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Empidonax trailliiwillow flycatcher

By Emily Brazil and Leila Siciliano Martina

Geographic Range

Willow flycatchers (Empidonax traillii) may be found in North, South or Central America, depending on the time of year. Their North American range extends from southern Canada throughout the United States. Willow flycatchers are often misidentified as alder flycatchers, and although these species may be found in overlapping ranges, willow flycatchers typically have a more southern range, nesting primarily south of Canada. During their migration, they are found in the southern United States. Willow flycatchers winter in Mexico, Central America and as far south as northern Columbia. (Crossley, 2011; Kaufman, 1996; Phillips, 1948; Sedgwick, 2000; Sibley, 2003)

Habitat

Willow flycatchers are found in moist environments where their main prey source, flying insects are abundant. They may also be found in semi-arid landscapes, the borders of forests, dry, upland areas, mountain meadows and riparian forests. Willow flycatchers inhabit a variety of areas, their preferred habitat is within low growing willow thickets, but they also favor short, shrubby areas. These areas are often near a water source, particularly among the endangered southwestern subspecies, which are generally found near rivers. Their breeding behavior generally occurs near areas with brushy thickets. During their wintering period, willow flycatchers are often found in clearings near tropical secondary growth forests. (Alderfer, 2006; Kaufman, 1996; Kaufman, 2000; Sedgwick, 2000; Yong and Finch, 1997)

Physical Description

Willow flycatchers are relatively small compared to other passerines, but they are larger than many other members of their genus (Empidonax). Adults are generally 13.3 to 17.0 cm in length, with an average bill size of 8.85 mm among females and 9.5 mm among males. Adults weigh between 11.3 to 16.4 g. There are no obvious differences in appearance between the sexes, with little differentiation between male and female plumage. Although their coloration may vary based on a population’s range, adults have brownish olive backs, dull off-white breasts and white throats. Their head is relatively flat, with a fairly long bill and a weak white eyering. Their wing tips are typically long, but the wings themselves are generally short. Their wings are black, with two white bars across the top and their tail is fairly long and thick. Juvenile willow flycatchers have a similar appearance, although they also have off-white wing bars. Due to the slight crest on their crown, willow flycatchers resemble eastern wood-pewees, although willow flycatchers have a smaller body size. Likewise, willow flycatchers are extremely similar in appearance to alder flycatchers, although willow flycatchers generally have browner plumage and a less noticeable eyering. In both cases, these species are best distinguished by their songs. (Alderfer, 2006; Crossley, 2011; Kaufman, 2000; Phillips, 1948; Sedgwick, 2000; Sibley, 2003)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    11.3 to 16.4 g
    0.40 to 0.58 oz
  • Average length
    8.85 (female); 9.5 (male) mm
    in

Reproduction

Willow flycatchers tend to be monogamous, but there are a few recorded instances of polygyny. Polygynous males usually divide their time between females. Song is likely important when maintaining a pair bond. Willow flycatchers seem to have a resource based mating system. When resources are abundant at a breeding site, it is to the bird’s advantage to remain there, rather than searching for a territory. Willow flycatchers frequently return to previous breeding sites after successful breeding years, however, dispersal is also common. It is less costly for females to disperse because, unlike males, they do not defend their territory. (Sedgwick, 2000; Sedgwick, 2004)

Willow flycatchers tend to have about a month and a half window, during which egg laying, hatching and fledging occurs, but it varies by location. Their cup nests are composed of grass, bark and other plant materials and are generally built in deciduous trees about 1.2 to 4.5 meters from the ground. They lay eggs from June to August and typically have clutches of 3 to 4 whitish eggs with brown spots. Willow flycatchers incubate their eggs for an average of 12 to 14 days. In some instances, such as in the northern Sierra Nevada region, fledglings were observed as late as the beginning of September, so there is great variation in breeding seasons, but it is mostly concentrated in the summer to early fall. (Kaufman, 1996; Sedgwick, 2000; Sedgwick, 2004)

  • Breeding season
    Willow flycatchers typically breed from June to August, but it may vary based on their range.
  • Average eggs per season
    4
  • Average time to hatching
    12 to 14 days
  • Average fledging age
    2 to 3 weeks

Females do the majority of the egg incubation; sitting on the eggs mostly during the day, and at an unknown stage, sitting on the eggs overnight as well. While females watch the nest, they turn the eggs using their beaks and feet. Once the eggs hatch, females remove the eggs shells from the nest. The female is the main presence in the nest, shading their altricial nestlings during the first few days after hatching. Both parents feed the young, although females do so at a much higher frequency. The nestling stage may last anywhere from 13 to 16 days, followed by the fledgling stage. By about two weeks of age, fledglings are able to fly short distances, but remain near the nest for three to four more days. Juveniles leave natal territories 14 to 25 days after hatching. (Sedgwick, 2000)

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

Lifespan/Longevity

There is currently very little information available regarding the lifespan of willow flycatchers. The lifespan of the endangered southwestern subspecies has been estimated at 1.02 to 1.63 years. The oldest known willow flycatcher was caught 11 years after its original tagging, although its age at the time of tagging is unknown. Similarly, several willow flycatchers have been caught 5 years after their initial banding. (Finch and Stoleson, 2000)

  • Range lifespan
    Status: wild
    11 (high) years
  • Typical lifespan
    Status: wild
    1.02 to 1.63 years

Behavior

Willow flycatchers are solitary birds that come together and usually form monogamous bonds during the breeding season, although there are noted cases of polygyny. They are also relatively aggressive birds, especially when another bird is in close proximity to their nest. They often attack larger birds that approach their nest when nestlings are present. Male willow flycatchers often defend a territory larger than what they need to survive. This may accommodate the possibility of a sudden reduction in food and/or an increase in predators in their territory. Males defend territories that provide sufficient energy needs for all present in the area during the breeding and nesting season. Males engage in a “sit and wait” foraging strategy, where they rest, search for prey and watch for intruders all at the same time. These birds travel over gulf coast areas to migrate in the spring, generally from mid- to late May. They also travel in the early fall, generally during August and September. (Alderfer, 2006; Kaufman, 1996; Prescott and Middleton, 1988; Sedgwick, 2000)

  • Range territory size
    1,000 to 5,000 m^2
  • Average territory size
    3,000 m^2

Home Range

The territory sizes maintained by willow flycatchers can vary greatly. Their reported home range sizes range from 3,000 meters squared (+/- 2,000 m2) up to 18,000 m2. Their wintering territories are smaller, generally around 1,100 m2, allowing for a specific foraging area to be defended with less energy expenditures. (Prescott and Middleton, 1988; Sedgwick, 2000)

Communication and Perception

There are three vocalizations typically used by willow flycatchers including sounds like 'fitz-bew', 'creet,' and 'fizz-bew'. Males mainly vocalize; however, females respond to mates and vocalize when they perceive a threat. Willow flycatchers perform a distinct call when they first arrive in the spring, through the pre-nesting season. When mating season begins, a new call is used. The season begins with long calls, or songs, and as the season progresses, the songs become shorter. Their breeding call is related to mate attraction and they rarely sing while migrating. (King, 1955; Sedgwick, 2000)

Food Habits

Willow flycatchers are mainly insectivorous aerial foragers, with up to a 96% insectivorous diet. Their other diet components generally include berries such as raspberries, blackberries and dogwood berries. Their main prey items are bees, wasps, ants, flies, butterflies and moths, with specific species differing by territory location. In order to catch prey, they generally perform either a perch to prey or hovering behavior. During perch to prey behaviors, birds swoop from their perch down to prey. Willow flycatchers are often seen hunting near brushy habitats. (Bakian, et al., 2012; Sedgwick, 2000; Sibley, 2003)

  • Animal Foods
  • insects
  • Plant Foods
  • fruit

Predation

Willow flycatchers may be preyed on by various mammals, birds and reptiles. Their eggs have been found in the stomach contents of dissected milk snakes and common king snakes. Cooper's hawks, great horned owls, long tailed weasels, voles, and common ravens are also known predators, although their specific mammalian predators may vary by territory location. When willow flycatchers encounter the brood parasites brown-headed cowbirds, they often chase them and vocalize less. (Cain, et al., 2003; Sedgwick, 2000)

Ecosystem Roles

Willow flycatchers are parasitized by brown-headed cowbirds, which lay their eggs in willow flycatcher nests. When cowbirds successfully parasitize their nests, flycatchers build a new nest directly over the cowbird eggs, abandon the nest and build a new one, or on occasion, raise the cowbird and flycatcher chicks together. Likewise, flycatchers from genus Empidonax may be parasitized by mites and ticks. By consuming berries, willow flycatchers may also act as minor seed dispersers. (Hamer, et al., 2012; Sedgwick, 2000; Skoracki, et al., 2008; Uyehara and Narins, 1995; Willson and Whelen, 1993)

Commensal/Parasitic Species
  • brown-headed cowbirds (Molothrus ater)
  • mites (Syringophilopsis)
  • ticks (Acarina)

Economic Importance for Humans: Positive

There is little information available on the positive economic importance of willow flycatchers, although due to their primarily insectivorous diet, they may help control pest populations. (Prescott and Middleton, 1988)

Economic Importance for Humans: Negative

There is little information available on the negative economic impacts of willow flycatchers.

Conservation Status

Currently, willow flycatchers are considered a species of least concern according to the IUCN Red List of Threatened Species, although their overall population trend is declining. The biggest concerns for the species are brood parasitism and habitat destruction. While the overall population remains stable, the southwestern subspecies (Empidonax traillii extimus) is currently endangered. (Alderfer, 2006; BirdLife International, 2013; Sedgwick, 2000; Sibley, 2003)

Other Comments

Willow flycatchers are often mistaken for alder flycatchers. Although these species have different songs, are found in different habitats and do not interbreed, they were considered a single species until the 1970s. (Fergus, 2003)

Contributors

Emily Brazil (author), Northern Michigan University, Leila Siciliano Martina (author, editor), Animal Diversity Web Staff, Alec Lindsay (editor), Northern Michigan 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.

female parental care

parental care is carried out by females

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).

male parental care

parental care is carried out by males

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

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).

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.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

visual

uses sight to communicate

References

Alderfer, J. 2006. Complete Birds of North America. Washington, D.C: National Geographic.

Bakian, A., K. Sullivan, E. Paxton. 2012. Elucidating Spatially Explicit Behavioral Landscapes in the Willow Flycatcher. Ecological Modeling, 232: 119-132.

BirdLife International, 2013. "Empidonax traillii" (On-line). IUCN RedList of Threatened Species. Accessed December 10, 2013 at http://www.iucnredlist.org/details/22699848/0.

Cain, J., M. Morrison, H. Bombay. 2003. Predator Activity and Nest Success of Willow Flycatchers and Yellow Warblers. The Journal of Wildlife Management, 67.3: 600-610.

Crossley, R. 2011. The Crossley ID Guide of Eastern Birds. Princeton, NJ: Princeton University Press.

Fergus, C. 2003. Wildlife of Virginia and Maryland and Washington D.C. Mechanicsburg, PA: Stackpole Books.

Finch, D., S. Stoleson. 2000. Status, Ecology, and Conservation of the southwestern willow flycatcher. Ogden, Utah: United States Department of Agriculture.

Garcia-Hernandez, J., O. Hinojosa-Huerta, V. Gerhart, Y. Carrillo-Guerrero, E. Glenn. 2001. Willow flycatcher (Empidonax traillii) surveys in the Colorado River delta: implications for management. Journal of Arid Environments, 49.1: 161-169.

Hamer, S., T. Goldberg, U. Kitron, J. Brawn, T. Anderson, S. Loss, E. Walker, G. Hamer. 2012. Wild birds and urban ecology of ticks and tick-borne pathogens, Chicago, Illinois, USA 2005-2010. Emerging Infectious Diseases, 18:10: 1589-1595.

Kaufman, K. 2000. Field Guide to Birds of North America. New York: Hillstar Editions L.C.

Kaufman, K. 1996. Lives of North American Birds. New York: Houghton Mifflin Books.

King, J. 1955. Notes on the Life History of Traill's Flycatcher (Empidonax traillii) in Southeastern Washington. The Auk, 72.2: 148-173.

Koronkiewicz, T., M. Sogge, C. III van Riper, E. Paxton. 2006. Territoriality, Site Fidelity, and Survivorship of Willow Flycatchers Wintering in Costa Rica. The Condor, 108.3: 558-570.

Phillips, A. 1948. Geographic Variation in Empidonax traillii. The Auk, 65.4: 507-514.

Prescott, D., A. Middleton. 1988. Feeding-Time Minimization and the Territorial Behavior of the Willow Flycatcher (Empidonax traillii). The Auk, 105.1: 17-28.

Sedgwick, J. 2004. Site Fidelity, Territory Fidelity, and Natal Philopatry in Willow Flycatchers (Empidonax traillii). The Auk, 121.4: 1103-1121.

Sedgwick, J. 2000. "Willow Flycatcher (Empidonax traillii)" (On-line). The Birds of North America Online. Accessed February 01, 2013 at http://bna.birds.cornell.edu/bna/species/533/articles/introduction.

Sibley, D. 2003. The Sibley Field Guide to Birds of Eastern North America. New York: Andrew Stewart Publishing.

Skoracki, M., M. Flannery, G. Spicer. 2008. Quill mites of the genus Syringophilopsis Kethley, 1970 (Acari: Syringophilidae) from North American birds. Folia Parasitologica, 55: 291-300.

Uyehara, J., P. Narins. 1995. Nest Defense by Willow Flycatchers to Brood-Parasitic Intruders. The Condor, 97.2: 361-368.

Willson, M., C. Whelen. 1993. Variation of dispersal phenology in a bird-dispersed shrub, Cornus drummondii. Ecological Monographs, 63-2: 151-172.

Yong, W., D. Finch. 1997. Migration of the Willow Flycatcher along the Middle Rio Grande. The Wilson Bulletin, 109/2: 253-268. Accessed February 12, 2013 at http://www.jstor.org/stable/4163809?seq=14.

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