Hooded Warbler

A regular breeding species and migrant; the Hooded Warbler was a rare species during the Minnesota Breeding Bird Atlas (MNBBA).

A species largely restricted to the eastern deciduous forest south of the Great Lakes, the Hooded Warbler has slowly been expanding its range northward. Along with several other southeastern warblers and vireos, it is also expanding to the west, particularly across the southwestern United States and into California. The core of the species’ distribution, however, remains in the southeastern United States (Figure 1).

Assigned a Continental Concern Score of 9/20 by Partners in Flight; officially listed as a Special Concern Species in Minnesota and designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources.

A medium- to long-distance migrant that spends winters in the Caribbean Islands and Central America.

An insectivore that obtains prey items by foliage gleaning, ground gleaning, and hover gleaning and by sallying out to catch insects in midair.

Open-cup nest usually placed in shrubs less than 1.5 meters above the ground.

When Roberts was actively documenting Minnesota’s bird life in the late 1800s and early 1900s, the Hooded Warbler had yet to be observed in the state. The first report of its presence would occur 10 years after he published his two-volume treatise, The Birds of Minnesota, in 1932. In an area now known as Roberts Bird Sanctuary, located on the north end of Lake Harriet in Minneapolis, an adult male was observed and heard singing on May 17, 1942 (Davidson 1942). Twenty years would pass before the second report was logged. On June 2, 1962, an adult male was caught in a mist net at a banding station at Lake Demontreville in Washington County (Olyphant 1962). Then, from 1969 through 1973, two additional reports were documented from Hennepin County (Arneson 1972; Stanley 1973) as well as one new report each from Ramsey County (Peake 1970) and Mille Lacs County (Goldberg 1973). All six records, from 1942 through 1973, were of individual birds, including four adult males and two adult females. Three of the records were in the spring season, two in the summer, and one in the fall. When Green and Janssen published their updated account of the species in 1975, the species’ status was classified as accidental.

Then, beginning in the spring of 1975, the Hooded Warbler has been documented somewhere in the state, during the spring or summer season, every single year except 1986 (Minnesota Ornithologists’ Union 2016). From 1975 through 1983, observations of single individuals continued to be reported in Hennepin County, from Roberts Bird Sanctuary and Theodore Wirth Park, and in Anoka and Scott Counties. Additional outstate reports were logged from Clearwater, Crow Wing, Le Sueur, Olmstead, Otter Tail, Rice, and Sherburne Counties (Table 1). Finally, 42 years after the Hooded Warbler was first observed in the state, a pair nested successfully in 1984 at Murphy-Hanrehan Park Reserve, which straddles the Scott-Dakota county line on the southern edge of the Twin Cities metropolitan region (Fall 1984).

Encompassing a matrix of woodlands and abandoned farmland, the reserve is unique because it includes a large, unfragmented tract of mature hardwoods. Approximately 3 km² in size, the site is a rare resource in a densely populated region that supports nearly 4 million people (Fall 1989). Since the first pair was reported nesting in 1984, Murphy-Hanrehan has become the core of the state’s breeding population, supporting as many as 35 territorial males during the 2001 breeding season (Minnesota Ornithologists’ Union 2016).

During the 1980s and 1990s, as numbers continued to build in Dakota and Scott Counties, an increasing number of reports were logged from the Twin Cities metropolitan region, including Anoka and Carver Counties, and from several southeastern counties, including Fillmore, Freeborn, Goodhue, and Winona. Scattered reports were also documented as far as Pipestone County in the spring of 1992, Cook County in the fall of 1995, and St. Louis County in the spring of 1999 (Minnesota Ornithologists’ Union 2016). Most surprising was a well-documented report of a nesting pair at Camp Ripley in Morrison County during the 1996 breeding season (Merrill 1996), two summers after a male was first heard singing and was observed on the facility grounds (Merrill 1994).

Then, in the following decade, in the years leading up to the initiation of the MNBBA (from 2000 to 2008), more spring reports came from the northwest region, including Beltrami and Kittson Counties, and the southwest region, including Lac qui Parle, Lyon, and Murray Counties. There was a summer report from Pennington County in 2008 and Pine County in 2002 (Minnesota Ornithologists’ Union 2016). Nesting was confirmed in Anoka County in 2003. The report followed several seasons where birds were observed at multiple sites in the county, including at Linwood Lake, where nesting was eventually confirmed (Bardon 2003).

Despite all the widely scattered reports of birds since 1942, Murphy-Hanrehan remained the one site where the birds were most consistently documented during the summer months.

The Hooded Warbler’s range expansion into Minnesota coincided with its expansion in the neighboring states of Wisconsin and Iowa. In Wisconsin, at least 1 to 10 birds were reported nearly every year since 1939, the first year records were published in The Passenger Pigeon, the state’s ornithological field journal (Robbins 1991). Most records were confined to eastern counties near Madison and Milwaukee and were males that were briefly present from May 5 to May 25. The first nesting record was documented in 1975 in Manitowoc County in southeastern Wisconsin. Five more breeding records were reported in 3 southeastern counties by June 1980 (Robbins 1991). During Wisconsin’s first breeding bird atlas (1995–2000), the species was confirmed nesting in 13 counties, with probable or possible breeding in 15 additional counties, all within the southern two-thirds of the state (Cutright et al. 2006).

In Iowa, the species was reported as common in the floodplain forests of southeastern Iowa in the late 1800s, when two nests were found in Lee County (Currier 1895). But that may have been a rare and localized occurrence. In the late 1800s there were a few scattered reports from eastern counties, and between 1900 and 1967 there were only four records from 4 counties bordering the Mississippi River (Kent and Dinsmore 1996). Beginning in 1968, the number of reports slowly but steadily increased. When the first breeding bird atlas was conducted in Iowa (1985–1990), the Hooded Warbler was reported in seven blocks as far west as the outskirts of Des Moines, including one confirmed nesting report just west of Iowa City. During the second atlas (2008–2012), the species was reported in 14 blocks, including 4 where nesting was confirmed (Iowa Breeding Bird Atlas, Iowa Ornithologists’ Union 2016).

Recent range expansions and increasing populations have been noted in other localities as well, including Michigan (Chartier et al. 2013), Ontario (Cadman et al. 2007), and in the southwestern United States and California (Patten and Marantz 1996).

During the MNBBA, the Hooded Warbler was reported in only 15 surveyed blocks and in 5 priority blocks. Breeding evidence was documented in 5 blocks (Figures 2 and 3; Table 2). The birds were reported from 9 of Minnesota’s 87 counties: Chisago, Dakota, Hennepin, Mower, Morrison, Scott, Sherburne, St. Louis, and Washington. Nesting was reported in 3 counties: Dakota, Scott, and Washington. The specific nesting localities were Afton State Park in Washington County, Murphy-Hanrehan Park Reserve in Dakota and Scott Counties, and Lebanon Hills Regional Park in Dakota County.

In addition to these atlas records, a number of observers reported the Hooded Warbler at three other localities during the five breeding seasons that spanned the MNBBA (2009–2013). Details were not specific enough to assign to a given atlas block but included the following: 1) an observation at the Rapids Lake unit of the Minnesota Valley National Wildlife Refuge in Carver County on July 1, 2009, and 2) observations on June 4 and June 7, 2011, at Hyland Lake Park Reserve and Cedar Lake, respectively, in Hennepin County. Following the MNBBA, a pair also was reported nesting in 2015 at the Woodland Trails Park in Elk River, Sherburne County, where the species was first reported in 2009 (Minnesota Ornithologists’ Union 2016, MNBBA).

Although the Hooded Warbler has become a regular breeding resident and migrant in Minnesota over a period of nearly 75 years, it remains a relatively rare to uncommon species. While records are scattered across all regions of the state, its primary breeding range remains restricted to the Twin Cities metropolitan area, bounded by Dakota and Scott Counties in the south, Washington County in the east, and Anoka and Sherburne Counties in the north. Further expansion outward from this core is likely in the coming years. Nevertheless, for a species that has expanded its range from the core of its distribution in the southeastern United States, it is interesting that there are so few reports within Minnesota along the Mississippi River and its tributaries, particularly from some of the regions’ larger forest tracts on state forest lands and state parks.

*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.

The Hooded Warbler’s breeding habitat is broadly described as mature forest interspersed with canopy gaps that allow a dense growth of small saplings and shrubs (Figure 4) (Chiver et al. 2011). In the core of its breeding range in the southeastern United States, it is frequently associated with mesic to wet lowland forests dominated by cypress and tupelo with an understory dominated by palmetto and cane. Further north, the species is found in a variety of mesic to dry upland forests. These include large tracts of mature hardwoods, selectively harvested stands of conifers and hardwoods, and former pastures with a sparse tree layer and a dense growth of shrubs and young trees (Dunn and Garrett 1997; Chiver et al. 2011; Danz et al. 2007; Bielefeldt and Rosenfield 2001).

The composition and age of the canopy appears less important than the structure and composition of the understory. Here a dense growth of shrubs and/or young saplings, vines, or semi-woody herbaceous plants is critical (Bielefeldt and Rosenfield 2001). The small woody stems provide support for the warbler’s nests, which are usually less than 1.5 m above the ground and provide foraging habitat. Most foraging is restricted to an area less than 3 m above the ground (Dunn and Garrett 1997; Chiver et al. 2011; Bielefeldt and Rosenfield 2001).

In extensive forest tracts, pockets of dense shrubs occur where canopy gaps are created by natural disturbances, such as lightning, tree falls, and windstorms or by selective harvesting. Trails and roads also may simulate such conditions, allowing enough light to penetrate to encourage a dense understory. Some have referred to the Hooded Warbler as a “gap specialist.” But such conditions may be simulated in a variety of modified habitats, including selectively harvested conifer plantations and old pastures with scattered trees and shrubs (Chiver et al. 2011; Bielefeldt and Rosenfield 2001).

The literature on whether or not the species requires large forest tracts is conflicting. Many studies conducted further east suggest that large forests are required. Robbins (1979), for example, found the Hooded Warbler was absent from sites less than 485 ha in size, while in Illinois sites larger than 600 ha were required (Blake and Karr 1984). Another study found the most significant factors influencing abundance were the height of the canopy, the percentage of forest within 2 km, and the foliage density between 0.3 and 1 m above the ground (Robbins et al. 1989). Forest area, however, was not an important predictor of abundance. Other studies have found that, despite the species’ apparent preference for large forest tracts, the warbler successfully nests in tracts smaller than 5 ha (Norris et al. 2000; Eaton 1988), particularly when the small fragments are located near much larger tracts of continuous forest (Norris and Stutchbury 2002; Rush and Stutchbury 2008). Often subject to high rates of brood parasitism by Brown-headed Cowbirds, at least one study has shown that rates of parasitism are higher on forest tracts less than 30 ha in size than on tracts greater than 150 ha (Chiver et al. 2011).

The most comprehensive description of the Hooded Warbler’s breeding habitat in Minnesota is provided by Fall (1989), who has closely monitored the local breeding population at Murphy-Hanrehan Park Reserve since nesting was first documented there in 1984 (Fall 1984). Fall’s examination of old aerial photos shows the presence of this mature woodlot as early as 1937. The mixed hardwood stand is dominated by a variety of oak and elm species but includes a variety of other species, including sugar maple, basswood, and bigtooth aspen. The understory is dense and includes oak, elm, and ironwood saplings plus a diversity of shrubs, including dogwoods, prickly ash, and hazel. Fall speculated that the size and maturity of this forest tract were likely the major reasons a small, loose colony of Hooded Warblers has been attracted to the area.

Federal Breeding Bird Survey (BBS) data have been used to generate a North American population estimate of 5.2 million birds (Rosenberg et al. 2016). Given the number of birds reported each year at Murphy-Hanrehan as well as a handful of other sites in the state, the statewide estimate for Minnesota is likely less than a couple hundred birds.

Across its breeding range, densities are relatively low north of a line that runs from southeastern Texas and Louisiana northeast to Ohio and Pennsylvania (Figure 1). Indeed, with an average of 1.14 warblers per BBS route, Ohio is the only northern state where the average number of birds observed on each route exceeds 1 (Sauer et al. 2017). The greatest densities are found in Louisiana, where observers report an average of 9 birds per route per year (Sauer et al. 2017). Site-level reports of breeding densities are available primarily from northern study sites, where densities range from 3 to 28 territorial males per 40 ha (Chiver et al. 2011).

Since population monitoring began in the mid-1960s, the Hooded Warbler has experienced a significant annual increase in North America of 1.36% per year through 2015 (Figure 5). Since 1970, the continental population has experienced a cumulative increase of 103% (Rosenberg et al. 2016). Florida is the only state that has experienced a statistically significant decline of 2.55% per year since 1966.

Factors responsible for this increase are largely unknown. On the breeding grounds, predation and brood parasitism are the most significant factors impacting local productivity, and both are exacerbated by landscape fragmentation. Donovan and her colleagues (Donovan and Flather 2002), however, found that the greatest proportion of the Hooded Warbler’s population occurs in relatively unfragmented landscapes, where productivity is presumably higher. As long as a sufficient amount of large, unfragmented forest tracts remain available, population increases may continue.

The Hooded Warbler has been assigned a moderate Continental Concern Score of 9/20 (Rosenberg et al. 2016). Despite an increasing population, there remain concerns regarding threats to its breeding and nonbreeding habitat. It was officially listed as a Special Concern Species in Minnesota in 1996 because of its small and restricted breeding population in the state (Minnesota Department of Natural Resources 1995). It also has been designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources (2015).

Although the research is somewhat conflicting, the Hooded Warbler should benefit from conservation measures that ensure the protection and management of large, continuous blocks of mature forest that are at least several hundred hectares in size. The long-standing population that resides at Murphy-Hanrehan Park Reserve occupies a site that is at least 300 ha in size. Disturbances, natural or prescribed, that maintain small openings with dense shrub growth are essential. Management guidelines in Wisconsin recommend maintaining forest tracts that are a minimum of 100 ha in size, with a dense understory of shrubs and saplings and small (0.03–1 ha) canopy gaps. The guidelines note that high deer densities can negatively impact the suitability of sites by reducing growth in the understory through browsing (Wisconsin Department of Natural Resources 2013).

Numerous factors can impact breeding numbers, including nest predation, nest parasitism, habitat loss, and collisions with communication towers (Chiver et al. 2011; Longcore et al. 2013). A Neotropical migrant that utilizes mature forests on its wintering grounds, the Hooded Warbler is vulnerable to wide-scale deforestation underway in Central America (Chiver et al. 2011). Warming temperatures also are predicted to negatively impact the species. The 2010 State of the Birds report classified the species as being moderately vulnerable to climate change (North American Bird Conservation Initiative 2010), while a recent analysis by the National Audubon Society classified the species as “climate threatened” and predicted that the species could lose 63% of its current summer breeding range by the year 2080 (Langham et al. 2015; National Audubon Society 2016).

In the short term, however, the Hooded Warbler appears to be a species on the move. An increasing population trend and a northward range expansion have established this once‑elusive species as a regular breeding resident in Minnesota. Biologists and birders alike should continue to keep an eye out for the species in suitable habitats throughout the state.