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Connecticut Warbler

Oporornis agilis
Overview
Minnesota Seasonal Status:

A regular breeding resident and migrant; the Connecticut Warbler was an uncommon species during the Minnesota Breeding Bird Atlas (MNBBA).

North American Breeding Distribution and Relative Abundance:

The Connecticut Warbler is one of the rarest and most narrowly distributed wood warblers in the northern coniferous forest, second only to the federally endangered Kirtland’s Warbler. Its breeding range stretches across a narrow band of the Canadian boreal forest from western Quebec to eastern British Columbia. South of Canada it is restricted largely to the northern Great Lakes states, specifically, northern Minnesota, northern Wisconsin, and the Upper Peninsula of Michigan. Despite its name, the Connecticut Warbler does not breed in Connecticut, only passing through as an uncommon migrant. The species is sparsely distributed throughout its restricted breeding range, and the densest breeding populations are concentrated in a very small region of southern Ontario and northern Minnesota (Figure 1).

Conservation Concern:
Conservation Status Score 13

Assigned a Continental Concern Score of 13/20 and designated a Yellow Watch List species by Partners in Flight; designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources.

Life History
Migration:

A long-distance migrant that winters in South America.

Food:

An insectivore that gleans food from foliage, downed logs, and the ground.

Nest:

Open-cup nest placed on or near the ground, often in a mossy hummock.

Connecticut Warbler Connecticut Warbler. Oporornis agilis
© Sparky Stensaas
Figure 1.

Breeding distribution and relative abundance of the Connecticut Warbler in North America based on the federal Breeding Bird Survey, 2011-2015 (Sauer et al. 2017).

Minnesota Breeding Bird Distribution*

A large portion of Roberts’s (1932) account of the Connecticut Warbler is spent describing the years of ornithological exploration that ensued between the time of the bird’s original discovery in North America in the early 19th century and documentation of its presence as a nesting species in Minnesota. Indeed, over 70 years passed between the year 1812, when the species was first described from a migrant specimen collected in its namesake, Connecticut, to 1883, when the first nest was discovered in Manitoba. Forty-three more years passed before a second nest was found, in Alberta.

Although the Connecticut Warbler was known as an uncommon migrant in Minnesota, summer records were scanty. The first was a bird collected on the state border at Pembina, North Dakota, in 1879. The second was a male collected in 1886 in St. Louis County. Summer reports of singing birds in Aitkin, Isanti, and Marshall Counties and in Itasca State Park soon followed. Adults carrying food were observed in both Isanti County and Itasca State Park, but the discovery of a nest eluded observers until the summer of 1929, when two nests were found in an extensive black spruce–tamarack bog in Aitkin County. Relying on these accounts, Roberts (1932) described the species’ breeding range as stretching from Isanti County in east-central Minnesota to Itasca State Park and eastern Marshall County in northwestern Minnesota. The species, he wrote, “so far as discovered, makes its summer home only in cold tamarack and spruce swamps of typical Canadian Zone character. Such places are numerous and wide-spread in northern Minnesota.”

Forty years later, Green and Janssen (1975) described the warbler as a summer resident throughout northeastern and north-central Minnesota. They considered it rare throughout much of the region with the exception of an area stretching “from Koochiching County northwestward,” where it was “plentiful” in lowland black spruce–tamarack bogs and in upland jack pine and aspen stands. In addition to the confirmed nesting records that Roberts (1932) reported in Aitkin County, records were now available from Hubbard and Lake Counties as well. Farther south, the birds had not been reported from Isanti County since the early 20th century.

When Janssen (1987) provided an updated account several years later, he included a distribution map that delineated 4 counties where nesting had been confirmed since 1970: Aitkin, Beltrami, Hubbard, and Lake. The southern periphery of the species’ Minnesota breeding range was identified as northern Pine County in the east and Hubbard County in the west. Hertzel and Janssen (1998) later added Itasca County to the list of counties with confirmed nest records since 1970.

Fieldwork conducted by the Minnesota Biological Survey (MBS) in northern Minnesota further confirmed the species’ restricted distribution. Although work had yet to be conducted in the far north-central counties of the state, the most southern breeding locations were in northern Pine County and southern Aitkin County. To the west, breeding locations were documented in eastern Kittson County, Roseau County, and northeastern Marshall County (Minnesota Department of Natural Resources 2017).

MNBBA participants tallied a total of 263 Connecticut Warbler detections in 3.4% (161/4,734) of the surveyed atlas blocks and in 3.6% (84/2,337) of the priority blocks. Breeding evidence was documented in only 2 blocks (Figures 2 and 3; Table 1). The birds were reported from 16 of Minnesota’s 87 counties and were confirmed breeding in 2 counties: Aitkin and St. Louis. The Aitkin County record was an adult carrying food; the St. Louis County record was an observation of an adult carrying nesting material.

The predicted breeding distribution map of the Connecticut Warbler, generated using MNBBA data, shows relatively low abundances in much of the southern and eastern portions of the species’ breeding range (Figure 4). However, moderate breeding densities are predicted in portions of southern and central St. Louis County, where extensive peatlands occur, and throughout the entire Northern Minnesota and Ontario Peatlands Section, north and east of Red Lake. The flat, poorly drained landscape in this ecological section is dominated by peatland communities, including black spruce bogs and tamarack swamps, the Connecticut Warbler’s preferred habitat.

Overall, the species’ distribution during the MNBBA was nearly identical to that documented by the MBS, with records as far south as central Pine County and as far west as eastern Kittson County. Other than the loss of the small nesting population in Isanti County, the distribution of this rare little warbler is remarkably unchanged over the past 100 years. The same is true throughout its very narrow and restricted breeding range in the northern Great Lakes and Canada (Pitocchelli et al. 2012). Wisconsin’s first breeding bird atlas confirmed what had been known regarding the species’ distribution for at least the previous few decades (Cutright et al. 2006). Michigan, however, documented a significant decline in the number of records in the northern reaches of the Lower Peninsula between their first atlas, conducted in 1983–1988 and their second atlas, conducted in 2002–2008. There also were fewer detections in the Upper Peninsula in the second atlas (Chartier et al. 2013). By contrast, Ontario documented a 94% increase in the number of detections during their second atlas. The change, they noted, could have been attributed to a real population increase or to an increase in atlas efforts, or it could have simply been “an artifact of a small data set” (Cadman et al. 2007).

*Note that the definition of confirmed nesting of a species is different for Breeding Bird Atlas projects, including the definition used by the Minnesota Breeding Bird Atlas, compared with a more restrictive definition used by the Minnesota Ornithologists’ Union. For details see the Data Methods Section.

Figure 2.

Breeding distribution of the Connecticut Warbler in Minnesota based on the Breeding Bird Atlas (2009 – 2013).

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Figure 3.

Summary statistics of observations by breeding status category for the Connecticut Warbler in Minnesota based on all blocks (each 5 km x 5 km) surveyed during the Breeding Bird Atlas (2009-2013).

Breeding statusBlocks (%)Priority Blocks (%)
Confirmed2 (0.0%)2 (0.1%)
Probable46 (1.0%)27 (1.2%)
Possible113 (2.4%)55 (2.4%)
Observed0 (0.0%)0 (0.0%)
Total161 (3.4%)84 (3.6%)
Table 1.

Summary statistics for the Connecticut Warbler observations by breeding status category for all blocks and priority blocks (each 5 km x 5 km) surveyed during the Minnesota Breeding Bird Atlas (2009-2013).

Figure 4.

Predicted breeding distribution (pairs per 40 hectares) of the Connecticut Warbler in Minnesota based on habitat, landscape context, and climate data gathered during the Minnesota Breeding Bird Atlas (2009-2013) using the General Linear Modeling method with an adjustment for detectability.

Breeding Habitat

An inhabitant of North America’s boreal forests, the Connecticut Warbler is found in a range of different forest habitats. In Minnesota the species is most abundant in mature, lowland coniferous forests comprised of widely scattered black spruce and tamarack trees ( Figure 5; Danz et al. 2007). A rich understory of sphagnum moss underlying a low shrub understory of two peatland species, Labrador tea and swamp laurel, is also characteristic of their habitat. Occasionally birds are found in upland forests, primarily upland pine stands, including jack pine barrens and mixed aspen–jack pine woodlands.

These broad habitat preferences were confirmed by the National Forest Bird (NFB) Monitoring Program in northern Minnesota and Wisconsin, where the warbler was most strongly associated with black spruce–tamarack bogs. Some birds were found also in jack pine stands and in young, regenerating conifer stands (Niemi et al. 2016). A study in the Agassiz Lowlands of north-central Minnesota, located within the Northern Minnesota and Ontario Peatlands Section, found that Connecticut Warblers were most strongly associated in this region with semiproductive black spruce–tamarack bogs (Bednar et al. 2016). Habitat within 200 m of MNBBA point counts where Connecticut Warblers were detected demonstrated a strong preference for bogs, followed by lowland coniferous forest stands and shrub wetlands (Figure 6).

An in-depth assessment of the species’ breeding habitat was conducted in Minnesota utilizing data collected over an 18-year period on the Chippewa and Superior National Forests (Lapin 2010). Habitat within 100 meters of Connecticut Warbler occurrence records was characterized by a high density of ground cover and low canopy cover, features typical of sparsely forested peatlands. Within 500 meters, they were positively associated with lowland spruce-tamarack forests and with low tree density and high tree density variance. At the landscape scale of 1,000 meters, Connecticut Warblers demonstrated a preference for large, unfragmented landscapes of both upland coniferous and lowland black spruce forest. Overall Lapin concluded that the species appeared to rely more on large landscape characteristics for selecting suitable habitat than on smaller-scale features at the stand level, confirming the observation that the species appears to be area sensitive (Green 1995). In the Agassiz Lowlands of north-central Minnesota, however, habitat features at both the stand and landscape level were deemed important predictors of suitable habitat, specifically the proportion of black spruce/tamarack forests within 200 m, the proportion of stagnant black spruce/tamarack forests within 1,000 m, and the absence of white cedar forests and sedge meadows within 1,000 m (Zlonis et al. 2017).

Figure 5.

Typical breeding habitat of the Connecticut Warbler in Minnesota (© Lee A. Pfannmuller).

Figure 6.

Habitat profile for the Connecticut Warbler based on habitats within 200 m of point counts where the species was present during the Minnesota Breeding Bird Atlas (2009-2013).

Population Abundance

Throughout its breeding range, the Connecticut Warbler is uncommon, local, and present in low densities. As for most land birds, the federal Breeding Bird Survey (BBS) is the most robust source of range-wide population data and was used to generate a North American population estimate of approximately 1.3 million breeding adults (Rosenberg et al. 2016). In 2013, Minnesota was estimated to support 4.7% of the continental population (Partners in Flight Science Committee 2013). Applying that percentage to the current estimate generates a statewide population of approximately 61,000 individuals. This is considerably lower than the median estimate of approximately 561,000 breeding adults derived from the MNBBA  point count data. The confidence interval for the latter estimate, however, was quite large.

Connecticut Warblers are sparsely distributed throughout their breeding range; an average of only 1 bird is detected per BBS route throughout the BBS survey area (Sauer et al. 2017). In Minnesota, the average number of Connecticut Warblers on each BBS route is less than 1 (0.79). Although densities are low, the Agassiz Lowlands of north-central Minnesota provide habitat for some of the densest populations in North America (Figure 1). Data from the Agassiz Lowlands study showed that Connecticut Warblers were one of the most abundant species in their preferred habitats of semiproductive black spruce–tamarack bog (density estimate of 16.7 pairs per 40 ha) and in stagnant black spruce–tamarack bogs (density estimate of 5.1 pairs per 40 ha). They were outnumbered only by Nashville Warblers (Bednar et al. 2016). Although similar habitat was not sampled by the NFB Monitoring Program on the Chippewa and Superior National Forests, the species was not one of the 20 most common species detected in mature black spruce–tamarack stands on either the Chippewa or Superior National Forest (Niemi et al. 2016). Breeding densities were 0.6 pair per 40 ha on the Chippewa, and 0.2 pair per 40 ha on the Superior.

Since the BBS began in the 1960s, it has documented a statistically significant, survey-wide population decline averaging 1.93% per year from 1966 to 2015 (Figure 7). This represents a cumulative decline of 60% since 1970 (Rosenberg et al. 2016). In Minnesota, the species’ relatively low abundance results in a population trend that lacks statistical precision. Nevertheless, the BBS data suggest a steep downward trend line, declining an average of 2.70% per year from 1967 to 2015 (Sauer et al. 2017). This is corroborated by NFB Monitoring Program data that demonstrate significantly declining populations on both the Chippewa (7.35% per year) and Superior National Forests (6.6% per year) from 1995 to 2016 (Figure 8).

The species’ decline is broadly attributed to an array of factors, including nest predation and loss of habitat on both the breeding and wintering grounds (Niemi et al. 2016). Although nearly 50% of the state’s wetlands have been lost due to cultivation and development, the loss is significantly less in the far northern regions of the state. Even on public lands, timber harvest pressures on lowland conifers are less than those on upland stands. Other factors that may be affecting populations on the breeding grounds or during migration include long-term changes in the climate, local weather events, and collisions with buildings and communication towers (Longcore et al. 2013; Pitocchelli et al. 2012). Recently the species was classified as “climate threatened” by the National Audubon Society after models predicted that none of the species’ current breeding range would be climate suitable by the year 2080 (National Audubon Society 2017; Langham et al. 2015).

Minnesota also is on the southern edge of the species’ continental breeding range and changes are often seen in the state before they manifest themselves elsewhere. The Connecticut Warbler’s decline is occurring range-wide, however, so factors throughout its range appear to be at play. Considerably more study of this rare warbler is warranted.

Figure 7.

Breeding population trend for the Connecticut Warbler in North America for 1966–2015 based on the federal Breeding Bird Survey (Sauer et al. 2017).

Figure 8.

Breeding population trends of the Connecticut Warbler in the Chippewa and Superior National Forests and the combined regional trend, 1995–2016 (Bednar et al. 2016).

Conservation

A restricted breeding range coupled with a small and declining population led to the assignment of a moderately high Continental Concern Score of 13/20 by Partners in Flight (Rosenberg et al. 2016). PIF has also designated the species as one of 12 Yellow Watch List species, a designation reserved for species that “require constant care.” The Connecticut Warbler is designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources (2015), and a Sensitive Species on both the Chippewa and Superior National Forests (U.S. Forest Service 2012). It is a focal species for the Upper Mississippi River & Great Lakes Region Joint Venture, which covers much of the species’ U.S. breeding range: the upper Great Lakes states of Minnesota, Wisconsin, and Michigan (Potter et al. 2007).

A conservation assessment for the species was prepared for the Eastern Region of the U.S. Forest Service in 2002 (Kudell-Ekstrum 2002). General habitat management recommendations in this and other documents are very general and simply focus on maintaining large blocks of key habitat. The Upper Mississippi Valley & Great Lakes Region Joint Venture plan (Potter et al. 2007), however, has established conservation goals to protect species dependent on conifer forests, with a special focus on Connecticut Warblers, Cape May Warblers, and Olive-sided Flycatchers. To meet the habitat needs of the Connecticut Warbler, the plan recommends protecting 7,400 ha of habitat and restoring 3,800 ha in Minnesota.

Niemi and his colleagues (2016) also point out the need to protect and conserve large tracts of black spruce–tamarack forest and emphasize the importance of selecting stands found adjacent to upland coniferous stands, which also provide suitable habitat for the species.

Minnesota’s northern peatlands are a unique landscape full of rare plants and animals that deserve careful attention. The Connecticut Warbler is one of many species that make these areas so special. Continued protection of many of these sites as state natural areas is critical for the future of the Connecticut Warbler and the entire peatland landscape.

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