Oporornis tolmiei

Geographic Range

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.

• Biogeographic Regions
• nearctic nearctic
  • native native
• neotropical neotropical
  • native native

Habitat

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.

• Habitat Regions
• temperate temperate
• tropical tropical
• terrestrial terrestrial

• Terrestrial Biomes
• chaparral chaparral
• forest forest
• rainforest rainforest
• scrub forest scrub forest

• Other Habitat Features
• riparian riparian

Physical Description

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).

• Other Physical Features
• endothermic endothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes colored or patterned differently
• male more colorful

Reproduction

MacGillivray’s warblers are thought to be seasonally monogamous and solitary on their winter grounds. Males establish territories and advertise for females with song. Currently, little information exists concerning pair fidelity.

• Mating System
• monogamous monogamous

MacGillivray’s warblers usually breed in secondary growth coniferous forests, along riparian corridors, or in clear-cuts. In Oregon, breeding density was calculated to be 0.56 birds per hectare. Courtship is thought to begin soon after arrival on the breeding grounds, once males have established territories. Nests are made in dense thickets, with a mean nest height of 46.3 cm off the ground. They are cup shaped, composed of an outer layer of stripped leaves and twigs that fasten the nest to the dense understory and an inner layer of softer grasses. Eggs are laid from May to early June, with a mean laying date of June 5th. Clutch sizes range from 2 to 6 eggs, with only one clutch per season. Eggs appear creamy with brown speckles and are usually 17.8 by 13.6 mm. Incubation begins after the first egg is laid and lasts 11 to 13 days.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous oviparous

Females develop brood patches and are the sole incubators of the clutch. The young are unfeathered and altrical at birth, demanding constant feeding and care from both the male and female parents. Hatchlings are left alone for only short intervals, as the females and males forage for insects to feed their offspring. Though both sexes participate, research indicates that females play a larger role in obtaining food for offspring. Young fledge after 8 to 9 days, though they continue to forage as a family for some time after. Unfortunately, no information exists as to how long this dependency period lasts.

• Parental Investment
• altricial altricial
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • male
    • female
• pre-weaning/fledging
  • provisioning
    • male
    • female
• pre-independence
  • provisioning
    • male
    • female

Lifespan/Longevity

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.

Behavior

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.

• Key Behaviors
• flies
• diurnal diurnal
• motile motile
• migratory migratory
• solitary solitary
• territorial territorial

Communication and Perception

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.

• Communication Channels
• visual visual
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

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.

• Primary Diet
• carnivore carnivore
  • insectivore insectivore

• Animal Foods
• insects

Predation

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.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

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

Economic Importance for Humans: Positive

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

• Positive Impacts
• ecotourism ecotourism
• controls pest population

Economic Importance for Humans: Negative

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

Conservation Status

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.

Other Comments

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.

Additional Links

Encyclopedia of Life

Contributors

Tanya Dewey (editor), Animal Diversity Web.

Daniel Karp (author), Stanford University, Terry Root (editor, instructor), Stanford University.

References

Howell, S. 1995. A Guide to the Birds of Mexico and Northern Central America . Oxford: Oxford University Press.

Hutto, R. 1981. Seasonal variation in the foraging behavior of some migratory western wood warblers. The Auk , 98: 765–777.

Klimkiewicz, M. 1983. Longevity records of North American birds: Remizidae through Parulinae. Journal of Field Ornithology , 54: 287-294.

Morrison, M. 1983. Bird community structure on early growth clear cuts in western Oregon. Am. Midl. Nat , 110: 129-137.

Pitocchelli, J. 1995. "The Birds of North America" (On-line). Accessed July 27, 2007 at http://bna.birds.cornell.edu/BNA/account/MacGillivrays_Warbler/ .

Sibley, D. 2000. The Sibley Guide to Birds . New York: Chanticleer/Knopf.