Connecticut Warbler Oporornis agilis Scientific name definitions

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

Conservation and Management

Classified globally as a species of Least Concern by BirdLife International (263) owing to its large range and population size (1,800,000 individuals [238]; see Population Status). Although the global population is believed to have declined by 0.8% annually from 1966–2019 (258), the decline was not sufficiently rapid to approach the thresholds for Vulnerable by BirdLife International (263). The Connecticut Warbler received a G4 (Apparently Secure – low risk of extinction but cause for concern) Global status ranking from NatureServe (264). The Connecticut Warbler is currently considered a Group 2 - Mid-priority candidate for extinction by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). Connecticut Warbler is on the Yellow Watch List of Partners in Flight owing to a combination of vulnerability scores: a high vulnerability score due to population decline and moderate vulnerability scores for population size and threats on the breeding and overwintering ranges (265).

NatureServe (264) provided a regional status for individual provinces and states: British Columbia–S4B (Apparently Secure breeding population), Alberta–S4B, Saskatchewan–S4B, Manitoba–S3S4B (Range of uncertainty from Vulnerable to Apparently Secure breeding population), Ontario–S5B (Secure breeding population), Quebec–S3S4B, Minnesota–SNRB (currently not assessed), Wisconsin–S2B (Imperiled – breeding population at high risk of extirpation), Michigan–S3 Vulnerable). There were status designations from other authorities for some provinces and states with minor differences from NatureServe. It is considered a Species at Risk in British Columbia (241), but is ranked as Secure in Alberta (266). The Minnesota Department of Natural Resources (267) considered it a Species in Greatest Conservation Need.

Ornithologists are beginning to assess the status and vulnerability of birds to climate change. Shank et al. (268) studied the relative vulnerability of species breeding in Alberta to climate change that included the Connecticut Warbler. They used NatureServe’s Climate Change Vulnerability Index (CCVI) and Ensemble climate change projections to predict how this species will respond to the effects of climate change by the 2050s. The Connecticut Warbler fell into the Medial Vulnerability category, in between the Most and Least Vulnerable quartiles for all bird species throughout Alberta. It also fell into the Medial Vulnerability category when they analyzed individual Natural Regions of Alberta (e.g., Boreal, Foothills, Parkland, Rocky Mountain) where it breeds. When they used different general circulation models (GCM), instead of Ensemble, to estimate projected climate changes, the CCVI results were equivocal. Two of five different GCM’s indicated higher levels of vulnerability versus one model that showed less vulnerability. The Connecticut Warbler was expected to be able to shift its range in response to climate change but would be limited by the amount of suitable habitat. Hoving et al. (269) conducted a vulnerability to climate change assessment of 180 species in Michigan. The state ranking of extinction risk for the Connecticut Warbler was a 2 from a range of 1 (imperiled) to 5 (secure). This ranking uses various data such as recent population trends and other factors besides climate change at the state level. The NatureServe’s Climate Change Vulnerability (CCVI) for this species was a Presumed Stable status in Michigan. This CCVI status takes into account the fact that this species may leave the state to colonize suitable habitat elsewhere in its breeding range (269).

Effects of Human Activity

Habitat Loss and Degradation

Hallworth et al. (64) found a strong correlation between population declines and habitat fragmentation on the breeding grounds. Habitat loss had a greater impact on eastern and southern populations breeding in jack pine (Pinus banksiana), tamarack (Larix laricina), and black spruce (Picea mariana) forests compared to northwestern populations that breed in upland aspen forests. Some of the leading causes of population decline in Minnesota are habitat loss and fragmentation (267). Loss of important jack pine forest has occurred in Wisconsin due to outbreaks of spruce budworm (Choristoneura fumiferana) and fire suppression (80). There was no difference in occurrence between continuous forest and forests fragmented by agriculture in Saskatchewan; however, more common in larger forest fragments or fragments within 5 km of areas with high local forest cover versus smaller fragments (79).

Impacts of Forest Management and Silviculture

An important focal species in studies of the effects of silviculture, forest management, and forest fragmentation due to logging. Retention levels of residual trees affected abundance in logged areas of mixed-wood forest in Alberta. Tittler et al. (270) found a decrease in abundance with an increase in basal area of trees left behind in cut blocks; results suggest that the Connecticut Warbler benefits from logging practices involving retention of lower densities of residual trees. However, low to moderate levels of tree retention had a negative impact on Connecticut Warbler breeding in mature deciduous-dominated forests in Alberta. Tree retention levels of 0% (clearcut), 20%, and 50% resulted in decreases in abundance or disappearance of Connecticut Warbler versus 100% retention levels (no cutting) that showed an increase in abundance (271).

Adults crossed clearcuts as they moved from one forest fragment to another, juveniles were found more often than adults using buffer strips to make crossings around clearcuts (272). Also reported to exhibit habitat compensation (e.g., ability to respond to loss of deciduous forest due to logging by exploiting other habitats; 273).

Corridors linking forest fragments caused by logging do not appear to affect abundance of Connecticut Warbler. Abundances were relatively stable in fragments surrounded by clearcuts on three sides, but connected to similar fragments by riparian corridors in Alberta. But, abundances increased in isolated fragments completely surrounded by clearcuts, creating temporary overcrowding followed by decreasing abundance (78). Hannon and Schmiegelow (274) also found equivocal evidence of the benefits of corridors. Abundance levels among control plots, isolated plots, and plots connected by corridors varied annually from 1995–1998 with no obvious trend suggesting the benefits of corridors.

Impacts of Wildfire

The impact of wildfires on species breeding in the boreal forest has been a major concern in the Prairie Provinces. Hobson and Shieck's (275) study showed increased abundance in post-fire stands that have fewer residual trees and shrubs. Zlonis et al. (276) studied the impact of fire severity in boreal forests of northeastern Minnesota on bird communities. Burns ranged along a continuum from severe (all trees destroyed, leaving area barren of vegetation) to low (mosaic of vegetation mortality from understory to canopy and most trees surviving). Abundance of most species in low-severity burns was similar to control areas versus decreased abundance in high-severity burns. The Connecticut Warbler was among the one-third of the species in this study whose post-fire abundance was unaffected by burn severity.

Comparisons of the Impacts of Wildfire versus Silviculture

There are numerous studies comparing the impact of wildfires versus logging and silviculture on breeding bird communities. These studies have produced mixed results for Connecticut Warbler. Cadieux et al. (277) modeled the potential impact of timber harvesting and the effects of climate change on fire activity and forest growth on bird communities in the Boreal Plains of northern Alberta. Climate-induced fire activity and forest growth were less important factors affecting warbler densities in this study. Change in abundance due to timber harvesting was the most important contributing variable predicting > 20% increases in Connecticut Warbler densities. This positive change was largely due to an increase in deciduous forests which are the preferred breeding habitat in the western part of the breeding range.

Charchuk and Bayne (278) studied the effects of timber harvesting techniques on species richness in boreal forest communities in Alberta. Natural Disturbance Harvesting (NDH) is an approach that attempts to mimic the effects of fires and or other natural disturbances. Understory Protection (UP) is a relatively new harvesting method that protects understory conifers, especially white spruce (Picea glauca), during timber harvesting. The effects of both techniques were compared to unharvested control plots. Their results showed that UP harvesting and the retention of white spruce was more beneficial to the Connecticut Warbler and other mature forest bird species. The destruction of white spruce and other understory plant species during NDH is less attractive to the Connecticut Warbler.

In contrast, comparisons of post-fire versus post-harvest stands showed higher densities of Connecticut Warbler in the post-fire stands; post-fire stands had fewer shrubs and live trees (275). Results were opposite those of the Mourning Warbler (Geothlypis philadelphia), a species that preferred post-harvest stands over the post-fire stands. Fuel breaks are part of a forest management strategy to mitigate the impact of wildfires on human communities. It involves clearing and or thinning combustible vegetation in large blocks surrounding these communities. The initial clearing is often followed by recurring visits to remove or burn regenerating second growth that could be fuel for future fires. Hobson and Kardynal (279) studied the response of boreal forest species to a fuel break in Waskesiu, Saskatchewan. They found that Connecticut Warbler density increased on the fuel breaks over a 16-year study period compared to densities from uncut reference sites.

Impacts of Energy Development, Powerlines, Artificial Noise, Artificial Light

Vegetation regeneration at former oil well sites in Alberta did not immediately benefit the Connecticut Warbler. While some boreal species began to colonize these well sites, only 6 Connecticut Warblers were detected using an acoustic location system, and they were in adjacent, mature forest, not on reclaimed well sites (280).

May be affected by powerline right-of-ways on breeding grounds (217): mean perpendicular distance between power lines and breeding territories was 144 m; greater than the 108 m between territories and control plots with similar characteristics but lacking power lines. The average height of trees near power lines was lower compared to control plots, which might have influenced habitat choice.

Occupancy rates were not affected by chronic noise from compressor stations at well pads and gas pipelines near breeding sites in Alberta (281). In Labrador, Hennigar et al. (48) studied the effects of noise and light pollution on boreal breeding birds, including Connecticut Warbler. Neither artificial light nor noise exposure during the night affected the start of the dawn chorus. There was some evidence that birds were attracted to and moved towards experimental traffic noise, but not the experimental artificial light during these experiments (48).

Effects of Invasive Species

Information needed.

Shooting and Trapping

Griscom (282) described declines based on observations in the area of Boston, Massachusetts. The number of Connecticut Warblers he observed declined from 4 birds per year to less than 3 birds every 10 years. Griscom and Snyder (283) speculated that scientific collecting may have played a role in population declines based on 60 specimens collected in 1870 and greater numbers (no data given) in 1871. These data should be interpreted with caution owing to inadequate and informal sampling.

Pesticides and Other Contaminants/Toxics

On the breeding grounds, herbicides are commonly used by forest managers to manage vegetation to enhance timber production. Logging of aspen (Populus) stands combined with herbicides that reduce understory vegetation was reported as the most serious threat in British Columbia (93, 89). Dacosta et al. (284) found reduction of the use of herbicides could benefit Connecticut Warblerin the eastern part of the breeding range. They modeled the ecological, economic and social impacts of reducing herbicides in the Spanish Forest and Romeo Malette Forest in southeastern Ontario. The Connecticut Warbler was among 21 species whose habitat requirements were taken into consideration when applying various levels of herbicides in these models. Results indicated that reducing herbicide levels would result in less habitat for early-successional species like American Kestrel (Falco sparverius), but more habitat for mature, older forest species like Connecticut Warbler, Black-backed Woodpecker (Picoides arcticus), and mammals like moose (Alces alces) and American marten (Martes americana). There was also less area harvested in plots with reductions in herbicides that may have contributed to the increased habitat availability in these mature, older stands.

Collisions with Stationary/Moving Structures or Objects

There are numerous accounts of birds colliding with human-made structures during migration. Loss et al. (285) conducted an extensive metanalysis of 23 studies from the published literature and found that Connecticut Warbler has a high vulnerability for building collisions, with the eighth highest risk of colliding with any size building. Their estimate of vulnerability was 22.9 times higher than species with an average risk for collision. The risk for high-rise buildings (greater than 12 stories) was 52.0 while the risk for low-rise buildings (4–11 stories) was 35.7. Longcore et al. (286) estimated that 14,324 individuals or 1.2% of the entire Connecticut Warbler population was killed annually by collisions with communication towers in North America.

Highest mortality reported by Robbins (102) in Wisconsin: 300 birds in one season, 140 in one night at Eau Claire; Connecticut Warbler accounted for 2% of all warblers killed by towers in Eau Claire. Two fall migration records of building strikes through Chicago, Illinois occurred from late August through mid-October (287). Two fatalities were reported from collisions with communications towers in Michigan from 2003–2005, one in spring 2004 and one in fall 2005; these birds represented less than 1% of the total fatalities in this study (288). A single victim of a window collision was reported in a study in Minneapolis/St. Paul, Minnesota from spring 2007 to fall 2009 (289).

Larger numbers were reported killed in New York from the late 1800s to the mid 1990s (290, 113): 10 striking Empire State Building in New York City (11 September 1948); 13 hitting Westhampton Air Force Base tower on Long Island (Suffolk County), New York (5 October 1954); 57, 16, and 18 at Fire Island lighthouse on 3 different days, respectively, in September and October in 1877 and 1883. Seven birds were killed from collisions with buildings in lower Manhattan from 1997–2008, primarily buildings with glass windows reflecting outdoor vegetation or transparent windows with indoor vegetation (291).

Eleven fatal collisions were reported at the John F. Kennedy Space Center in east-central Florida during spring migration from 1970–1981 (292), and 6 spring migrants killed by TV towers over 25 years in northwestern Florida (293). Single or few birds were recorded striking lighthouses on Sombrero Key and Mosquito Inlet during spring and fall migration in Florida, and a building at Seabreeze High School, Daytona Beach, Florida during fall migration (170).

Other Human/Research Impacts

Three of 7 nests were abandoned by parents in eastern Quebec. Abandonment might have been caused by researchers near the nests (27). Needs further study.


Conservation Measures and Habitat Management

Niemi et al (96) suggested that forest management plans that result in increased conifers would benefit the Connecticut Warbler in Minnesota and Wisconsin. Beaudry et al. (239) built habitat suitability models as part of a study identifying important areas for conservation planning for the Connecticut Warbler and 19 other species breeding in northern Wisconsin. Models developed for land-use managers emphasize the use of pine barrens and woody wetlands with dense shrub and herb layers.

Habitat suitability models were compared to metadata from previous studies and actual data from the Wisconsin Natural Heritage and Wisconsin Breeding Bird Atlas program. Abundance predicted by the model was significantly correlated with the metadata and actual detections (239). The estimated amount of habitat needed to maintain current breeding populations in the Laurentian Mixed Forest Ecoregion of northern Wisconsin is 1,697 ha (239). In a follow up study, Beaudry et al. (294) developed a conservation plan for reserves for at-risk species like Connecticut Warbler in this same area of northern Wisconsin. Instead of the minimal habitat needed by each species in this region, this study identified the potential habitat for each species in the area. Another important part of the plan was to find public lands that would minimize economic and land use conflicts versus land conservation. They estimated that approximately 1 million ha would provide the best minimal habitat requirements for a majority of species in this region. The potential habitat in this area for Connecticut Warbler was estimated to be 231,822 ha of which 103,861 ha (45%) were located on public lands.

Beaudry et al. (295) also proposed using models that link socioeconomic and ecological variables for conservation planning and mitigating habitat loss in northern Wisconsin. They developed three econometric scenarios for different, simulated land use policies on private lands: Baseline (net revenue from land use was held constant), Forest Incentive (subsidy for planting new forest and tax on deforestation) and Urban Growth (net increase in revenue for urban development). Each scenario provided estimates of land use and habitat changes, and their effects on breeding birds. Increasing subsidies for landowners under the Forest Incentive model led to a decrease in projected habitat loss for most species in this study but did not affect the Connecticut Warbler. However, Connecticut Warbler was one of 16 species projected to lose habitat with development under the Urban Growth scenario. The results were significant and negative for this species when compared to results for the Baseline Scenario.

Cooper and Beauchesne (89) made recommendations for preserving breeding habitat in British Columbia: retain mixed-woods forest and/or pure stands of aspen ≥ 5 ha, retain dense understory of herbs and shrubs, maintain habitat corridors, limit amount and timing of grazing (after nesting) in these stands, restrict access routes and pesticide use. Saulnier (27) studied the effects of a forest/blueberry management plan on the Connecticut Warbler breeding in jack pine forests in eastern Quebec. This agroforestry system consisted of alternating 40‒60 m bands of blueberry fields with jack pine forest. The 7 nests discovered in this study were within or very near the forest bands. Saulnier (27) hypothesized that the alternation of blueberries and forest bands could support boreal species like Connecticut Warbler, but suggested that harvesting of jack pine forest bands should be limited to a third (20 m) or less of each forested band to sustain these populations.

Effectiveness of Measures

Information needed.

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