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Kentucky Warbler
Geothlypis formosa
NatureServe conservation status
Global (G-rank): G5
State (S-rank): SNA
External links
Species range
BREEDING: from southeastern Nebraska, east across central Iowa, southwestern Wisconsin, northeastern Illinois, central Indiana, north-central Ohio, southern Pennsylvania, northern New Jersey, and southeastern New York, to southwestern Connecticut, south to Texas, Gulf Coast to northwestern Florida, central Georgia, and South Carolina, and west to eastern Kansas and central Oklahoma (AOU 1983). NON-BREEDING: tropical zones of southern Veracruz and Oaxaca, through Chiapas, the base of the Yucatan Peninsula, primarily on the Caribbean slope of northern Central America, throughout Costa Rica and Panama, and into northern Colombia and northwestern Venezuela (AOU 1983, McDonald 1998). Uncommon transient through the West Indies; some may overwinter on eastern and southern West Indies islands (McDonald 1998). Birdlife International (2014) estimates the range size to be 1.1 million square kilometers.
Migration
Arrives in Costa Rica early to mid-September, departs by late April (Stiles and Skutch 1989). Present in South America mostly October-March (Ridgely and Tudor 1989). See also McLaren (1981) and Moore (1990).
Habitat
BREEDING: Humid deciduous forest (Hamel 1992), dense second growth, swamps. Occurs in stands of various ages but is most common in medium-aged forests (Shugart et al. 1978). Prefers forests with a slightly open canopy, dense understory, and well-developed ground cover (Bushman and Therres 1988). Seldom found in conifers. In Virginia, McShea et al. (1995) found that forest type, streams, and the density of deer were significant variables in territory selection, but forest age (within a reasonable span of years) and the presence of a habitat boundary did not contribute significantly. Specifically, warblers selected cove hardwoods and avoided oak/hickory overstory. The avoidance of oak/hickory overstory is surprising, since a nonquantified gestalt impression of distribution at this site would at first suggest they prefer oak/hickory. Areas with streams and low white-tailed deer (ODOCOILEUS VIRGINIANUS) density also were selected. NON-BREEDING: In migration, habitats include forest, woodland, scrub, and thickets. In winter, habitat includes the floor of rain forests; also second growth, forest edge, undergrowth (AOU 1983, Bushman and Therres 1988). This species was found in wet forest (most commonly), moist forest (less commonly), and dry forest (rarely) on the Yucatan Peninsula (Lynch 1992); birds were also captured in mid-successional Acahual habitat. From studies in various Latin American countries, Robbins et al. (1992) concluded that wintering birds are ground foragers that require forest. Some birds were found in early successional habitats, but only an occasional bird was captured in pine woods or agricultural habitats. In Belize, found to prefer broadleaved forest edge and interior habitats (Petit et al. 1992).
Food habits
Walks rapidly over ground overturning leaves with bill, searches under sticks and in crevices, leaps up to snatch insect or spider from overhanging leaf or branch (Terres 1980). In winter in Mexico, gleans insects from the undersurfaces of low herbaceous growth typical of dense humid forests; sometimes climbs into low vegetation; uncommonly takes food items from forest floor (Rappole and Warner 1980). Also forages along twigs in low shrubbery (Stiles and Skutch 1989). Regularly attends army ant swarms in Panama (Ridgely and Tudor 1989).
Ecology
POPULATION DENSITY: Published information on densities from breeding bird censuses in the southeastern U.S. between 1947 and 1979 were summarized by Hamel et al. (1982): mean (se) density was 4.8 (0.56) pairs per 40 ha with a range of 1-8.5 pairs per 40 ha. In Missouri, density was 1.82 males per 10 ha in continuous forest, 0.91-1.29 per 10 ha in isolated forest fragments (Wenny et al. 1993). Wenny et al. (1993) studied two forest fragments, one smaller (300 ha) and one larger (> 800 ha) in Missouri. Kentucky warblers were found to have significantly higher densities in the larger forest area than in both fragments. In the largest contiguously forested site (> 800 ha), the estimated total population size was 243 birds with 98 breeding pairs. Gibbs and Faaborg (1990) found an average 2.2 males per 10 ha in larger forest tracts compared with 1.4 males per 10 ha in smaller fragments. Whitcomb et al. (1981) reported a territorial density of 36 males per sq km in Maryland. In Virginia, densities of 30-55 pairs were observed over the years 1988-1997 in the 1200 ha core area of suitable habitat at the study site (McDonald 1998). Winter density was up to 5.5 birds per 10 ha in Panama, around 30 per 10 ha in Veracruz, Mexico (Mabey and Morton 1992). TERRITORY SIZE: Territory sizes were found to differ significantly between forest tracts of different size by Wenny et al. (1993): territories averaged 0.8 ha in a large forest (> 800 ha) and 1.08 ha in two smaller fragments (300 ha). In Virginia, territory sizes ranged from about 0.8 to 2 ha; for nearly all territories considered individually, configuration (boundaries) and size remained nearly constant over the 14-year study. With few exceptions, the same male returned to and occupied a given territory for as long as he lived, although returning females did shift from year to year (McDonald, unpubl. data). Territorial in winter (Stiles and Skutch 1989, Mabey and Morton 1992). Individuals commonly return to the same winter territory in successive years (Rappole and Warner 1980).
Reproductive characteristics
Eggs are laid mainly in May-June. Clutch size is 3-6 (usually 4-5). Incubation lasts 12-13 days, by female. Young are tended by both parents, leave nest at 8-10 days (before they can fly), fed by adults for up to 17 days more. Typically one brood, but sometimes two (Harrison 1975). A first clutch size of 4.12 eggs with 1-2 broods per year and a reproductive effort of 6.18 was reported by Whitcomb et al. (1981) in Maryland.
Threats or limiting factors
Probably adversely affected by forest fragmentation. Whitcomb et al. (1981) reported that populations were only 33% of their maximum potential in forests less than 173 acres (70 ha). Nesting was reported in higher densities at distances approaching 200 ft (61 m) from transmission-line corridors (Kroodsma 1982, Chasko and Gates 1982). Pairing success and territory density were examined in populations occupying small (< 140 ha), isolated forest fragments in an agricultural setting and larger (> 500 ha), contiguous forest tracts in central Missouri by Gibbs and Faaborg (1990). Densities did not differ between fragmented (2.1 males per 10 ha) and contiguous sites (1.9 males per 10 ha), and the proportion of paired and unpaired males was identical between the two forest types. Gibbs and Faaborg (1990) suggested two reasons for the lack of sensitivity to forest area. First, the fact that breeding occurs in heavily vegetated creek-bottoms where nests are inconspicuous may alleviate predation pressure. Second, food (herb and shrub invertebrates) is unlikely to be affected by the increased local evaporation rates and decreased soil and litter moisture levels associated with forest area reduction. Loss of tropical forests may also contribute significantly to the regional declines that have been observed in temperate North America. Some declines parallel deforestation in the tropics, and habitat fragmentation on the breeding grounds cannot account completely for these declines. 1980). According to Bent (1953), a common victim of the brown-headed cowbird (MOLOTHRUS ATER); in parts of Pennsylvania, historical records cite the warbler as the commonest victim of the cowbird. More recently, Robinson (1992) documented brood parasitism by cowbirds, although his data, as well as McDonald's in Virginia, indicate that ground-nesters appear to be less susceptible than shrub-nesting species. Of six nests in central Illinois, two were parasitized with an average 0.8 cowbird eggs per nest and an average 3.0 cowbird eggs per parasitized nest (Robinson 1992). In total, three warblers and one cowbird were raised; in no cases were warblers and cowbirds raised together at the Illinois site. At McDonald's Virginia site, the intensity of cowbird parasitism over 14 years has varied annually from 0% to about 15% of the known nests and fledged families. No correlates have been identified, however, to account for this variation (McDonald, unpubl. data). Unlike the Illinois study, in Virginia warblers and cowbirds were raised and fledged together successfully, with no species-specific survival differences. Predation on nests is probably more common than usually realized because the parents simply start re-nesting within a week unless the nest was destroyed very late in the season (McDonald, unpubl. data). Bent (1953) stated that snakes and other prowling predators have been known to rob nests. At McDonald's site in Virginia, about one-fourth of the nests were depredated before fledging. Indirect evidence, including disturbance of the nest cup, and the results of experiments at the same site on artificial nests, suggest that the major predators at this site are small mammals (e.g., eastern chipmunk [TAMIAS STRIATUS]) and medium-sized mammals (raccoon [PROCYON LOTOR], skunk [MEPHITIS MEPHITIS], and opossum [DIDELPHIS VIRGINIANA]). It is not unlikely that snakes and corvids also prey on these nests (McDonald, unpubl. data). The only documented cases of predation on adults include a bizarre report of one being captured and consumed by a box turtle (TERRAPENE CAROLINA) and McDonald's finding remains of a banded female at her nest of four 7-day-old nestlings, also mostly consumed. A medium-sized mammalian predator, such as a raccoon or opossum, common at the Virginia site, were suspected in the latter predation event.
