Magnolia Warbler

A regular breeding resident and migrant. The Magnolia Warbler was common during the Minnesota Breeding Bird Atlas (MNBBA).

Found throughout the upper midwestern and northeastern United States and Canada and south in the Appalachian Mountains to Virginia. Also patchily distributed in the central and western provinces of Canada to British Columbia (Figure 1). High densities are observed in northeastern Minnesota, Ontario, Quebec, Labrador, and British Columbia.

Assigned a Continental Concern Score of 7/20 by Partners in Flight.

Long-distance migrant, overwinters from the southern United States, to Mexico and south in Central America to Panama, also throughout the Caribbean.

Arthropods gleaned from foliage.

Low to mid-canopy in coniferous trees near trunk, especially balsam fir and spruce.

Historically described as a common summer resident in the northern evergreen forests as far south as northern Mille Lacs County and west to Itasca State Park and eastern Marshall County (Roberts 1932). Roberts included nesting observations at Cass Lake (nest with one cowbird egg), Itasca Park (male feeding young out of the nest), Itasca County (nest with four young), and St. Louis County (feeding young out of the nest). The paucity of nesting observations was likely a consequence of limited coverage in the late 1800s and early 1900s in the northern coniferous forests. Roberts stated the Magnolia Warbler was “nowhere really abundant in Minnesota during the summer months,” though he found it “common about Island Lake, in northwestern Itasca County, in June 1923.” He considered it strictly a species of the “Canadian Zone.”

More than 40 years later, Green and Janssen (1975) expanded inferred or confirmed nesting to Aitkin, Cook, Hubbard, and Lake Counties, plus they emphasized the Magnolia Warbler was most numerous in the eastern parts of its range. A few years later, Janssen (1987) included confirmed nesting from 6 counties since 1970: Aitkin, Beltrami, Clearwater, Cook, Hubbard, and Lake Counties. He also defined the breeding range as south to central Carlton, southern Crow Wing, and Hubbard Counties and west to central Roseau County. Hertzel and Janssen in 1998 added no new confirmed nesting records to Janssen’s list since 1970.

Observations by the Minnesota Biological Survey (MBS) included 619 records of breeding season locations for the Magnolia Warbler (Minnesota Department of Natural Resources 2017). The MBS included locations from southeastern Roseau County, eastern Becker County, and northern Aitkin and Carlton Counties. However, as emphasized by Green and Janssen (1975) and Janssen (1987), the bulk of breeding observations are found in northern St. Louis and Lake Counties and throughout Cook County.

The MNBBA reported 1,683 records and clearly emphasized the major distribution of the Magnolia Warbler was found in the Laurentian Mixed Forest Ecological Province. Magnolias were especially common to abundant in eastern and northern St. Louis County, northern Lake County, and Cook County (Figure 2). The observations were comprised of probable nesting in western Roseau County and northern Lake of the Woods County, as well as possible nesting in northern Marshall, eastern Mahnomen, central Becker, and northern Pine Counties. Collectively, these records slightly expand the breeding range of this species to the northwest and western portions of Minnesota from that presented by Janssen (1987).

The MNBBA added no new confirmed nesting activity for the Magnolia Warbler in counties not previously reported. Confirmed nesting blocks only represented 3.4% (23 of 676) of all blocks where the species was detected (Figure 3; Table 1). Nests or observations of young are very difficult to find among the dense, young coniferous trees. Few observers have the patience or stealthy skills necessary to discover its nest.

The predicted probability map (Figure 4) also emphasizes the highest densities occur in the northeastern region of the state, especially northern Lake and Cook Counties, plus scattered high-density areas in St. Louis County. Low densities were predicted farther south, west, and to the northwest beyond where observations currently exist in Minnesota.  However, MBS and MNBBA included breeding evidence as far west as Becker, Clearwater, and Roseau Counties and the Wisconsin Breeding Bird Atlas (Cutright et al. 2006) had probable breeding evidence for the Magnolia Warbler as far south as the extreme southwestern part of the state in Grant County.  The Magnolia Warbler should be looked for in areas of dense, young conifers during the breeding season in many areas on the periphery of its current range.

In their review of the Magnolia Warbler in North America, Dunn and Hall (2010) identified range extensions southward in Ontario from 1985 to 2005. They also reported on range expansions in several New England states as forests regenerated following clearing in the 1800s and early 1900s. Some range contractions may have occurred in the western fringes of the species’ range in Minnesota because balsam fir and spruce have become less common in these areas.

Although the Wisconsin breeding bird atlas primarily found this species in coniferous forests in the northern tier of counties, it confirmed nesting in Pierce and Grant Counties (Cutright et al. 2006); these counties are far south of any breeding observations found in Minnesota. Whether this is due to more intense coverage in the latest atlas or to more appropriate habitat for the species on the Wisconsin side is unclear. More focused efforts in eastern and even southeastern forests of Minnesota may be warranted where suitable patches of young conifers exist.

*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 Magnolia Warbler has very distinctive breeding habitats and is among the most easily recognized of Minnesota’s warblers. Its habitats are young, dense coniferous tree species usually dominated by balsam fir, young white spruce, and less often jack pine (Collins et al. 1982; Niemi and Hanowski 1992; Dunn and Hall 2010) (Figure 5). The National Forest Bird (NFB) Monitoring program has recorded 2,287 observations from 2001 to 2010 at over 612 locations, most in the Superior National Forest. The predominant locations were in pole-sized and mature mixed aspen-spruce-fir forest cover types. These habitats were especially characterized by balsam fir or white spruce in the understory. Additional forest cover types commonly used were 10- to 20-year-old upland red pine forests and dense jack pine forest cover types.

The Magnolia Warbler was more sparingly observed in the Chippewa National Forest. In most areas it tended to avoid lowland coniferous forest cover types and regenerating forests less than 10 years of age. Bednar et al. (2016) also found the species in low abundance in mixed white cedar and black spruce-tamarack forest in the extensive coniferous forests of the Agassiz Lowlands Ecological Subsection north of Upper Red Lake. Some use of lowland forest types was also noted by Green and Niemi (1978) and Niemi and Pfannmuller (1979). MNBBA detections during the point counts primarily observed the species in upland coniferous forests, bogs, mixed upland forests, and in pine forests (Figure 6).

The Partners in Flight program (Rosenberg et al. 2016) estimated a global population of 39 million breeding adults. MNBBA estimates for Minnesota’s breeding population were about 1.43 million (95% confidence interval was 1.28 – 1.70 million), or a relatively small proportion of its overall population (3.7%). The population is vastly higher in Canada, where it is broadly estimated to be between 5 and 50 million adults (Environment Canada 2011).

The federal Breeding Bird Survey (BBS) population trends in Minnesota from 1967 to 2015 showed a tendency toward decline, but the trend was not significant (Figure 7). The Magnolia Warbler was not abundant enough to detect a trend in Michigan or Wisconsin, but Ontario had a stable population. Populations in the Boreal Hardwood Transition Region and the province of Quebec recorded significantly increasing populations of 1.56% per year and 3.60% per year from 1966 to 2015, respectively (Sauer et al. 2017). Overall, Partners in Flight (Rosenberg et al. 2016) estimated a population increase of 51% from 1970 to 2014.

The NFB program has also recorded an overall stable population for the Magnolia Warbler from 1995 to 2016 in the Chippewa and Superior National Forests as well as for both forests combined (Figure 8). Recent trends in the Chippewa National Forest have been increasing since 2009.

Mean population density estimates from the NFB program within the Chippewa and Superior National Forests range 8-fold, from 0.9 to 6.8 pairs per 40 ha, respectively (Niemi et al. 2016). These are consistent with the broad increase in the species’ population from the southwestern portion of its range to the northeast in Minnesota (Figure 4). Within specific habitats, mean density estimates in the Superior National Forest range from 4.7 pairs per 40 ha to 7.5 pairs per 40 ha in aspen-spruce-fir and 16.9 in 10- to 20-year-old, pole-sized red pine. Densities were much lower in the Chippewa National Forest and in other habitat types of the Superior National Forest.

Dunn and Hall (2010) provided density estimates broadly within the ranges reported in Minnesota, including population estimates of 17 males per 40 ha in an open forest stand with little understory in Maine to 314 males per 40 ha in second-growth spruce forests.

The Magnolia Warbler is of minimal conservation concern in North America, especially with an overall score of 7/20 by Partners in Flight (Rosenberg et al. 2016). This is primarily due to its wide breeding geographic range, its substantial breeding population, and the lack of major threats to its current population. Morse (1979) reported that it occurred on islands as small as 1.5 ha in Maine.

The Magnolia Warbler tends to be killed frequently at communication towers (Dunn and Hall 2010; Longcore et al. 2013). However, in a window kill study at Minnesota Point, Bracey (2011) found no Magnolia Warblers over 5 migration seasons and 108 individual bird mortalities.

Hobson and Bayne (2000) found forest clearing to negatively affect population, but the effect was short-term if regeneration included an adequate coniferous component. Pearson and Niemi (2000) emphasized that management for mixed stands of aspen, spruce, and fir was an effective strategy. In addition, intermediate stages of red pine plantations prior to thinning can provide breeding habitat for the species; this is temporary if the site is thinned and it matures. Niemi et al. (2016) emphasized that forest management that focuses on fuel reduction and clear-cuts that remove young balsam fir and white spruce would have negative effects on population levels of this species. In addition, the Magnolia Warbler appears to be negatively affected by interactions with many of the spruce budworm specialists (e.g., Cape May and Bay-breasted Warblers) during outbreaks (Patten and Burger 1998)

In its review of climate change sensitivity for North American birds, Langham et al. (2015) and the National Audubon Society (2015) identified the species as “climate threatened” because of a potential 91% loss of its current summer range northward. Moreover, the society states that “arctic tundra probably won’t turn into spruce forest quickly,” thereby creating a habitat availability problem for the species over the long term. Therefore, climate change may be one of the most important factors to negatively affect this species in the future.