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Empidonax difficilisPacific-slope flycatcher

By William Love Anderegg

Geographic Range

Pacific-slope flycatchers winter in southern Mexico from the southern end of Baja California, along the coastal lowlands of the Pacific coast, to Oaxaca. They breed along the Pacific coast, from northern Baja California to southeastern Alaska. Their range stretches east to the Rocky Mountains in British Columbia, the Cascades in Oregon, and the Sierra Nevada mountains in California. (American Ornithologists’ Union, 1998; Lowther, 2005)

Habitat

Pacific-slope flycatchers breed in humid coniferous, dense second-growth, and mixed deciduous-conifer woodlands. They have been found throughout British Columbia, Washington and Oregon in old-growth Douglas fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), sugar pine (Pinus lam-bertiana), and incense cedar (Calocedrus decurrens) forests. They are associated with shady, riparian habitats. They are primarily found at elevations from 0 to 1500 m. In British Columbia they have been found in red cedar (Thuja plicata) forests, primarily along dense creek vegetation and ravines, often near lakes and ponds. In Mexico, they winter in mountainous conifer forests, tropical deciduous forests, and tropical lowland evergreen forests. (American Ornithologists’ Union, 1998; Campbell, et al., 1997)

  • Range elevation
    0 to 1500 m
    0.00 to 4921.26 ft

Physical Description

Pacific-slope flycatchers are small perching birds around 14 to 17 cm in length and with a mass of 9 to 12 g. They have a relatively large head in comparison to their body, with a faint white to yellow teardrop-shaped patch around each eye. They have broad bills with a lower mandible that varies from yellow to light pink, distinguishing them from other flycatchers. Dull olive or brown feathers comprise the upperparts and back, with more pale and yellow feathers beneath. These flycatchers have relatively short wings (60 to 70 mm), longer tails, gray legs, and faint yellow wing bars.

Western flycatcher species (Pacific-slope and Cordilleran flycatcers) are difficult to distinguish from other flycatchers and each other. The olive-green back, almond-shaped pale eye patch, and gray legs differentiate Pacific-slope flycatchers from yellow-bellied flycatchers (Empidonax flaviventris), Acadian flycatchers (Empidonax virescens) and pine flycatchers (Empidonax affinis). The species are further differentiated by song. Pacific-slope flycatchers are indistinguishable from Cordilleran flycatchers (Empidonax occidentalis) in the field and almost impossible in the hand. However, Pacific-slope flycatchers are more often found in lower elevation, humid forests while Cordilleran flycatchers are often found in higher elevations, in dry coniferous forests. (Lowther, 2005)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    9 to 12 g
    0.32 to 0.42 oz
  • Range length
    140 to 170 mm
    5.51 to 6.69 in
  • Range wingspan
    60 to 70 mm
    2.36 to 2.76 in

Reproduction

Pacific-slope flycatchers are mostly monogamous. A single study done on mating Pacific-slope flycatchers in British Columbia reported that 1 in 7 males partnered with 2 females. (Ainsley, 1992)

Pacific-slope flycatchers usually hatch several broods in a given breeding season from mid April to mid July. In Monterey County, California they have been found to build nests from April 15 to May 1 for first broods and June 1 to July 15 for second broods and re-nesting. Mean clutch size is four eggs per brood. Incubation time lasts between 13 and 16 days and fledging time is around 14 to 17 days after hatching. Age is presumed to be one year at first breeding and they breed annually afterwards. (Lowther, 2005)

  • Breeding interval
    Pacific-slope flycatchers breed once annually.
  • Breeding season
    Pacific-slope flycatchers breed from mid-April through mid-July.
  • Average eggs per season
    4
  • Range time to hatching
    13 to 16 days
  • Range fledging age
    14 to 17 days
  • Range time to independence
    14 to 17 days
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Average age at sexual or reproductive maturity (male)
    1 years

The female alone incubates the eggs but both sexes bring food to the fledgling young and remove fecal sacs. (Lowther, 2005)

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

Lifespan/Longevity

Longest lifespan in the wild is recorded at 6 years based on museum specimens. Longevity in captivity and expected lifespan is unknown (Lowther, 2005)

Behavior

Like other flycatchers, Pacific-slope flycatchers feed by sitting on an exposed perch and sallying forth to catch insects from the air and from leaves. Pacific-slope flycatchers migrate annually between wintering and breeding ranges. They are mainly solitary outside of the breeding season. (Davis, et al., 1963; Lowther, 2005)

  • Range territory size
    1.5 to 3.5 km^2

Home Range

They are territorial and typical territory size is between 1.5 and 3.5 hectares. They typically defend their territory from conspecifics by giving chase, chirping, buzzing, snapping their bills and occasionally locking feet with a rival and fluttering to the ground. (Davis, et al., 1963; Lowther, 2005)

Communication and Perception

Only males sing and the song primarily seems to serve to attract mates and delineate territories. Males sing extensively during the breeding season, but both males and females can give alarm calls in the presence of danger or predators. (Ainsley, 1992; Davis, et al., 1963; Lowther, 2005)

Food Habits

Pacific-slope flycatchers feed almost entirely on insects caught in the air or on tree and bush leaves. They prefer to forage in the middle and lower canopy of forests. They use a hawking method of prey capture; sallying forth from a perch to catch insects on the wing, then returning to the perch. Their main foods are bees, wasps, moths, spiders, flies, and other insects, though they occasionally consume vegetation like blackberry and elderberry leaves. (Lowther, 2005)

  • Animal Foods
  • insects
  • Plant Foods
  • leaves

Predation

Little is known about predation on Pacific-slope flycatchers. However it is likely that they are vulnerable to the main predators of other small, forest birds, including hawks, squirrels, snakes, and jays. Records exist of both Steller's jays (Cyanocitta stelleri) and scrub jays (Aphelocoma californica) preying on Pacific-slope flycatcher nests. (Lowther, 2005)

Ecosystem Roles

Pacific-slope flycatchers are predators to many insects and spiders and in turn are consumed by hawks, squirrels, snakes, jays, and other predatory birds. They are parasitized by brown-headed cowbirds (Molothrus ater). Aside from this, little else is known about the role of these flycatchers in their ecosystem. (Lowther, 2005)

Economic Importance for Humans: Positive

Aside from controlling insect populations, there are few other documented benefits to humans. (Lowther, 2005)

  • Positive Impacts
  • controls pest population

Economic Importance for Humans: Negative

There are no known adverse effects of Pacific-slope flycatchers on humans.

Conservation Status

While long term population trends have not been quantified, Pacific-slope flycatchers are a species of Least Concern by the IUCN Redlist. They are not listed under the U.S. Endangered Species act or the Convention on International Trade in Endangered Species. They are listed and protected under the U.S. Migratory Bird Treaty Act. Breeding Bird Survey data show no significant declines of this species between 1966 and 1996. (Lowther, 2005)

Contributors

Tanya Dewey (editor), Animal Diversity Web.

William Love Anderegg (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.

arboreal

Referring to an animal that lives in trees; tree-climbing.

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

chemical

uses smells or other chemicals to communicate

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

polygynous

having more than one female as a mate at one time

riparian

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

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

taiga

Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.

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

"IUCN Redlist of Threatened Species" (On-line). Accessed May 11, 2007 at http://www.iucnredlist.org/search/details.php/49951/summ.

Ainsley, D. 1992. Vocalizations and nesting behaviour of the Pacific-slope Flycatcher, Empidonax difficilis. M.Sc. thesis, Univ. of Victoria, Victoria, BC.

American Ornithologists’ Union, 1998. Check-list of North American birds. Washington D.C.: American Ornithologists’ Union.

Campbell, R., N. Dawe, I. McTaggart-Cowan, J. Cooper, G. Kaiser. 1997. The birds of British Columbia. Vol. 3—Passerines: flycatchers through vireos. Victoria, Canada: R. Br. Columbia Mus..

Davis, J., G. Fisler, B. Davis. 1963. The breeding biology of the Western Flycatcher. Condor, 65: 337-382.

Lowther, P. 2005. "Pacific-slope Flycatcher" (On-line). The Birds of North America Online. Accessed May 11, 2007 at http://bna.birds.cornell.edu/BNA/account/Pacific-slope_Flycatcher/.

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