bitterbrush, and deer brush, along with numerous other flowering plants, forbs, and ferns. In the southeastern portion of Coahuila state in Mexico, an isolated breeding population inhabits high-elevation, chaparral-like habitat in forests of pine, juniper, oak, and madrone. Migrating MacGillivray’s gravitate toward habitats that resemble breeding habitat in structure, with plenty of underbrush, especially near water. In Central America, migrants also use shade coffee plantations and shrubby margins of other agricultural areas. Wintering birds seem to occupy almost every vegetated habitat from mangroves to thorn forest to cloud forest, so long as second growth or scrub is present.

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Food

Food Insects

MacGillivray’s Warblers pluck insects and other arthropods from foliage and branches within their dense habitats. They take some prey on the ground as well. Most of their foraging is low, less than 3 feet off the ground, and they seldom forage more than 6 feet high. MacGillivray’s Warblers eat bugs, leafhoppers, weevils, beetles, bees, wasps, ants, moths, and butterflies (including larvae). There is one report of a bird drinking sap from a sapsucker well.

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Nesting

Nest Placement

Nest Shrub

Nests are set low in shrubs, often at a branch fork.

Nest Description

The female builds a cup nest of grass, bark, leaves, and straw, lined with grasses, root fibers, and hair. Nests measure on average 5 inches across, with interior cup 2 inches across and 1.6 inches deep.

Nesting Facts
Clutch Size:2-6 eggs
Egg Description:

Creamy white, with variable tints and speckling.

Condition at Hatching:

Helpless and naked.

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Behavior

Behavior Foliage Gleaner

The breeding habits of MacGillivray’s Warbler are poorly known. Males sing on their return to breeding grounds, often from conspicuous perches, to claim territory and advertise to females, which arrive slightly later in breeding areas than males. Males defend territories by chasing rival males, and females also defend the area around the nest. Defended areas of territories are generally 2 to 4 acres in extent, but it is likely that some pairs defend larger areas. Courtship displays are unknown. Females build the nest, and both male and female share incubation and chick-rearing duties.

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Conservation

Conservation Low Concern

According to the North American Breeding Bird Survey, populations of MacGillivray's Warbler declined by about 1.7% annually between 1968 and 2015, indicating a cumulative decline of 56% over that period. Partners in Flight estimates a total breeding population of 11 million and rates the species a 12 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. If current rates of decline continue, the species will lose another half of its population by 2086. MacGillivray's Warblers have a preference for regenerating habitats and thus have benefited locally from some types of logging and mining. The reasons for its population declines are unclear. In areas where timber interests have introduced pine plantations, MacGilllvray’s Warblers are no longer able to breed, as these artificial forests lack sufficient understory.

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Credits

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

Pitocchelli, Jay. (2013). MacGillivray's Warbler (Geothlypis tolmiei), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

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.

Sauer, J. R., J. E. Hines, J. E. Fallon, K. L. Pardieck, Jr. Ziolkowski, D. J. and W. A. Link. The North American Breeding Bird Survey, results and analysis 1966-2013 (Version 1.30.15). USGS Patuxtent Wildlife Research Center (2014b). Available from http://www.mbr-pwrc.usgs.gov/bbs/.

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.

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