Animal Diversity Web

Geographic Range

Kentucky warblers (Oporornis formosus) are found in the Nearctic and Neotropical regions. Their breeding range includes much of the eastern United States, as far north as Illinois and eastward to New Jersey, and along the east coast of the United States. This range continues south through the Carolinas and into the Florida panhandle. They breed as far west as northeastern Texas, and the far eastern parts of Oklahoma, Kansas, and Iowa.

States such as Florida, Texas, Louisiana, and Georgia serve as migratory stopover sites. Overwintering locales include western Mexico, southward to and including the Yucatan peninsula. They inhabit the Florida keys southward to Cuba, Dominica, and much of the Caribbean islands. This overwintering range includes nearly all of Central America and northern South America (northern Colombia and Venezuela).

They are occasional migrants through the western United States, having been documented in California, New Mexico, Arizona, Colorado, and as far north as Alaska. ("Kentucky warbler (Geothlypis formosa) species guidance", 2018; Kilgo, et al., 1996; Magee and Clef, 2016; Matsuoka, 1997; McDonald, 2020; McShea, et al., 1995; Pimm and Askins, 1995)

• Biogeographic Regions
• nearctic
  • native
• neotropical
  • native

Habitat

Kentucky warblers inhabit riparian forests, bottomlands near creeks and rivers, ravines in deciduous forests, with deciduous forests serving as the primary habitat. Swampy edges provide suitable conditions for them to breed and survive, with the deciduous forests serving as the habitat for Kentucky warblers to breed in native areas and riparian forests used in atypical breeding areas (e.g., California). In Arkansas, these birds established breeding territories in areas with thick ground cover (0-1m high). Areas lacking slope also were selected more often than hilly sites.

While in the tropics for the winter, Kentucky warblers will use low-elevation forested areas, and will also inhabit shaded coffee and cacao plantations.

In their breeding season, nests are made from leaves, grass stems, and weeds, and they are built at the base of bushes and trees or directly on the forest floor. In swamps, they will construct their nests around the edges of the environment to avoid the risk of water levels rising and potentially destroying their nests. While the majority of Kentucky warblers forage close to or on the ground, singing males will do so high up in mature trees. Kentucky warblers are absent from areas with human disturbance that would disturb the ground level. The average elevation for this species is 1050 m. (Adams and Barrett, 1976; Halley, 2012; "Kentucky warbler (Geothlypis formosa) species guidance", 2018; McDonald, 2020; Moorman, 2007; Pimm and Askins, 1995)

• Habitat Regions
• temperate
• tropical

• Terrestrial Biomes
• forest

• Wetlands
• swamp

• Other Habitat Features
• agricultural
• riparian

• Average elevation

  1050 m

  3444.88 ft

Physical Description

Male Kentucky warblers are slightly larger than females, with males typically having an average mass of 14.0 g, length of 14.0 cm, and wingspan of 22.0 cm. Females have an average mass of 10.0 g, length of 12.0 cm, and wingspan of 18.0 cm.

During the breeding season, males possess what look like a black mask that is incomplete due to the yellow stripe in the supercilium region that resemble eyebrows. Females have a similar mask, but it is broken up a bit more or lighter in color (gray or brownish). These birds appear yellow on their belly and breast and olive colored on their mantle, coverts, and tail. Females lack the black head pattern in males, and females' colors are generally duller. Males, especially, may lose their bright colors in the non-breeding season, typical of what are termed confusing fall warblers.

Hatchlings do not have any feathers upon hatching and their eyes are closed. After 8-10 days, offspring begin to develop feathers and start displaying the yellow and olive colors, with adult resemblance appearing after 17 days. Their initial plumage colors are muted versions of the adults.

Kentucky warblers are often mistaken for hooded warblers (Setophaga citrina) or Canada warblers (Cardellina canadensis), because hooded warblers possess the same yellow on their belly and breast and olive on their coverts and mantle. Canada warblers also have the same shade of yellow on their belly and breast, but both species are missing the yellow eyestripe patterns that Kentucky warblers develop. ("Kentucky warbler (Geothlypis formosa) species guidance", 2018; McDonald, 2020)

• Other Physical Features
• endothermic
• bilateral symmetry

• Sexual Dimorphism
• male larger
• sexes colored or patterned differently
• male more colorful

• Range mass

  10.0 to 14.0 g

  0.35 to 0.49 oz

• Range length

  12.0 to 14.0 cm

  4.72 to 5.51 in

• Range wingspan

  18.0 to 22.0 cm

  7.09 to 8.66 in

Reproduction

Female Kentucky warblers find potential mates by remaining in breeding areas where males are consistently singing. Breeding males sing from perches at heights of 5 to 20 m away from the trunks of trees during the early breeding season, often on exposed horizontal or near-horizontal perches. Kentucky warblers are mostly monogamous, meaning they only have one female and one male in a pair, and this bond lasts just for one season. Due to having conspecific neighbors, these birds participate in extra-pair copulations, which contribute to their mixed reproductive strategies. Males not yet paired sing more than 5 times more often than paired males, as those without a mate use songs to signal a female to their territory. Males will usually sing around perches in dominated areas on a day to day basis, usually during the day. Following the formation of a pair, males will follow and maintain visual contact with their mate during the incubation process. (Halley, 2012; "Kentucky warbler (Geothlypis formosa) species guidance", 2018; Magee and Clef, 2016; McDonald, 2020)

• Mating System
• monogamous

Kentucky warblers breed once or twice during their breeding season, from May to August. Female clutch size averages to 4 eggs (range 3-6), and the incubation process lasts 11-13 days. Upon hatching, juvenile Kentucky warblers weigh 2.5 to 3.5 grams. After 8 or 9 days, the young Kentucky warblers become capable of flying. Juveniles leave the nest after about 9 days, and are considered fully fledged. Parental care is still in effect after juveniles have made it through the fledging process. It usually takes 8-10 days post-fledging for juvenile Kentucky Warblers to become independent (16-19 d post-hatching). Females reach sexual maturity after 1 year, while it is unknown how long it takes for males. (Halley, 2012; "Kentucky warbler (Geothlypis formosa) species guidance", 2018; Magee and Clef, 2016; Matsuoka, 1997; McDonald, 2020)

• Key Reproductive Features
• iteroparous
• seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual
• fertilization
• oviparous

• Breeding interval

  Kentucky warblers breed once or twice during the warmer months

• Breeding season

  Kentucky warblers breed from May to August

• Range eggs per season

  3 to 6

• Average eggs per season

  4

• Range time to hatching

  11 to 13 days

• Range fledging age

  8 to 9 days

• Range time to independence

  16 to 19 days

• Average age at sexual or reproductive maturity (female)

  1 years

During pre-fertilization, females will begin nest construction while males observe the nest for the entire duration of the nest building process. The incubation period begins once females have laid their last egg into the nest, followed up by females brooding the eggs and males staying within the vicinity of the nest and singing. Females on occasion will forage during the incubation period, and males rarely bring females food. Hatchlings are responsible for breaking out of the eggs themselves. Empty eggshells will be discarded by both parents once all the young have hatched. As the hatchlings are altricial, food and protection are provided by both parents up until the fledging process is complete. Parents, both males and females, will distribute the caring roles for the fledglings until they are independent. Approximately half the brood is raised by each parent after fledging, and if the nest is late in the breeding season, the young and the two parents may never be reunited within the season due to drifting. (Halley, 2012; "Kentucky warbler (Geothlypis formosa) species guidance", 2018; Matsuoka, 1997; McDonald, 2020)

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

Lifespan/Longevity

The lifespan of Kentucky warblers in the wild is estimated to be 6.9 years. The longest known lifespan of Kentucky warblers has been reported to be 11 years. Kentucky warblers are not kept in captivity. Predators prey on about 60% of 87 nests containing juvenile Kentucky warblers before they fledge in Virginia. In Virginia, the survival of adult banded birds varies annually, with the average survival rate of six years being 62%, which was obtained from the re-sightings of adult Kentucky warblers that were color-banded. ("Kentucky warbler (Geothlypis formosa) species guidance", 2018; Matsuoka, 1997; McDonald, 2020; Peak, 2004; Pimm and Askins, 1995)

• Range lifespan
  Status: wild

  11 (high) years

• Average lifespan
  Status: wild

  6.9 years

Behavior

Adult Kentucky warblers are normally solitary, except when they are nesting. These birds are primarily diurnal. They are mobile with the exception of females remaining sedentary during incubation periods. Males often claim a territory for future breeding, females are able to locate these territories by listening for the males' advertisement songs. Only males can sing their species-specific advertisement calls. Females and males will hop after each other to begin initiating their mating ritual, followed by the birds playfully chasing each other. Kentucky warblers are very aggressive, usually when predators approach their territory and when encountering other warblers in the vicinity. They will chase and aggressively chirp at any animal, even other Kentucky warblers, that invade their territory.

These migratory birds will settle in Central America in winter months. They typically depart their wintering ground in March or April, arriving at breeding grounds in April or May. In fall months, these birds depart their breeding territories in August and September. Migration flights occur at night, with Kentucky warblers forming small groups during the transition. These birds are aggressive even on their wintering grounds. (Adams and Barrett, 1976; Bryens, 1924; Coffin, 1915; Halley, 2012; "Kentucky warbler (Geothlypis formosa) species guidance", 2018; Kilgo, et al., 1996; Magee and Clef, 2016; McShea, et al., 1995; Moorman, 2007; O'Donnell, 2010)

• Key Behaviors
• arboreal
• terricolous
• flies
• diurnal
• nocturnal
• motile
• migratory
• solitary

• Average territory size

  158 m^2

Home Range

Home range for Kentucky warblers in Virginia was reportedly 22100 square meters. The defended territory size of these birds was estimated at 158 square meters. Kentucky warblers are very defensive of territory. Within this area, Kentucky warblers construct their nests and forage for food and other resources. (Halley, 2012; "Kentucky warbler (Geothlypis formosa) species guidance", 2018; Kilgo, et al., 1996; Magee and Clef, 2016; McDonald, 2020; McShea, et al., 1995; O'Donnell, 2010; Pimm and Askins, 1995)

Communication and Perception

Only male Kentucky warblers sing advertisement calls, with the frequency ranging from 2-5 kHz. During mating season, males establish a territory and sing to attract females to their location. McDonald (2020) describes the sound as a "chuurup" or a "chuuree" that is sounded out as two syllables, and trilling and increasing in frequency over 3 to 8 seconds. The offspring and adults of both sexes can produce generic alarm calls, a high-pitched chirping noise or a barrage of chirps, if predators are nearby. During nesting and breeding periods, they will utilize these sounds to deter potential predators and alert the others of possible danger.

Kentucky warblers will often make use of their tactile and visual senses for foraging, along with their sense of taste. These warblers will follow each other once a pair has been formed, which is initiated when the female makes physical contact with the male. Each of the parents will take care of approximately half of the young, remaining near them and always maintaining visual contact. Kentucky warblers tend to be aggressive towards other bird species, and males will pursue each other during the early stages of the mating season. The brighter hue present on males allows females to recognize them, along with the songs being conveyed during breeding periods. These birds can see colors and can even view their surroundings in ultraviolet. (Barrett, 1913; Halley, 2012; "Kentucky warbler (Geothlypis formosa) species guidance", 2018; Magee and Clef, 2016; McDonald, 2020; McShea, et al., 1995)

• Communication Channels
• visual
• tactile
• acoustic

• Perception Channels
• visual
• tactile
• acoustic
• chemical

Food Habits

Although classified as omnivores, Kentucky warblers' primary diet is insectivorous. During breeding season, small spiders, caterpillars (order Lepidoptera), and insects are typically being consumed, along with fruits form the genus Cecropia for birds overwintering in Mexico. These warblers rarely consume seeds. They forage on the ground for insects both out in the open and in piles of leaves.

Besides ground-foraging, these warblers feed in basal sections of trees and bushes. Spiders, beetle (order Coleoptera) grubs, caterpillars and adult moths (order Lepidoptera), grasshoppers and locusts (order Orthoptera), and Hemipterans like aphids make up the majority of these birds' diet. In their nonbreeding tropical environment, Kentucky warblers generally pursue crowds of army-ants (Eciton burchellii) and takeover any game that the ants routed. ("Kentucky warbler (Geothlypis formosa) species guidance", 2018; McDonald, 2020; Moorman, 2007)

• Primary Diet
• carnivore
  • insectivore
  • eats non-insect arthropods

• Animal Foods
• insects
• terrestrial non-insect arthropods

• Plant Foods
• seeds, grains, and nuts
• fruit

Predation

Common predators that prey on Kentucky warblers include raccoons (Procyon lotor), striped skunks (Mephitis mephitis), Virginia opossums (Didelphis virginiana), cats (Felis catus), eastern box turtles (Terrapene carolina), eastern chipmunks (Tamias striatus), red foxes (Vulpes vulpes), blue jays (Cyanocitta cristata), and American crows (Corvus brachyrhynchos). Young Kentucky warblers and eggs are most prone to predators, with over 60% of nests in Virginia losing nestlings to predation. Females will walk swiftly in a zigzag pattern and feign wing injuries as a distraction when predators are a threat to a nest. Males do not have any methods for dealing with predators. (Bryens, 1924; "Kentucky warbler (Geothlypis formosa) species guidance", 2018; Matsuoka, 1997; McDonald, 2020; O'Donnell, 2010; Peak, 2004)

• Known Predators

  • Raccoons (Procyon lotor)
  • Skunks (Mephitis mephitis)
  • Virginia opossums (Didelphis virginiana)
  • Cats (Felis catus)
  • Eastern box turtles (Terrapene carolina)
  • Eastern chipmunks (Tamias striatus)
  • Red foxes (Vulpes vulpes)
  • Blue jays (Cyanocitta cristata)
  • American crows (Corvus brachyrhynchos)

Ecosystem Roles

Kentucky warblers primarily eat insects and are prey for larger carnivorous vertebrates, such as snakes and foxes. Brown-headed cowbirds (Molothrus ater) will lay their eggs into the nests of Kentucky warblers, resulting in the Kentucky warblers taking care of young cowbirds.

Kentucky warblers serve as hosts for some ectoparasites, such as bird fleas (Ceratophyllus gallinae), chewing lice (parvorder Phthiraptera), and ticks (Haemaphysalis leporis-palustris). In Virginia, Kentucky warblers with arthropod parasites and embedded blowfly larvae (family Calliphoridae) have been discovered, and any nestlings with heavy concentrations of blowfly larvae are abandoned by their parents. Revels (1996) reported blowflies identified as Protocalliphora braueri in Arkansas, found in nests and embedded in nestlings. (Coffin, 1915; "Kentucky warbler (Geothlypis formosa) species guidance", 2018; Kilgo, et al., 1996; Magee and Clef, 2016; Matsuoka, 1997; McDonald, 2020; McShea, et al., 1995; Moorman, 2007; O'Donnell, 2010; Revels, 1996)

Economic Importance for Humans: Positive

Birdwatching is a source of ecotourism within the range of Kentucky warblers. The Bernheim Arboretum and Research Forest is a hot spot outside of Louisville, Kentucky that provides people with the opportunity to birdwatch various species of birds, including Kentucky warblers. In the United States, birdwatching generates $36 billion in annual revenue. ("Kentucky warbler (Geothlypis formosa) species guidance", 2018; Magee and Clef, 2016; McDonald, 2020; Moorman, 2007; O'Donnell, 2010; Peak, 2004)

• Positive Impacts
• ecotourism

Economic Importance for Humans: Negative

Kentucky warblers have no reported negative economic impacts on humans. (Adams and Barrett, 1976; "Kentucky warbler (Geothlypis formosa) species guidance", 2018; Magee and Clef, 2016; Matsuoka, 1997; McDonald, 2020; O'Donnell, 2010; Peak, 2004; Pimm and Askins, 1995)

Conservation Status

Kentucky warblers are listed as a species of "Least Concern" on the IUCN Red List. These birds are protected by the US Migratory Bird Act. Under the US Migratory Bird Act, Kentucky warblers cannot be killed, captured, sold, traded, or transported without US Fish and Wildlife Service authorization. No special status is in place for Kentucky warblers on the US Federal List. CITES does not have a special status for Kentucky warblers. The State of Michigan List has not placed a special status on Kentucky warblers.

The biggest threat towards Kentucky warblers is the loss, modification, and fragmentation of habitat, exacerbated by an overabundance of white-tailed deer (Odocoileus virginianus). These birds are not hunted or sold on the black market. Kentucky warblers are not severely affected by climate change related events. Brown-headed cowbirds (Molothrus ater) nest parasitism negatively impacts these birds. There have been observations of dead Kentucky warblers underneath TV towers in Florida, possibly during migration, as well as communications tower deaths. In a 2000 study, the American Bird Conservancy reported 568 Kentucky warblers killed by these collisions. Human disturbance near nesting sites results in parents abandoning their nests and nestlings, and potentially increases the chances for predators to prey on young Kentucky warblers. In an Indiana, mature oak (Quercus) forest, controlled fires eliminated Kentucky warblers from the site for years, even though they can still be found in similar unburnt areas.

These birds can be found in national forests, state forests, and more. White-tailed deer population management is being considered as a practice that could help stabilize or reduce fragmentation of forests. By maintaining vertical heterogeneity of forests, the ideal habitat for Kentucky warblers, there should be a reduction in brown-headed cowbirds infesting nests. Studies conducted across 12 forests in Ohio found that increasing habitat heterogeneity increased density concentrations of Kentucky warblers. Reducing the populations of potential predators of Kentucky warblers is another method that could help reduce the risk of population decline in these birds. When selective logging is performed appropriately, Kentucky warblers can benefit from it. According to the Institute for Bird Population, a fair amount of fragmentation within contiguous forests is needed to preserve bushy habitats, which helps with the conservation of Kentucky warblers. (BirdLife International, 2021; "Kentucky warbler (Geothlypis formosa) species guidance", 2018; Magee and Clef, 2016; McDonald, 2020; Peak, 2004; Pimm and Askins, 1995)

• IUCN Red List

  Least Concern

• US Migratory Bird Act

  Protected

• US Federal List

  No special status

• CITES

  No special status

• State of Michigan List

  No special status

Contributors

Jonah Kennedy (author), Radford University, Sierra Felty (editor), Radford University, Bianca Plowman (editor), Radford University, Karen Powers (editor), Radford University, Victoria Raulerson (editor), Radford University, Christopher Wozniak (editor), Radford University, Genevieve Barnett (editor), Colorado State University.

References

Wisconsin Department of Natural Resources. Kentucky warbler (Geothlypis formosa) species guidance. PUB-ER-694. Bureau of Natural Heritage Conservation: Wisconsin Bird Conservation Initiative. 2018.

Adams, D., G. Barrett. 1976. Stress effects on bird-species diversity within mature forest ecosystem. The American Midland Naturalist, 96/1: 179-194.

Barrett, H. 1913. Kentucky warbler in Massachusetts. The Auk, 30/4: 587.

BirdLife International, 2021. "Geothlypis formosa" (On-line). The IUCN Red List of Threatened Species 2021: e.T22721812A137319618. Accessed April 19, 2022 at https://dx.doi.org/10.2305/IUCN.UK.2021-3.RLTS.T22721812A137319618.en.

Bryens, O. 1924. Kentucky warbler in northern Michigan. The Wilson Bulletin, 36/3: 137.

Coffin, L. 1915. Nesting habits of Kentucky warbler. The Wilson Bulletin, 27/2: 348.

Halley, M. 2012. Direct evidence of Kentucky warbler (Geothlypis formosus) breeding success from a disturbed mid-Atlantic Piedmont forest in Delaware. Delmarva Ornithologist, 41: 7-9.

Kilgo, J., R. Sargent, K. Miller, B. Chapman. 1996. Nest sites of Kentucky warblers in bottomland hardwoods of South Carolina. Journal of Field Ornithology, 67/2: 300-306.

Magee, S., M. Clef. 2016. Plant structure of hooded and Kentucky warbler breeding sites in New Jersey. Cassinia, 76: 30-35.

Matsuoka, S. 1997. Nesting ecology of Townsend's warblers in relation to habitat characteristics in a mature boreal forest. The Condor, 99/2: 271-281.

McDonald, M. 2020. "Kentucky warbler (Gerothlypis formosa), version 1.0" (On-line). In Birds of the World (A. F. Poole, Editor). Accessed January 30, 2022 at https://doi-org.radford.idm.oclc.org/10.2173/bow.kenwar.01.

McShea, W., V. McDonald, E. Morton, R. Meier, J. Rappole. 1995. Long-term trends in habitat selection by Kentucky warblers. The Auk, 112/2: 375-381.

Moorman, C. 2007. Seasonal diets of insectivorous birds using canopy gaps in a bottomland forest. Journal of Field Ornithology, 78/1: 11-20.

O'Donnell, S. 2010. Army ant raid attendance and bivouac-checking behavior by neotropical montane forest birds. The Wilson Journal of Ornithology, 122/3: 503-512.

Peak, R. 2004. Factors affecting songbird nest survival in riparian forests in a midwestern agricultural landscape. The Auk, 121/3: 726-737.

Pimm, S., R. Askins. 1995. Forest losses predict bird extinctions in eastern North America. Proceedings of the National Academy of Sciences of the United States of America, 92/20: 9343-9347.

Revels, M. 1996. Eight new host species for the parasitic blow fly genus Protocalliphora (Diptera: Calliphoridae). Wilson Bulletin, 108/1: 189-190.