Oporornis tolmiei
MacGillivray's warbler

MacGillivray’s warblers breed in western North America as far north as southeastern Alaska and as far south as Central Arizona and parts of southern California. There have been records of them nesting as far east as the Cypress Hills of South Dakota, but they are more commonly found in the Rocky Mountains and west to the Pacific Ocean. MacGillivray’s warblers winter primarily along the pacific slope of Central America from north Mexico to Panama, though they occur as far south as Colombia. Some birds have been known to over winter as far north as southern California (San Diego), but this is quite rare. (Howell, 1995; Pitocchelli, 1995; Sibley, 2000)

MacGillivray’s warblers breed primarily at moderate elevations (up to 3000 m) in secondary growth coniferous forests and riparian corridors. In the north parts of their range they inhabit some deciduous forests and will sometimes breed in chaparral. During migration, they are found in dense shrubs in mountain ranges. In their wintering range, they are generally encountered in humid or semiarid second growth forest. (Howell, 1995; Morrison, 1983; Pitocchelli, 1995)

MacGillivray’s warblers are small warblers, weighing between 8.6 and 12.6 grams (mean 10.4 g). They are 10 to 15 cm long with an average wingspan of 7.5 cm. Adult males in alternate plumage exhibit a broken eye ring, jet black hood, yellow underparts, distinctive black lores, and an olive back. There is considerable sexual dimorphism, as females’ hoods are always a light grey with a whitish chin. Juvenile birds appear similar to adult females, but may show slight black lores. Similar species include both mourning warblers (Oporornis philadelphia) and Connecticut warblers (Oporornis agilis). Though the broken eye-ring is usually diagnostic, mourning Warblers may have a thin broken eye-ring. For definitive identification, a wing length minus tail length value of less than 10 mm will distinguish MacGillivray's warblers from mourning warblers (wing-tail > 19mm). (Pitocchelli, 1995; Sibley, 2000)

The longevity record for a wild MacGillivray warbler is four years and one month. Little information exists concerning the average lifespan or likely causes of mortality, though predation is an obvious threat. (Klimkiewicz, 1983; Pitocchelli, 1995)

MacGillivray’s warblers are highly migratory, arriving in California from Central America during mid-April and departing for the tropics in mid-August to early October. Their migration usually follows the Rocky Mountains. On the breeding grounds males are quite territorial, singing to advertise their presence to possible competitors. Though males may chase each other, fighting rarely occurs. Studies of territory size range from 0.8 to 1.7 hectares. On their winter grounds, these birds are generally solitary, forming seasonally monogamous pairs only upon returning to the breeding grounds. Birds are found hopping along the ground, gleaning insects from vegetation. Besides migration, flight is characterized by short bursts from one piece of vegetation to another. Little interaction with other species occurs, though observations do exist of MacGillivray’s warblers bathing with other warblers. Richard Hutto observed some agonistic interaction with other warbler species; however, such behavior was rare. (Hutto, 1981; Pitocchelli, 1995)

Most communication occurs via vocalization. Males sing during the breeding season to define their territories and determine species identity of other individuals. Occasionally, they may emit a flight song as well. Singing is most frequent at dawn and dusk during the breeding season, usually while sitting 5 to 7 m off the ground. In addition to singing, both males and females may use harsh chip calls to scare intruders away from the nests. Calls may be vocalized year-round. Nestlings also emit noise, using a “buzzy” food-begging call. (Pitocchelli, 1995)

MacGillivray’s warblers are insectivorous, gleaning insects from bark and other vegetation at lower levels in the forest. Food items include beetles, bees, wasps, ants, true bugs, caterpillars, and weevils. Most foraging is in dense, wet thickets along the forest floor. (Hutto, 1981; Pitocchelli, 1995)

MacGillivray’s warblers are preyed upon by accipiters, snakes, and small mammalian predators nest predators, such as squirrels. Females often engage in an injury display to draw potential predators away from their nest. Like other warbler, species, they are cryptically colored in their dense foliage habitats. (Pitocchelli, 1995)

MacGillivray’s warblers are parasitized by brown-headed cowbirds (Molothrus ater), though there is little information as to frequency of occurrence. (Pitocchelli, 1995)

There are no known adverse effects of MacGillivray’s warblers on humans.

MacGillivray’s warblers are exclusively insectivorous, and thus could possibly control a component of pest populations. Warbler species serve as ecotourism draws for birders.

The IUCN evaluated MacGillivray’s warblers as a species of “least concern” in 2004. This classification is justified by the fact that the range is quite broad (2.6 million kilometers squared), and the global population is estimated at 5.4 million individuals. However, MacGillivray’s warblers are listed as protected under the US Migratory Bird Act Treaty, a piece of legislating protecting all North American migratory bird species. Surprisingly, since MacGillivray’s warblers often nest in secondary growth and clear-cuts, evidence exists that populations are actually expanding in response to deforestation both in the winter and breeding ranges. In addition, replanted pine forests, used for logging, often provide suitable habitat for nesting. However, the long-term consequences of deforestation may have negative impacts on this species, as forest composition eventually changes to unsuitable nesting habitat. (Pitocchelli, 1995)

Interestingly, controversy exists surrounding the name “MacGillivray’s warbler.” John Townsend first named the bird Tolmie’s Warbler after Dr. T. Tolmie, an esteemed ornithologist. John James Audubon renamed the species MacGillivray’s warbler, after his close friend Dr. W. MacGillivray, a Scottish ornithologist. MacGillivray’s lack of experience with American ornithology and Audubon’s disregard for Townsend’s priority are sources of controversy for some veteran bird watchers. (Pitocchelli, 1995)