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Oporornis tolmieiMacGillivray's warbler

By Daniel Karp

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. (Howell, 1995; Pitocchelli, 1995; Sibley, 2000)

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. (Howell, 1995; Morrison, 1983; Pitocchelli, 1995; Howell, 1995; Morrison, 1983; Pitocchelli, 1995; Howell, 1995; Morrison, 1983; Pitocchelli, 1995; Howell, 1995; Morrison, 1983; Pitocchelli, 1995)

  • Range elevation
    3000 (high) m
    9842.52 (high) ft

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). (Pitocchelli, 1995; Sibley, 2000)

  • Sexual Dimorphism
  • sexes colored or patterned differently
  • male more colorful
  • Range mass
    8.6 to 12.6 g
    0.30 to 0.44 oz
  • Average mass
    10.4 g
    0.37 oz
  • Range length
    10 to 15 cm
    3.94 to 5.91 in
  • Average wingspan
    7.5 cm
    2.95 in

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. (Pitocchelli, 1995)

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. (Morrison, 1983; Pitocchelli, 1995)

  • Breeding interval
    MacGillivray’s warblers have one clutch per year.
  • Breeding season
    Breeding occurs from May to early June.
  • Range eggs per season
    2 to 6
  • Range time to hatching
    11 to 13 days
  • Range fledging age
    8 to 9 days

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. (Pitocchelli, 1995)

  • Parental Investment
  • 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. (Klimkiewicz, 1983; Pitocchelli, 1995)

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. (Hutto, 1981; Pitocchelli, 1995)

  • Range territory size
    8000 to 17000 m^2

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. (Pitocchelli, 1995)

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. (Hutto, 1981; Pitocchelli, 1995)

  • 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. (Pitocchelli, 1995)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

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

Commensal/Parasitic Species

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
  • 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. (Pitocchelli, 1995)

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. (Pitocchelli, 1995)

Contributors

Tanya Dewey (editor), Animal Diversity Web.

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

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

diurnal
  1. active during the day, 2. lasting for one day.
ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

insectivore

An animal that eats mainly insects or spiders.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

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.

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