MacGillivray’s Warbler

Famous ornithologists disagreed, then compromised on the name of MacGillivray’s Warbler!

The MacGillivray’s Warbler is hard to distinguish from its doppelganger, the Mourning Warbler. Males of both species have gray heads, dark coloring from the top of their bills to their eyes, and black patches at the base of their necks. Their overall coloring and size are nearly identical and experts have long been divided about how very closely they are related. If it weren’t for the fact that their ranges only barely overlap, most birders wouldn’t be able to tell them apart at all. MacGillivray’s Warblers do have distinct partial eye rings, which appear as white arcs above and below their eyes. And their song is different from the Mourning Warbler if you know what to listen for as you approach thickets or forest undergrowth.

With a range that extends from Alaska and the Yukon Territory all the way to Costa Rica, MacGillivray’s Warblers can be found in most of the western part of North America. They live in the Rocky Mountains and to the west, all the way to the coast. They stick close to the ground and usually pop out only to sing or chase prey over short distances.  

Incredible MacGillivray’s Warbler Facts

• MacGillivray’s Warblers were named by John James Audubon for his friend and coauthor, the Scottish ornithologist, William MacGillivray.
• These birds were also named by ornithologist, John Kirk Townsend, who collected the first recorded specimen and named it for his friend, William Tolmie.
• A compromise allowed MacGillivray’s Warblers to have the specific epithet, tolmiei instead of macgillivrayi.
• Audubon and Townsend died within days of one another in 1851.
• MacGillivray’s Warblers are almost identical to Mourning Warblers.
• The MacGillivray’s Warbler were moved to the Geothlypis genus from the Oporornis genus in 2011.
• These warblers migrate each year from as far north as the Yukon Territory and southern Alaska to Costa Rica.

Where to Find MacGillivray’s Warblers

Like other wood warblers, the MacGillivray’s Warbler spends most of its time on or near the ground. It tends to prefer the dense new undergrowth in clearcuts and disturbed or burned forests. It also frequents clearings with dense shrubs, briars, or woody thickets in both its breeding and winter areas. These warblers can be found along forest edges and cultivated fields or thick grassy areas throughout their migratory range.

MacGillivray’s Warblers breed in the western portion of Canada and the United States, and they migrate south to winter grounds in Mexico and Costa Rica. They live primarily in mountainous regions from the Rocky Mountains west. Their breeding range extends from northern Mexico through much of the western United States, north through British Columbia and Alberta to the southern reaches of the Yukon Territory and the southern tip of Alaska. Their breeding range overlaps slightly with the Mourning Warbler in the Peace Region of British Columbia.

In the United States, MacGillivray’s Warblers stay mostly within the Rocky Mountains and west to the coast. Some migrating birds do spill over to the far western side of the Great Plains, along the western edges of Nebraska and Kansas, and the tip of the Oklahoma panhandle. They migrate south through west Texas, into Mexico, and as far south as Central America, where they spend their winters mostly in the foothills and mountains.

MacGillivray’s Warbler Nests

These warblers nest mainly in forks of low trees or shrubs. They build their nests in a variety of deciduous and evergreen trees and shrubs. These include short maples, alders, fir saplings, scrub oaks, and flowering bushes. They place their cup-shaped nests within dense, low vegetation usually about 2 to 3 feet above the ground. The nests are made of grass, bark, and weeds, and lined with soft grasses, rootlets, or hair.

Scientific Name

The story of how the MacGillivray’s Warbler got its name involves four men and a compromise. The common name for this bird was bestowed by the famous artist, naturalist and ornithologist, John James Audubon. He received a specimen collected in the field by John Kirk Townsend, an ornithologist and naturalist who was working at the time in the Pacific Northwest. Townsend sent lots of specimens to Audubon during that time for further inspection and classification, many of which he named, including the Townsend’s Warbler and others.

Whether Audubon realized it or not, Townsend had apparently already named this particular specimen for his friend, Willian Fraser Tolmie. Tolmie was a young physician and fur trader who was exploring the same region as Townsend. Audubon, upon painting the bird and adding its description to his work, gave it a new name. He named it for his friend and co-author, William MacGillivray, a Scottish ornithologist.

MacGillivray’s Warbler, originally called both Sylvia macgillivrayi and Sylvia tolmiei, was soon given the scientific name Oporornis tolmiei. The genus name Oporornis is derived from Greek words meaning “autumn bird.” In recent years, this warbler along with along with most of the other birds from the Oporornis genus, were moved to the genus, Geothlypis. Today, the MacGillivray’s Warbler is known as Geothlypis tolmiei. There are two recognized subspecies: G. tolmiei tolmiei, named in 1839, and G. tolmiei monticola, named in 1947.

Appearance

MacGillivray’s Warblers are small birds, averaging 3.9 to 5.9 inches in length with a wingspan of approximately 7.5 inches. They weigh only 0.4 to 0.5 ounces. Their size and appearance is very similar to that of a Mourning Warbler, so much that they are primarily differentiated by their separate ranges. In places where they do overlap, only the most experienced birders can tell them apart.

Both MacGillivray’s Warblers and Mourning Warblers have gray heads with black markings at the base of the neck. Their upper parts are olive, their underparts are yellow, and their legs and feet are pink. Like other Passeriformes, they have three toes facing forward and one facing back. They have long, pointy pinkish bills that are black on top. Males have black lores which go from the top of their bills to their eyes, giving them a somewhat spectacled appearance. Both male and female MacGillivray’s Warblers, unlike Mourning Warblers, have a distinct white, partial eye ring that appears as arcs above and below the eye.

MacGillivray’s Warbler Behavior

These warblers are known as skulking birds. They hide in the undergrowth or in dense thickets and are much more easily heard than seen. Sticking close to the ground, they spend much of their time hopping around, foraging for food or darting after prey in short, quick flights. They are a diurnal species, more active during the early morning hours or near dusk. During these times, males are more likely to pop up out of the foliage and sit atop a thicket or on a branch higher in a nearby tree to sing their song. The song is one way birders can tell the difference between the MacGillivray’s Warbler and other similar species. It is described as a rapid, two-note warbling song that increases in volume and ends on a pair of single notes. Recordings of the song sound like, “jeeter-jeeter-jeeter-JEETER-JEETER, jeet-jeet.”

MacGillivray’s Warblers arrive at their northern breeding grounds as early as April, and males begin to search for a mate. They sing frequently, not only to attract a mate but also to defend their territory throughout breeding season. Both males and females use short, harsh calls to defend their nests. They migrate south to the winter grounds from late summer through October and stay there until spring comes again.

Diet

MacGillivray’s Warblers eat mostly insects and insect larvae that it can find near the forest floor and up to about 3 to 5 meters above the ground. It uses its sharp, pointy bill to forage in the leaf litter. It also easily plucks prey from the bark and the foliage of low branches and from the stems and leaves of weeds and grasses. Some of its favorite food items include a wide variety of beetles, true bugs, bees, wasps, ants, and caterpillars. This clever bird has also been seen eating sap from holes in willow trees poked by the sapsuckers which share its territory.

Reproduction

MacGillivray’s Warblers are seasonally monogamous, so they may or may not select the same mate from season to season. Pairs form upon arriving in the breeding territory in the spring. They have one brood with typically 3 to 5 eggs. The female lays her eggs in a cup-shaped nest, usually placed in the fork of a small tree or shrub just 2 or 3 feet off the ground. She incubates the eggs alone for a period of 11 to 13 days.

The chicks are altricial, meaning they hatch helpless and without feathers. They develop quickly though, with both parents feeding them. The chicks fledge after 8 to 9 days, and the parents continue to look after them for an undetermined time until they reach independence.  

Predators & Threats

MacGillivray’s Warblers, like most other warblers, are vulnerable to birds of prey such as owls and hawks. Domestic and feral cats can also pose a threat, since the birds spend so much of their time on or near the ground. Nest predators are perhaps the biggest danger to these birds. They include snakes, and different sorts of mammalian predators such as squirrels, chipmunks, skunks, weasels and raccoons. 

According to research, losses to nest predators may be decreased when these warblers are able to stake out territory in areas with a greater number of suitable nest sites, such as dense, short maple stands, as opposed to more elevated firs.

In addition to predators, the Brown-headed Cowbird is a threat to MacGillivray’s Warblers. This bird parasitizes the nests of other birds, laying its egg in the nest and then leaving the host bird to raise its young. Because Brown-headed Cowbirds are larger than the warblers, they can use up vital resources the warbler chicks need.

Lifespan & Population

Little is known about the average lifespan of MacGillivray’s Warblers. The oldest recorded individual was a little over 4 years of age. There are approximately 11 million adult individuals and the population is declining. The IUCN Red List for Threatened Species lists this bird as a species of least concern.

Similar Animals

• Mourning Warbler – This warbler from the Geothlypis genus is almost identical to the MacGillivray’s Warbler. They are differentiated mainly by their ranges, which mostly do not overlap.
• Kentucky Warbler – Another member of the Geothlypis genus, this warbler has distinctive black spectacles that resemble cat eye glasses.
• Common Yellowthroat – This bird is a warbler from the Geothlypis genus. It has a prominent black mask rimmed in white.

1. American Bird Conservancy, Available here: https://abcbirds.org/bird/macgillivrays-warbler/
2. Wyoming Game and Fish Department, Available here: https://wgfd.wyo.gov/WGFD/media/content/PDF/Habitat/SWAP/Birds/Mac-Gillivrays-Warbler.pdf
3. Bruce Lagerquist, Available here: https://xeno-canto.org/762298
4. Thomas E. Martin, Available here: https://www.umt.edu/mcwru/documents/Martin_Publications/Martin%201993%20BioScience.pdf
5. IUCN Red List, Available here: https://www.iucnredlist.org/species/22721830/138884523#population