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.
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 ; acoustic
Perception Channels: visual ; tactile ; acoustic ; chemical
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.
US Migratory Bird Act: protected
US Federal List: no special status
CITES: no special status
IUCN Red List of Threatened Species: least concern
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.
Positive Impacts: ecotourism ; controls pest population
MacGillivray’s warblers are parasitized by brown-headed cowbirds (Molothrus ater), though there is little information as to frequency of occurrence.
Commensal/Parasitic Species:
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.
Animal Foods: insects
Primary Diet: carnivore (Insectivore )
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 (Native ); neotropical (Native )
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.
Range elevation: 3000 (high) m.
Habitat Regions: temperate ; tropical ; terrestrial
Terrestrial Biomes: chaparral ; forest ; rainforest ; scrub forest
Other Habitat Features: riparian
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.
Range lifespan
Status: wild: 4.03 (high) years.
Average lifespan
Status: wild: 86 months.
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).
Range mass: 8.6 to 12.6 g.
Average mass: 10.4 g.
Range length: 10 to 15 cm.
Average wingspan: 7.5 cm.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: sexes colored or patterned differently; male more colorful
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.
Known Predators:
Anti-predator Adaptations: cryptic
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
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.
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.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; 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 ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Male, Female); pre-weaning/fledging (Provisioning: Male, Female); pre-independence (Provisioning: Male, Female)
