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Wilsonia pusillaWilson's warbler

By Claudia Paniagua-Ugarte

Geographic Range

Wilson’s warblers consist of three subspecies, separated geographically in the Nearctic region. Wilsonia pusilla pusilla winter south in Caribbean slope, northeastern Mexico and south into Costa Rica. These Wilson’s warblers begin their spring migration northward through southern Texas traveling through the United States until reaching their breeding grounds. Their breeding grounds extend from northern western Alaska, eastward into Canada, Newfoundland, New England states and south along the Pacific coastal states.  During the fall migration Wilson’s warblers can be found in the states of Oregon, Idaho, and Arizona. Members in the subspecies Wilsonia pusilla pileolata overwinter in Sinaloa and Durango, Mexico south into Veracruz, Mexico. In the spring, they migrate north through the United States into their breeding grounds that extend from Alaska to Yukon. After the breeding season, they migrate south in the fall through Mississippi and Louisiana to their wintering grounds. Members of the subspecies Wilsonia pusilla chryseola spend their winter months throughout northern Mexico to western Panama. During the spring they migrate north through the United States to reach their breeding grounds, which extend as far north as British Columbia southward along the Pacific coast to southern California. They can also be found breeding further south covering the eastern part of Washington to northern New Mexico. Their fall migration path crosses the Pacific coastal states and Arizona. (Dunn and Garrett, 1997)

Habitat

Wilson's warblers winter in highlands of central America that contain humid forests or dense shrubbery. The breeding habitat of Wilson's warblers includes ponds, bogs, and streams that provide a humid habitat and dense ground coverage.

The subspecies (Wilsonia pusilla pileolata) lives in thickets of willows (Salix) and alder trees (Alnus). They can also be found breeding in open moist habitats near streams or ponds. (Wilsonia pusilla chryseola) breeds in coniferous forests because of the canopy gaps that produce dense shrubbery on the ground. They also reside in moist habitats that allow the growth of willows western poison oaks (Toxicodendron diversilobum), and alders. (Dunn and Garrett, 1997; Henderson, 2002; Sterry, 2017)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams

Physical Description

Wilson's warblers are 121.65 in length from head to tail and weigh approximately 11 grams. They have thin tan-pink legs and thin beaks. The maxilla is black and the tip on their break ranges from yellow to brown. Spring male Wilson’s warblers have a bright yellow head with a black patch on their crowns. Their back and wings are olive yellow color, while their dorsal side is bright yellow. Early spring males are very similar but they have more green tips towards the ends of the black crown of their head. Early spring males also have more brown in their wings. Female Wilson’s warblers are smaller and thinner than the males. Spring adult female warblers also have a cap on their crown that can range in coloration from black to olive green. The cap on female warblers is smaller and not as shiny and slick as that on males. First spring female warblers have similar colors except for the cap on the crown of their head ranges from light olive to almost absent. They have a darker olive yellow coloration in their wings. Fall female warblers have an olive yellow forehead. The side of their face is olive color. The subspecies Wilsonia pusilla pileolated is brighter yellow than Wilsonia pusilla pusilla on their ventral sides and have a more vibrant olive-green color on their dorsal sides than Wilsonia pusilla pusilla and Wilsonia pusilla chrysoulaWilsonia pusilla chrysoula is the smallest subspecies. They have the most vibrant-colored forehead that can range from bright yellow to almost orange. (Dunn and Garrett, 1997; Harrison, 1984; Sterry, 2017)

  • Sexual Dimorphism
  • male larger
  • sexes colored or patterned differently
  • male more colorful
  • Average mass
    11 g
    0.39 oz
  • Average length
    121.65 mm
    4.79 in

Reproduction

Wilson's warblers are monogamous, individuals mating with one partner during the breeding season. Pairing can occur when a female warbler enters into the territory of a male warbler. The male may follow her silently or while singing.

Pair formation begins in March-April. About 2 weeks after pairing occurs, females begin nesting. Nest building is typically April through late May. Nesting female Wilsonia pusilla pusilla and Wilsonia pusilla pileolata select their nesting sites on or near the ground. Their nest sites are placed in moss or sedges (Carex) near the base of trees or shrubs. They make their nests out of thin grass and leaves. The nests are very small compared to that of Wilsonia pusilla chryseola which are thicker because they incorporate twigs and fine stem. Female Wilsonia pusilla chryseola also choose to construct their nests up to 0.94 meters off the ground. Nest construction is completed by establishing a large foundation made of large leaves. The outer structural material is composed of long stems or moss and the nest lining is composed of dead grasses.

Egg-laying begins 1-3 days after the nest is fully built. They do not reuse or alter their nests after they have been constructed. Once the mating season is over, new pairs can form the following year. Extra pairing can also occur during the mating season, but it is uncommon. (Ammon and Gilbert, 1999; Harrison, 1951; Sterry, 2017)

Female warblers can lay between two to seven eggs in a clutch, averaging 4 per clutch. They typically have just one clutch per year, but if the first nest fails, they may re-nest if early in the season. The breeding season is from March-August, with pairing in March-April, nest-building in April-May, and raising of young from June-August. The eggs are 15-17 mm long length 12 mm wide. Their eggs are a white to creamy white with fine reddish-brown speckling. 10-13 days. Once the young hatch, they remain in the nest for 9-11 days. The fledglings can usually fly a day after leaving the nest, and they continue to communicate with their parents. Some clutches are fed by one of the parents up to 24 days after fledging. Both sexes are able to breed at 1 year of age, but breeding success that first year is typically very low. (Ammon and Gilbert, 1999; Harrison, 1951)

  • Breeding interval
    one nest a year
  • Breeding season
    June-August
  • Range eggs per season
    2 to 7
  • Range time to hatching
    10 to 13 days
  • Range fledging age
    9 to 11 days
  • Range time to independence
    36 (high) days
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Average age at sexual or reproductive maturity (male)
    1 years

Male Wilson's warblers invest their energy during parenting in protection of the nest and female warbler. They also help build the nest. Female Wilson's warblers spend their energy in the construction of the nest, fledgling, and hatching.

After fledging, parents usually split duties of feeding the young. For example, in a nest of 4, each parent would tend to 2 of the young. Occasionally, the ratios are 3:1 towards more female parental care. The parents care for their "assigned" young for up to 25 days post-fledging. (Ammon and Gilbert, 1999; Dunn and Garrett, 1997)

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

Lifespan/Longevity

The average life span of Wilson's warblers is six years in the wild. There are records of these birds living up to eight years. They are not held in captivity and therefore do not have a recorded lifespan in captivity.

Year-to-year survivorship has been calculated between 30-61% each year. Some of these estimates, however, are based on banded birds returning to the same spot to mate each year. So it's possible that survivorship is higher than reported here. (Ammon and Gilbert, 1999; Harrison, 1951; Sterry, 2017)

  • Range lifespan
    Status: wild
    8 (high) years
  • Average lifespan
    Status: wild
    6 years

Behavior

Wilson's warblers are active birds. They hoover from perch to perch, flick their wings, and are constantly moving their long tail in circular up-down motions. When perched, they also scratch their heads frequently by lifting their legs over their wings or under the wing. Female warblers preen before returning to their nests while male warblers preen between foraging and singing. Wilson’s warblers bathe in small areas of water such as small streams and puddles. There is no record of sunbathing, dust-bathing or rubbing of other insects on their wings. If they incorporate their flight song which is a high pitched sound that drops towards the end, their wings begin to beat in a fluttering motion.

Mating behavior begins when a male establishes his territory and begins to sing. Female warblers when found flying in the territory of a male warbler are followed by the male. Pairing occurs during the mating season but can be terminated in the summer months. Extra paring has also been noted.

Nesting female Wilsonia Pusilla pusilla and Wilsonia pusilla pileolata select their nesting site on or near the ground. Male warblers are very territorial. If other male warblers attempt to intrude into their territory, it results in chases by resident males. Fighting between intruders and residents can be shown with locked feet, frequent singing and calling, and flutter their wings on the ground.

Intrusion of predators into the territory of Wilson's warblers can result in frantic callings or a broken wing display to distract predators. (Ammon and Gilbert, 1999; Dunn and Garrett, 1997; Harrison, 1951; Stephenson and Whittle, 2013; Sterry, 2017)

  • Range territory size
    1800 to 17000 m^2

Home Range

Home ranges have not been reported, but territory sizes are known to vary widely across their geographic range. Ammon and Gilbert (1999) summarize a few studies that report territory sizes: Colorado reported 0.18 ha ± 0.02 SE, California reported 2 studies with averages of 0.31 ha ± 0.02 SE, 0.19 ha ± 0.01 SE, and 0.48-0.57 ha. In Sierra Nevada, home ranges were 2-3 times these ranges, with a mean of 1.17 ha. (Ammon and Gilbert, 1999)

Communication and Perception

Wilson’s warblers communicate through a series of chattering noises. Individuals from Wilsonia pusilla pusilla start off the call with a weak chattering noise that drops in tone towards the end ("Chi chi chi chet chet"). Individuals from Wilsonia pusilla pileola have a high pitched call that starts off slow and becomes louder and faster towards the end.

Females actually sing a song, as well, but it does not have anything in common with the male warblers songs. Typically, females have higher-pitched calls than male warblers.

Wilson’s warblers use the timing, pitch, and pattern of their calls to communicate between the young, old, male, and females. Male warblers sing primarily for mating. Once male warblers have mated, singing diminishes to almost no singing. Mating does not impact the song of female warblers. Wilson’s warblers sing while perched and rarely during flight. Their singing is most intense during the early morning before sunrise.

These warblers also interact by touch. During territorial periods, males defend their territories with squawks plus attacks with feet-to-feet contact. Both bird may actually fall to the ground in fighting. (Ammon and Gilbert, 1999; Sterry, 2017)

Food Habits

Wilson’s warblers use their wings to hover over insects such as beetles (Coleoptera), stoneflies (Plecoptera), bees (Hymenoptera), flies (Diptera), aphids, mayflies (Ephemeroptera), spiders, and caterpillars of butterflies (Lepidoptera). They will consume adults and non-aquatic larva of these orders, if available. Nestlings have been fed small caterpillars.

They also eat berries primarily during the fall season. Wilson’s warblers capture their food through hovering, 0-9.8 meters off the ground in low ground shrubbery and up to seventeen meters in environments with tall trees. They also catch their foods while flying between branches. Because their wings are strong enough to allow them to hover, they use their beak to reach for insects located under leaves. (Ammon and Gilbert, 1999; Dunn and Garrett, 1997; Sterry, 2017)

  • Animal Foods
  • insects
  • terrestrial non-insect arthropods

Predation

Adult Wilson's warblers are primarily known to be preyed on by domestic cats (Felis catus) and ridge-nose rattlesnakes (Crotals willardi). Their predation list also expands to include snakes, small hawks (Accipiter), jays of multiple species (Cyanocitta), and weasels (Mustela).

Wilson's warblers are known to defend their nests from predators by calling frantically from a high point in a tree or shrub. They will raise the feathers on their crowns. When their nests are being raided, predators will fully consume the eggs, young , and even adults, from the nest. Partial consumption is rare. (Ammon and Gilbert, 1999; Griscom and Sprunt, 1957; Sterry, 2017)

Ecosystem Roles

The most common parasites of Wilson's warblers include apicomplexans like Leucocytozoon andrewsi, Leucocytozoon fringillinarum, Leucocytozoon dubreuili, and Leucocytozoon majoris. Another protozoan, an alveolate, is g. Haemoproteus.

There was one case of an individual Wilsonia pusilla pileolata that was in infected with a blowfly, Protocalliphora hirudo, and one case of mites in these warblers' nests.

Brown-headed cowbird (Molothrus ater) parasitism has also been reported which is more common in California. (Ammon and Gilbert, 1999; Dunn and Garrett, 1997)

Commensal/Parasitic Species
  • Protozoan in Apicomplexa clade Leucocytozoon andrewsi
  • Protozoan in Apicomplexa clade Leucocytozoon andrewsi
  • Protozoan in Apicomplexa clade Leucocytozoon fringillinarum
  • Protozoan in Apicomplexa clade Leucocytozoon dubreuili
  • Protozoan in Apicomplexa clade Leucocytozoon majoris
  • Protozoan in Alveolata clade Haemoproteus
  • mites
  • Brown-headed cowbird Molothrus ater

Economic Importance for Humans: Positive

Beyond birdwatching, Wilson's warblers have no positive economic importance for humans.

Economic Importance for Humans: Negative

There are no known adverse economic effects of Wilson's warblers on humans.

Conservation Status

Wilson's warblers a species of "Least Concern" on the IUCN Red List. Wilson's warblers are protected under the U.S Migratory Bird Act (they cannot be hunted) and are on several conservation listings in western U.S. states because there has been a decline in population due to phenoxy herbicides, which are used against deciduous trees when conifers are a harvested crop. The removing of shrub areas for cattle grazing and human use, also has an impact on population size.

There has also been record of migrant mortality due to collisions with television towers in Florida, Wisconsin, and Minnesota. Human disturbance has also causes these birds to abandon nests.

To manage the decline in population size in Wilson's warblers, restoration and protection of breeding areas is suggested, coupled with limiting cattle grading and the use of herbicides. However, no large-scale conservation efforts have been implement for these warblers. (Ammon and Gilbert, 1999; Dunn and Garrett, 1997)

Contributors

Claudia Paniagua-Ugarte (author), Radford University, Alex Atwood (editor), Radford University, Lindsey Lee (editor), Radford University, Karen Powers (editor), Radford University, Joshua Turner (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

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

acoustic

uses sound to communicate

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

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.

frugivore

an animal that mainly eats fruit

herbivore

An animal that eats mainly plants or parts of plants.

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.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

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).

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

visual

uses sight to communicate

References

Ammon, E., W. Gilbert. 1999. "Wilson's Warbler (Cardellina pusilla), version 2.0" (On-line). The Birds of North America (A. F. Poole and F. B. Gill, Editors). Accessed April 30, 2018 at https://doi.org/10.2173/bna.478.

Campbell, W. 2001. Birds of British Columbia, Volume 4: Wood Warblers through Old World Sparrows. Vancouver, Canada: UBC Press.

Dunn, J., K. Garrett. 1997. The Field Guide to Warblers of North America. New York: Houghton Mifflin Company.

Griscom, L., A. Sprunt. 1957. The Warblers of America. New York, NY: The Devin-Adair Company.

Hagar, J., K. Dugger, E. Starkey. 2007. Arthropod prey of Wilson's warblers in the understory of Douglas-fir forests. The Wilsons Journal of Ornithology, 119/4: 533-546.

Harrison, H. 1984. Wood Warblers World. New York, New York: Simon & Schuster Inc..

Harrison, H. 1951. Notes and observations on the Wilson's warbler. The Wilson Journal of Ornithology, 63/3: 143-148.

Henderson, C. 2002. Birds of Costa Rica: A Field Guide. Austin, TX: University of Texas Press.

Kroodsma, D. 2008. The Backyard Birdsong Guide: Western North America. San Francisco, CA: Chronicle Books.

Levang, P. 2014. The Bird Book: Alma's Story. Bloomington, IN: Xlibris Corporation.

Lockwood, M. 2007. Basic Texas Birds: A Field Guide. Austin, TX: The University of Texas Press.

Shewer, J., T. Blount. 2017. Birds of the Pacific Northwest. Portland, OR: Timber Press.

Stephenson, T., S. Whittle. 2013. The Warbler Guide. Princeton, NJ: Princeton University Press.

Sterry, P. 2017. Warblers and other Songbirds of North America: A Life-Size Guide to Every Species. New York City, NY: Harper Collins Publishers.

Stewart, R. 1973. Breeding behavior and life history of the Wilson's warbler. The Wilson Journal of Ornithology, 85/1: 21-30.

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