Connecticut Warbler Oporornis agilis Scientific name definitions

Jay Pitocchelli, Julie L. Jones, and David C. Jones
Version: 2.0 — Published June 2, 2023



Occupies jack pine forests, black spruce‒tamarack bogs, and muskeg in the eastern part of the breeding range and drier aspen and mixed aspen‒conifer forests in the western part of the breeding range. Migratory stopover habitat is highly variable, primarily wet, open deciduous forest with dense herbaceous or woody understory, occasionally in urban parks, scrublands, and cultivated fields— poorly known. Overwintering habitat, also poorly known, often consists of early successional stages with lower canopy and dense understory.

Habitat in Breeding Range

Habitat use is highly variable across the breeding range. In the eastern portion of the breeding range, it often occupies poorly drained, mature black spruce (Picea mariana) forest with nutrient-poor soils (73). Bogs are usually ≤ 40 ha in size in the Great Lakes region (74). They also breed in drier, jack pine (Pinus banksiana) forests in parts of Quebec (27, 75), Ontario (76) and around the Great Lakes (77). Western breeders are found in drier, more open forests dominated by aspen and poplar (78, 79). It has been observed in 15 different forest types, wetland bogs, and along the wooded margins of sedge meadows during the summer in the United States (80).

Habitat in northwestern Ontario is typical of the breeding habitat throughout most of breeding range: wet areas with black spruce and tamarack (Larix laricina), undergrowth and ground cover composed of mosses (Sphagnum), alder (Alnus), dogwood (Cornus), Labrador tea (Ledum), bog rosemary (Andromeda), bog laurel (Kalmia), and leatherleaf (Chamaedaphne) (81). The species has a strong association with black spruce and tamarack on poorly drained, wet, nutrient-poor soils with a deep organic component in Ontario (82), occasionally breeding at margins of damp woodlands or logged-over areas with low, thick undergrowth composed of raspberry (Rubus) vines, tangled brush, and fallen trees (83). Also found in drier, mature mixed-woods forest in north-central Ontario with jack pine, spruce, trembling aspen (Populus tremuloides), paper birch (Betula papyrifera), northern white cedar (Thuja occidentalis), and tamarack (76). In eastern Quebec, where blueberry harvesting has become an important industry, Connecticut Warbler was nearly restricted to residual jack pine forests surrounding blueberry fields; areas of timber harvesting within these forests were also used; it voided areas with lichens and mosses, as well as blueberry fields that offered no cover or protection against predators (75).

Connecticut Warbler breeds in or along the border of wet, deciduous forests in the western part of its breeding range, especially drier, well-drained upland sites (84, 73). Never far from an aspen‒poplar (Populus) forest or open areas in woods or margins of small meadows in the Prairie Provinces of Canada (85); it is common in pure aspen and mixed-woods dominated by aspen with shrub layer composed of aspen, rose (Rosa sp.), dogwood (Cornus stolonifera), beaked hazelnut (Corylus cornuta), Saskatoon serviceberry (Amelanchier alnifolia), and chokecherry (Prunus virginianum) in Saskatchewan (86, 79). It is rare but present in American elm (Ulmus americana) forests in northeastern Saskatchewan with average elm tree density of 166/ha, average diameter-at-breast-height (DBH) of elms 50 cm, 22% shrub cover, 91% forb cover, and 73% fern cover (87). Abundant in mixed-wood forests of Alberta with trembling aspen, balsam poplar (Populus balsamifera), white spruce (Picea glauca), black spruce, and understory dominated by fruiting shrubs (Rosa), alder, and willow (Salix) (78). In central Alberta, Connecticut Warbler occurred more often in aspen stands versus mixed-wood stands (88). The canopy composition was dominated by aspen (> 80%) with less than 10% conifers, whereas mixed-wood stands had greater than 50% aspen and at least 20% white spruce; both stand types shared a common understory comprised of green alder (Alnus crispa), Saskatoon serviceberry, and willows (88).

Often in pure stands of trembling aspen, but also mixed-woods comprised of aspen and spruce in British Columbia, forests with dense understory and high canopy with stand age varying from young to mature to older forests (89). Found in open forest, dense forest and muskeg/riparian areas (90); also found in groves of young aspens with poplars and white spruce (7). Typical elevation of these habitats occurs from 400–1,100 m (91). Dense understory is usually less than 3 m in height (92). Mid-canopy is open with few branches (93). A comprehensive study by Preston et al. (94) found the following characteristics of breeding habitat near Dawson Creek: wide age range of aspen stands with average age of 68 years, canopy containing gaps; low densities of spruce, average shrub cover less than 20%, less than 5% of cover by tall shrubs (less than 2 m tall), herbaceous cover greater than 75%, approximately 40% cover by grasses, terrain with an average slope 8.1°. Tall cover by herbaceous plants combined with low shrub cover were considered best predictors of breeding habitat. Most common shrubs were rose, soopolaillie (Shepherdia canadensis), raspberry, and spiraea (Spiraea spp.). Herbaceous species dominated by grasses (unknown species) along with fireweed (Epilobium angustifolium), willow, tall bluebell (Mertensia paniculta), aster (Aster spp.), American vetch (Vicia americana), and peavine (Lathyrus nevadensis) (94).

Several studies of habitat monitoring of boreal species in Minnesota have included breeding habitat descriptions. Grinde et al. (95) found that abundance was positively associated with lowland coniferous forests in the Chippewa National Forest. Wet, mature black spruce–tamarack forests made up the most important breeding habitat in the Chippewa and Superior National Forests in northern Minnesota (96). Warblers occupied wet coniferous forests in Rice Lake National Wildlife Refuge and Tamarac National Wildlife Refuge in Minnesota (97). Zlonis et al. (98) used Maxent modeling to find the best environmental predictor variables that correlated with species presence and breeding habitat in northern Minnesota. Stagnant stands of black spruce and tamarack, intermediate levels of basal area occupied by trees, stand age, and buffers from human disturbance like logging were positively correlated with species presence. Sedge meadow, upland forests within 200 m, and mature cedar forests were negatively correlated. They were also found breeding in productive black spruce and tamarack bogs with harvestable timber.

Bednar (99) described the site characteristics of three habitats that contained breeding warblers in the Agassiz Lowland Subsection of Minnesota: 1) stagnant stands of black spruce and tamarack bogs; 2) semi-productive black spruce and tamarack bogs; and 3) productive black spruce and tamarack bogs. The stagnant stands were characterized by an average DBH of 8 cm, average height of 5–6 m and average age of 65 years. Semi-productive black spruce and tamarack bogs were intermediate with an average age of 97 years, average DBH of 11 cm and average tree height of 11 m. Semi-productive black spruce and tamarack bogs were the most important habitat for the Connecticut Warbler. Productive black spruce and tamarack bogs were older with larger trees: average of 127 years old, average DBH of 15 cm, average height of 13 m. Bednar (99) also observed the Connecticut Warbler in forests disturbed by the eastern larch beetle (Dendroctonus simplex). The site characteristics of this habitat were similar to semi-productive black spruce and tamarack bogs due to the widespread destruction of larger tamarack trees by the beetle.

Lapin et al. (100) studied associations between habitat and landscape variables and Connecticut Warbler abundance at three different spatial scales, within buffers of radii of 100 m, 500 m, and 1,000 m. Higher densities of ground cover, lower percentage of canopy cover and lower percentage of deciduous forest were important at the 100 m spatial scale. Higher percentage of black spruce and lower density of all trees were important at the 500 m scale. The most important variables at the 1,000 m scale were higher percentages of upland and lowland coniferous forests, low percentage of upland deciduous forest, and larger forest patch size. They concluded that the most suitable habitat was found in large patches of lowland coniferous forest that were mixed together within a mosaic of lowland and upland conifer forests in northern Minnesota.

Fledglings moved from mature forest to disturbed habitat in Minnesota. Streby (101) caught fledglings with mist nets in regenerating clearcuts of different ages with average canopy height less than 5.1 m. Younger stands were composed of red raspberry (Rubus strigosus) and aspen (Populus spp.) saplings less than 2 m in height. Older stands were dominated by taller aspen saplings (4–6 m) with red maple (Acer rubrum), hazel (Corylus spp.), and pussy willow (Salix discolor).

In northern Wisconsin, the species is found in oak‒jack pine forests with dense underbrush, as well as margins of coniferous swamps (102). Dependent on conifer forests with understory of shrubs, dead pine branches, ferns (Pteridium), and tamarack bogs throughout most of the state (80). Based on breeding bird survey data, warblers were more common in mature jack pine forests with DBH > 12.7 cm and dense understory with small shrubs in Bayfield county, Wisconsin (103). In Michigan, there is a variety of forest habitats, but often consisting of aspen (104): dry mixed-woods forest with aspen‒birch, aspen‒pine‒fir; mesic mixed forests consisting of aspen, firs (Abies), and pines; wet coniferous spruce forest and tamarack bogs (80). Corace et al. (97) found Connecticut Warbler in wet coniferous forests on the Seney National Wildlife Refuge in northern Michigan. See also Breeding: Nest Site, and Food Habitats: Feeding.

Habitat in Nonbreeding Range

Habitat in Migration

During spring migration, predominantly recorded in brushy understory tangles in swamp forest, and on occasion in upland woods in Ohio; some males were noted to leave the dense tangles to perch and sing from small trees or bushes (105). It was observed more often in forest gaps created by tree falls with dense understory during spring migration in Illinois (106, 107). It occurs on or near the ground in mature floodplain forest in Kentucky (108). It is uncommon but regular in city parks and yards in residential areas with lilac bushes (Ceanothus), shrubs, and other low, dense vegetation in Wisconsin (102).

During fall migration along the East Coast, it frequents: wet brushy areas; low, damp woodlands with dense undergrowth; and weed and brier patches along the edges of moist fields, pastures, and banks along ditches and hedgerows (109). The Connecticut Warbler is one of several fall migrants that modifies its use of habitat patches based on the availability of ripe fruit, invertebrates, soil moisture, and defoliation at migratory stopover sites near Acadia National Park in Maine. Olsen et al. (110) determined that this bird is sensitive to changes in these variables and subject to potential phenological mismatches with stopover arrival times caused by climate change. In Massachusetts, occurs in large maples (Acer) in wooded area of swamps, thickets of clethra, shadbush (Amelanchier), black alder (Alnus glutinosa), on the ground and under or within rank vegetation in beds of jewelweed (Impatiens); it is also found along banks of intersecting ditches with deadly nightshade (Solanum) (109, 111, 112). Noted on occasion foraging in branches of taller trees during fall stopover; e.g., 20 m above ground in tall willows (Salix) (111). Along the coast of New York, it appears in dense undergrowth with ragweed (Ambrosia), sunflowers (Helianthus), asters (Aster), catbrier (Smilax), poison ivy (Rhus), and bayberry (Berberis) (113). It has a similar habitat in Pennsylvania: dense undergrowth of willows, ragweed, and jewelweed (114). Fetterman (115) described the vegetation characteristics of a fall stopover site for the Connecticut Warbler south at Rushton Woods Preserve near West Chester, Pennsylvania, as having an average canopy of 12.9 m consisting of box elder (Acer negundo) and black walnut (Juglans nigra), with the average shrub height of 3.4 m of amur honeysuckle (Lonicera maackii), spicebush (Lindera benzoin), and multiflora rose (Rosa multiflora). Thirty-three birds were observed using the capped Erie landfill at Meadowlands, in Lyndhurst, New Jersey where the habitat was composed of shrubs and herbaceous layer, with black locust (Robinia pseudoacacia) and eastern cottonwood (Populus deltoides) trees (116). During fall migration, Bystrak (117) banded 13 warblers in an upland deciduous forest with a 2–3 m canopy of shrubs along a powerline corridor in eastern Maryland. Fall vagrants in Colorado have been observed away from woods in willows up to 1.5 m tall (118).

It has been observed in mesquite woodland and ucar (Bucida) in Puerto Rico (119). It is found in a variety of different habitats in Bermuda, including overgrown or cultivated fields, scrubland, parks, gardens, and hedgerows (120). In Venezuela, transients have been recorded in parks, gardens, almost any habitat with trees and dense understory thickets, mostly at lower elevation, but up to 4,200 m (57). It appears primarily in tropical thorn scrub in northwestern Venezuela with blackbeads (Pithecellobium), mesquites (Prosopis), and cercidium (Parkinsonia), and undergrowth with prickly pear (Opuntia), Croton, and Lippia (121). In mesquite scrub in foothills of Coastal Cordillera of Venezuela, transient individuals were observed in late April and mid-to-late October in scrub on the ground near a dry water course and 1.5‒3 m high in mesquite trees (122, 123). It has been recorded in moist second-growth and forest edge at lower and middle elevations during fall migration in Costa Rica (124). It has occurred in second-growth of clearings in southeastern Peru (29). A presumed fall migrant was mist-netted on 18 November in white sand forest in northeastern Peru, where the habitat contained sandy soil with an average depth of 41.7 cm ± 20.5 SD, average canopy height of 14.3 m ± 3.5 SD, dense woody vegetation, and trees with average DBH of 19.8 cm ± 4.5 SD (125).

Habitat in Overwintering Range

Information needed. Poorly known since there are few definitive records of overwintering individuals. In South America, habitats reported to include humid forest undergrowth, woodland, forest edge, generally below 1,000 m elevation (58). There are records of overwintering birds from two areas in Bolivia. It has been reported from 220–370 m in dry, deciduous woodlands at several localities from the Chaco in southeastern Bolivia. An adult male and female from the Chaco were banded during the first week of April, 1997 in the Santa Cruz Botanical Gardens. Additional records from the Chaco region include Tucavaca, Estancia El Canon, Fortin Ravelo, Palmar de las Islas, dates unknown (63). In the upper Machariapo Valley of Bolivia at 1,200–1,500 m it occupies in dense deciduous scrub with a canopy height of 5–7 m; such habitats in the Machariapo and similar inter-Andean valleys are believed to be important overwintering areas in Bolivia (62).

Recommended Citation

Pitocchelli, J., J. L. Jones, and D. Jones (2023). Connecticut Warbler (Oporornis agilis), version 2.0. In Birds of the World (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.conwar.02