A regular breeding resident and migrant; the Scarlet Tanager was a common species during the Minnesota Breeding Bird Atlas (MNBBA).

The Scarlet Tanager is a bird of the eastern deciduous forest. Its breeding range extends across southeastern Canada south to the Carolinas in the east and the Great Plains in the west. Reports from the aspen parklands of southern Saskatchewan and Manitoba may represent a recent range expansion. Highest breeding densities are found in the Appalachian Mountains region from central Pennsylvania south through Virginia and West Virginia (Figure 1). A long-distance, Neotropical migrant, the Scarlet Tanager spend winters in northwestern South America, spanning from Panama through Columbia and south to Bolivia.

During the breeding season, the Scarlet Tanager relies primarily on an insectivorous diet secured by gleaning the forest foliage or by hawking flying insects from a perch.

A loosely woven, open-cup nest frequently placed in a mature deciduous tree 6 to 9 m above the ground and on a horizontal branch. Scarlet Tanager. Piranga olivacea
© Mike Lentz Breeding distribution and relative abundance of the Scarlet Tanager in North America based on the federal Breeding Bird Survey, 2011-2015 (Sauer et al. 2017).

In the late 1800s and early 1900s, Roberts (1932) described the Scarlet Tanager as a summer breeding resident throughout the forested regions of the state. He wrote, “It is a fairly common summer resident in all suitable places and is quite as numerous in the evergreen forests, as far north as Itasca Park, as it is in the deciduous woodlands of the south. Farther north it decreases in numbers but is found quite to the Canadian boundary.” Although far less common, the species was also found in natural woodland groves bordering lakes and streams throughout the prairie region. Confirmed nesting reports (nests with eggs or young) were available from Hennepin, Isanti, Jackson, and Wabasha Counties and Cass Lake. Inferred nesting reports (nests; adults with young or feeding young) were available from Goodhue, Itasca, and Sherburne Counties.

Years later, Green and Janssen (1975) commented that the species was most abundant in the state’s central woodlands and least abundant in the southern and western prairies. It could be found north as far as Beltrami, Cook, and Norman Counties. Janssen (1987) stated that it was abundant in the southeastern forests as well as in central and east-central Minnesota. Other than the floodplain forests of the upper Minnesota River valley, his distribution map eliminated the Prairie Parkland Province. Within the species’ primary range, nesting had been confirmed in 12 counties since 1970. Hertzel and Janssen (1998) identified 15 counties with confirmed nesting since 1970. Two counties delineated earlier by Janssen were not included: Clearwater and Winona.

The Minnesota Biological Survey has documented 1,149 breeding season locations for the Scarlet Tanager, including numerous records in southwestern Minnesota. The species was absent only from the intensely agricultural landscapes of west-central Minnesota and the Red River valley (Minnesota Department of Natura Resources 2016).

The MNBBA participants reported 1,470 Scarlet Tanager records from 20.6% (979/4,743) of the surveyed atlas blocks and from 26.4% (616/2,337) of the priority blocks. Nesting was confirmed in only 34 blocks, but probable nesting was confirmed in 239 blocks (Figures 2 and 3; Table 1). Tanagers were reported from 75 of Minnesota’s 87 counties and were common along the upper Minnesota River valley, where numerous blocks straddled multiple counties. Breeding was confirmed in 24 counties; 2 blocks straddled 2 counties each (Isanti and Anoka, and Scott and Dakota). All counties where the species had not previously been reported nesting since 1970 (16 in total) were well within the species’ primary breeding range.

Although they are broadly distributed, Scarlet Tanagers are indeed most abundant in the north-central region of the state. Even when the sampling bias in the metropolitan region and Brainerd Lakes region was eliminated and only the priority blocks were examined, Scarlet Tanagers were most abundant and widely distributed in the Northern Minnesota Drift and Lake Plains Section of the Laurentian Mixed Forest Province. The diverse topography of this region, especially in the western portion of the section, supports a mosaic of mesic hardwood forests and drier forests of pine and boreal hardwoods (quaking aspen and paper birch) that provide ideal habitat for Scarlet Tanagers (Minnesota Department of Natural Resources 2003). The MNBBA probability map reflects the importance of this region as well as the hardwood forests of southeastern Minnesota (Figure 4). Farther east in the Laurentian Mixed Forest Province, the mixed boreal forest uplands are predicted to support lower breeding densities with the exception of the mesic hardwoods along the North Shore of Lake Superior.

In his comprehensive review of the Scarlet Tanager, Mowbray (1999) noted few historical changes in the species’ distribution other than its apparent expansion westward in southern Manitoba and southeastern Saskatchewan. It was unclear whether new records in eastern South Carolina represented a range extension, better coverage, or improved habitat conditions. States and provinces in the Upper Midwest and Great Lakes region that have conducted two atlases note little distributional change in the 20 to 25 years between atlas efforts, including Ohio (Rodewald et al. 2016), Ontario (Cadman et al. 2007), Michigan (Chartier et al. 2013), and Iowa (Iowa Ornithologists’ Union 2017). Wisconsin’s first atlas (1995–2000) documented fewer birds in the state’s most southern tier of counties than Robbins’s (1991) earlier account depicted using BBS data (Cutright et al. 2006). In South Dakota, where the species is uncommon and local in distribution, the species was reported farther west during their first atlas. All but two records during their second atlas (2009–2012) were substantially east of the Missouri River (Drilling et al. 2016). Breeding distribution of the Scarlet Tanager in Minnesota based on the Minnesota Breeding Bird Atlas (2009-2013). Summary statistics of records by breeding status category for the Scarlet Tanager in Minnesota based on all blocks (each 5 km x 5 km) surveyed during the Breeding Bird Atlas (2009-2013).

Probable239 (5.0%)171 (7.3%)Possible703 (14.8%)423 (18.1%)Observed3 (0.1%)1 (0.0%)Total979 (20.6%)616 (26.4%) Table 1.

Summary statistics for Scarlet Tanager records by breeding status category for all blocks and priority blocks (each 5 km x 5 km) surveyed during the Minnesota Breeding Bird Atlas. Predicted breeding distribution (pairs per 40 hectares) of the Scarlet Tanager 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.

The Scarlet Tanager utilizes a variety of mature upland forest habitats. Deciduous forests are predominant, but a wide range of mixed deciduous-coniferous and coniferous forests are also used (Figure 5). Occasional use of residential woodlands with large shade trees is reported, including parks and cemeteries (Mowbray 1999; Danz et al. 2007). More specific details, apart from its dependence on large forest tracts, are lacking.

Monitoring work conducted on the four national forests in northern Wisconsin and Minnesota demonstrated the wide range of forests used by Scarlet Tanagers. Although oak forests were the predominant habitat utilized, the birds were found in at least 16 other cover types, including upland deciduous, coniferous, and mixed forests, lowland hardwoods, lowland conifers, and young successional stands (Niemi et al. 2016). Habitat data collected within 200 m of MNBBA point counts where Scarlet Tanagers were detected demonstrated a strong preference for pine forest stands followed by oak forest (Figure 6). The former includes the Laurentian pine-oak forest and pine forest communities that are common in the Northern Minnesota Drift and Lake Plains Section and the Northern Superior Uplands Section. The latter includes dry-mesic and dry oak forest communities that are found throughout the Eastern Broadleaf Forest Province.

Several studies have documented the sensitivity of Scarlet Tanagers to the size of a forest tract and their negative response to a fragmented forest landscape where stands are small and isolated from one another (Mowbray 1999; Rosenberg et al. 1999a). The degree of sensitivity varies geographically. In the Midwest and Atlantic coast regions, where the forest landscape is highly fragmented, Scarlet Tanagers require larger tracts than in the extensive forest landscapes of the northern Great Lakes and New England (Rosenberg et al. 1999a). Typical breeding habitat of the Scarlet Tanager in Minnesota (© Lee A. Pfannmuller). Habitat profile for the Scarlet Tanager based on habitats within 200 m of point counts where the species was present during the Minnesota Breeding Bird Atlas (2009-2013).

Data collected by the federal Breeding Bird Survey (BBS) have generated a continental population estimate of 2.7 million Scarlet Tanagers (Rosenberg et al. 2016). In 2013, Minnesota was estimated to support 2.2% of the continental population, or 50,000 breeding adults (Partners in Flight Science Committee 2013). Given the species’ broad distribution throughout the state, the estimate seems particularly low. The estimate generated with MNBBA data is significantly larger, at 852,000 individuals (95% confidence interval of 663,000 – 1.1 million). The discrepancy is due in part to the more intensive data set that the MNBBA provided. The Scarlet Tanager’s low detectability on BBS routes also may contribute to the low BBS estimate. On average less than 1 bird is detected on BBS routes in Minnesota each year. Many factors influence detectability of a given species, but the Scarlet Tanager’s song is one that can sound similar to many other forest songbirds, including American Robins, Yellow-throated Vireos, and even Rose-breasted Grosbeaks, potentially causing some confusion for observers. In the core of the tanager’s breeding range, the number of birds detected on BBS routes averages from 9 in Pennsylvania to as high as 17 in West Virginia (Sauer et al. 2017).

Local breeding densities range from a low of 3.2 pairs per 40 ha in oak-hickory forests in Wisconsin, to 16 pairs per 40 ha in mixed broadleaf forests in the Appalachian forests of Tennessee (Mowbray 1999). In Minnesota, breeding densities of 7.5 pairs per 40 ha were reported in a mature maple-basswood forest (Kelleher 1967), and 12.5 pairs per 40 ha were reported in mature oak stands (Back 1979). More comprehensive data are available from the National Forest Bird (NFB) Monitoring Program, which documented an average of 2.3 pairs per 40 ha on the Chippewa National Forest, an area of prime habitat for the tanager, and only 0.5 pairs per 40 ha on the Superior National Forest. The species reached its greatest abundance, 3.5 pairs per 40 ha, in the Chippewa’s mature oak forest communities (Niemi et al. 2016).

From 1966 to 2015, BBS data demonstrate a significant but small annual decline of 0.22% per year (Figure 7). Cumulatively, the population has declined 7% from 1970 to 2014 (Rosenberg et al. 2016). Because of the species’ low detection on Minnesota BBS routes, the statewide trend line lacks statistical precision but also suggests a small annual decline. On the Chippewa and Superior National Forests, the NFB Monitoring Program demonstrates a slow, but nonsignificant decline of 1.85% per year on the Chippewa and 0.93% on the Superior from 1995 to 2016 (Figure 8).

Across North America, some of the strongest population declines for the tanager are occurring in the northern forest regions, including the Boreal Hardwood Transition, the Atlantic Northern Forest, and the New England/mid-Atlantic Coast (Figure 9). Farther south, including central and southern Minnesota, many local populations are stable or significantly increasing.

General factors that influence local population levels include successional changes in habitat, food availability, nest predation and parasitism, and forest fragmentation (Mowbray 1999). During the spring, the birds are also particularly susceptible to periods of cold and wet weather, which can suppress breeding populations for several subsequent breeding seasons (Zumeta and Holmes 1978).

In their comprehensive assessment of 20 years of forest bird monitoring on the national forests of Wisconsin and Minnesota, Niemi et al. (2016) highlighted the status of the Scarlet Tanager and its declining population trend. The decline was most pronounced from 2000 to 2011. They noted that forest fragmentation and cowbird parasitism are frequently cited as factors that negatively impact population levels. Yet, in their study areas Brown-headed Cowbirds declined, and timber harvest levels decreased. Forest fragmentation should also be less of a factor in the northern forest region than farther south, where population centers are larger, agricultural landscapes are more common, and the landscape is more fragmented. Some authors have pointed to high mortality during migration and on the wintering grounds, but those factors should be impacting populations across the breeding range (Mowbray 1999). As Mowbray (1999) discusses, these large variations in local population trends are not uncommon for many species and likely reflect many regional differences in climate, food availability, and habitat conditions. Breeding population trend for the Scarlet Tanager in North America from 1966-2015 based on the federal Breeding Bird Survey (Sauer et al. 2017). Breeding population trends of the Scarlet Tanager in the Chippewa ad Superior National Forests of Minnesota from 1995-2016 (Bednar et al. 2016). Breeding Population Trend Map for the Scarlet Tanager based on federal Breeding Bird Survey data, 1966-2015 (Sauer et al. 2017).

Although the Scarlet Tanager’s rate of population decline is lower than that documented for many long-distance migrants that inhabit the eastern deciduous forest biome, biologists remain concerned about threats to the species’ breeding and wintering habitats and its relatively small population size. Partners in Flight assigned it a moderate Continental Concern Score of 12/20 (Rosenberg et al. 2016).

Given the species’ sensitivity to forest fragmentation, it has been the focus of numerous field studies designed to address management requirements. In 1993, the Cornell Laboratory of Ornithology launched a three-year, volunteer-based field study to assess how North America’s four tanager species are affected by forest fragmentation (Gregory et al. 2016; Rosenberg et al. 1999a). The results in turn led to the publication of “A Land Manager’s Guide to Improving Habitat for Scarlet Tanagers and other Forest-Interior Birds” (Rosenberg et al. 1999b). The study concluded that in landscapes (i.e., blocks 2,500 acres in size) with greater than 70% forest cover, tanagers are not area sensitive. But as the percentage of forest cover decreases, the amount of forest cover needed to sustain a breeding pair of tanagers increases. For example, in the Midwest, if the forest cover is only 40%, forest stands must be a minimum of 245 ha in size to achieve high sustainability for a breeding pair. If the forest cover is 60%, forest stands need to be only a minimum of 57 ha in size. As forest cover decreases, the impacts of cowbird parasitism and predation on reproductive success increase.

Climate change is another conservation concern for the Scarlet Tanager. The “State of the Birds 2010 Report on Climate Change” classified the species as having a medium vulnerability to climate change (North American Bird Conservation Initiative 2010). More recently, an extensive analysis of the response of nearly 600 North American birds to climate change by the National Audubon Society predicted that by the year 2080, the tanager would lose 93% of its current breeding habitat. The species was classified as “climate threatened” (Langham et al. 2015; National Audubon Society 2016).

Collisions with cars, residential buildings, and communication towers are another well-documented concern (Bracey 2011; Longcore et al. 2013; Longley 1981). Annual mortality due to collisions at communication towers alone is estimated to be 1.6% (Longcore et al. 2013).

In Minnesota, forest management practices that insure the availability of large tracts of mature upland deciduous forests, with some oak, and that support a moderate density of larger trees should help insure the long-term sustainability of this brilliant forest songbird (Niemi et al. 2016).
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