Brown Creeper

A summer resident in the forested region of the state and a regular spring and fall migrant; may occur statewide during the winter but is most frequently observed in the south. The Brown Creeper was an uncommon species during the Minnesota Breeding Bird Atlas (MNBBA).

Found from southern Alaska, east across southern Canada, south through the Pacific Northwest and the Rocky Mountains, and east across the Great Lakes states and New England. Difficult to detect, the Brown Creeper does not reach high densities anywhere within its breeding range (Figure 1).

Assigned a Continental Concern Score of 8/20 by Partners in Flight.

In many areas the species is a year-round resident. Birds that breed in the far north, however, move south for the winter, many overwintering in the U.S. interior. Birds that nest in high elevations may move to lower elevations.

Insectivore that forages primarily on tree trunks, systematically moving from the base of a tree upward.

One of the most unusual and difficult-to-locate bird nests, it is usually wedged between loose bark and the trunk of a dead or dying tree.

The Brown Creeper is a somewhat complicated species to describe. When Roberts wrote Birds of Minnesota in 1932, he characterized the Brown Creeper as a “half-hardy species” residing in “the northern evergreen forests as far south as Isanti County.” Although he classified it as a permanent resident, he noted that it was less common in the region during the winter and also an uncommon winter resident farther south. Given the difficulty of finding the species’ well-hidden nests, it is not surprising that there were only two confirmed reports: one from central Minnesota, “probably southern Crow Wing County” (1870; nest with six eggs), and one from Cook County (1930; five fully fledged young and one unhatched egg). An inferred nesting report was available from Itasca County, where adults were seen carrying food (1930).

Some summer residents in the northern regions remained into the winter months, but conditions must have been challenging for a species that depends on insect eggs and larvae for food. Commenting on those northern birds, Roberts (1932) wrote,

They are regular visitors in small numbers at winter feeding-stations where suitable food is supplied. It appears, however, that continued cold weather and perhaps a decreasing food supply take a toll from these venturesome birds that remain behind after the bulk of their kind has retreated to milder climes, for it frequently happens that, as the winter progresses, they become fewer and fewer.

Forty years later, Green and Janssen (1975) considered the Brown Creeper a regular summer resident throughout north-central and northeastern Minnesota. Documented nesting reports in Hennepin County in 1951 and 1952 (Jarosz 1952; Parmelee 1975) and in Goodhue County in 1956 (Titus and Titus 1956) led them to speculate that the species was probably a resident “throughout much of the wooded portion of the state.” They also described it as a winter visitant from Clay, Beltrami, and St. Louis Counties southward.

When Janssen (1987) prepared another updated account several years later, new breeding reports were available from Flandreau State Park in Brown County (1983) and Whitewater State Park in Winona County (1983). These reports, combined with numerous summer observations in Houston and Winona Counties, led him to conclude the species’ breeding range extended south along the Mississippi River valley to the Iowa border. In addition to the more western breeding record in Brown County, summer records were also reported in Cottonwood and Pope Counties. Since 1970, breeding records were now available from 8 counties: Aitkin, Anoka, Brown, Clearwater, Lake, Lake of the Woods, St. Louis, and Winona. Hertzel and Janssen (1998) later added 3 more counties to the list: Cook, Dakota, and Polk. Overwintering birds, Janssen (1987) noted, were most common in the east-central, south-central, and southeastern regions of the state.

Although the Minnesota Biological Survey (MBS) had yet to complete its statewide efforts, from 1987 to 2014 they collected 250 Brown Creeper breeding season observations. Perhaps most significant were the breeding season locations documented along the floodplain forests of the Minnesota River, west to Redwood County, as well as locations in the central Minnesota counties of Kandiyohi, Morrison, Stearns, and Todd (Minnesota Department of Natural Resources 2016).

MNBBA observers tallied 709 Brown Creeper detections from 10.1% (479/4,738) of the surveyed atlas blocks and from 11.6% (271/2,337) of the priority blocks. Breeding was confirmed in 28 blocks (Figures 2 and 3; Table 1). The birds were detected in 33 of Minnesota’s 87 counties and were confirmed breeding in 14 counties. Breeding records in 9 of the 14 counties were additions to the 1998 list published by Hertzel and Janssen: Carlton, Cass, Crow Wing, Goodhue, Hennepin, Hubbard, Itasca, Ramsey, and Scott. Ramsey and Scott Counties were added because 2 blocks with confirmed breeding each straddled two counties: Dakota/Scott and Dakota/Ramsey. The MNBBA failed to document Brown Creepers in the upper Minnesota River valley but did document considerably more birds than the MBS in the north-central and northwestern regions of the state and in the Twin Cities metropolitan region. Records suggest the core of the species’ breeding range, however, is in Cook, Lake, and northern St. Louis Counties.

The predicted distribution map generated using MNBBA data in combination with data on habitat, landscape context, and climate, predicts that Brown Creepers may be found in moderate breeding densities across much of the Laurentian Mixed Forest Province (Figure 4). Lower densities are predicted throughout the Tallgrass Aspen Parklands, in the northern portions of the Eastern Broadleaf Forest Province, and along the Mississippi River valley, from the Twin Cities south to the Iowa border.

Accurately assessing significant changes in the distribution and abundance of Brown Creepers has many challenges. This small, diminutive bird is well camouflaged against the tree trunks it inhabits, and its high-pitched, tinkling song is difficult to detect beyond close range. Nevertheless, the absence of reports during the summer months in southern Minnesota in the early 1900s suggested they were either absent or quite rare in abundance. Although they still are uncommon south of Crow Wing County, scattered locations in east-central Minnesota, particularly in the floodplains of the lower Minnesota River and the Mississippi River, provide suitable habitat as do extensive forest tracts located in southeastern Minnesota.

Aside from local changes in abundance due to changing habitat conditions, the only notable range expansion apparently occurred decades ago, when the species expanded south in the mid-Atlantic states (Poulin et al. 2013). In the Upper Midwest, Wisconsin has not detected any major change in distribution (Cutright et al. 2006). In Ontario the number of observations in two of their northern regions (the Northern Shield and Hudson Bay Lowlands) increased significantly between their first (1981–1985) and second (2001–2005) atlases, but the differences were thought to reflect the participation of more-skilled field observers (Cadman et al. 2007). In Michigan the species was believed to breed locally throughout the state in the early 1900s but to have later retreated from areas of the southern Lower Peninsula that were cleared for agriculture and industrial development. Populations were believed to rebound during the 1960s and 1970s when Dutch Elm disease left an abundance of dead and dying tress that temporarily provided new habitat opportunities for Brown Creepers. Biologists predicted the spread of gypsy moths could result in a similar beneficial die-off of mature trees in future years (Brewer et al. 1991). To date, at least, atlas efforts have not demonstrated any significant change in the detection of Brown Creepers in the southern Lower Peninsula (Chartier et al. 2013).

*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 Brown Creeper is most closely associated with mature upland and lowland forests. Deciduous, coniferous, and mixed forest stands are all utilized. The limiting factor is the availability of large trees for foraging and the presence of dead and dying trees with peeling bark for nesting (Poulin et al. 2013). In field studies conducted in New Brunswick, nesting territories had higher densities of mature trees (> 30 cm dbh) and snags (> 10 cm dbh) than forest stands where Brown Creepers were absent. Balsam fir was the preferred nesting substrate (Poulin et al. 2008). The authors speculated that the fir’s thin bark dries out more quickly than that of other trees, breaking off in large pieces that provide ideal cover for nest sites while they still remain attached to the tree. Although balsam firs were preferred nest trees in this study (Poulin et al. 2013), a wide range of deciduous and coniferous trees are used throughout their breeding range.

In Minnesota, Apfelbaum and Haney (1977) described the characteristics of six different forest stands where Brown Creepers were nesting. Fire had affected each stand at least once in the past, varying from 2 to 367 years ago. Each stand also retained mature and/or old-growth pine, spruce, and fir trees that were large enough to support good populations of bark-inhabiting insect populations and suitable nesting sites (Figure 5). In stands where large trees were more sparsely distributed, the creeper’s territories were larger. Clearly age of the stand was less important than its retention of large, mature trees. Parmelee (1975) described the characteristics of nine nests found in or near Itasca State Park in the early 1970s. All were within 0.3 to 3.6 meters of the ground, and all were on coniferous trees (red pine, white pine, jack pine, and balsam fir). Seven of the nine nests were in dead trees; eight of the nine nests were located behind the tree’s loose bark (one was nestled within an old fire scar).

In long-term monitoring studies conducted on the four national forests in northern Minnesota and Wisconsin by the Natural Resources Research Institute, Brown Creepers were found using a wide diversity of forest stands, but their strongest association was with lowland hardwood stands, followed by white pine and aspen-birch-fir (Niemi et al. 2016). Habitat data collected within 200 m of MNBBA point counts where Brown Creepers were detected showed the strongest association with pine forests followed by upland coniferous stands (Figure 6).

The Brown Creeper’s light, wispy song is only detected at close distances, so the reliability of common roadside counts, such as the federal Breeding Bird Survey (BBS), to accurately detect population trends is low. Regardless, the survey remains the only range-wide data available for the species. With this constraint in mind, the BBS data have been used to generate a North American population estimate of 10 million breeding adults (Rosenberg et al. 2016). In 2013, when the continental population was 8.5 million birds, Minnesota’s statewide population was estimated at 120,000 birds (Partners in Flight Science Committee 2013). The population estimate derived from the more intensive MNBBA point count data was significantly higher at approximately 780,000 breeding adults (95% confidence interval of 638,000 to 1.3 million adults).

Because of the Brown Creeper’s low detectability, the BBS data indicated a relatively low abundance of Brown Creepers throughout the species’ range (Figure 1). In all but one state where it was detected, the average number of Brown Creepers per BBS route is less than 1; the one exception is California, where the average number detected is 1 to 2 birds per route (Sauer et al. 2017). Local breeding densities are quite variable and range from approximately 3 breeding pairs per 40 ha to as high as 63 pairs per 40 ha (Poulin et al. 2013). On the Chippewa and Superior National Forests, the average number of pairs per 40 ha was 3.4 on the Chippewa, and 2.7 on the Superior (Niemi et al. 2016).

Because of the species’ low abundance on BBS routes, statistical analyses of long-term population trends lack precision. Survey-wide, the data suggest a stable population, while BBS data in Minnesota suggest population numbers have been steadily increasing at a rate of nearly 6% per year (Sauer et al. 2017). Biologists estimate the continental population has increased 30% since 1970 (Rosenberg et al. 2016). The statewide BBS data may be suspect in light of the long-term monitoring data that are available for the Chippewa and Superior National Forests, which include the primary breeding range of the species in Minnesota. From 1995 to 2016, the data reveal a nonsignificant, relatively stable population trend (Figure 7).

Other than presumed population declines due to the loss of old-growth forests, especially in the Pacific Northwest, there is little to suggest what factors may be influencing large-scale population trends, either positive or negative. As noted earlier, the species is susceptible to cold winter temperatures, so difficult winters could affect survivorship and result in local population declines. The same may be true when prolonged wet or cold spells occur during the breeding season. Timber management activities that reduce the availability of mature trees and snags also will make habitat less suitable.

Given its relatively stable population, its wide distribution, and only moderate threats to its breeding and winter habitats, the Brown Creeper is not a conservation priority at either the national or state level and has been assigned a low Continental Concern Score of 8/20 by Partners in Flight (Rosenberg et al. 2016).

The species’ reliance on large trees for both foraging and nesting points to the major conservation concern for the species: the protection and management of mature and old-growth forest stands. In Minnesota, robust old-growth forest management guidelines on public lands, particularly on state and national forest lands, should help to insure that old-growth stands are protected now and into the future (Minnesota Department of Natural Resources 2002). Equally important is the need to manage forest landscapes to provide a diversity of age classes and cover types that provide for the needs of all forest birds

Other potential challenges include warmer temperatures and window collisions. Classified as a “climate endangered” species by the National Audubon Society, models suggest the Brown Creeper’s breeding range could decline more than 40% by the year 2080 (Langham et al. 2015; National Audubon Society 2016). The little creeper also comprised 3.7% of the fatal window collisions recorded on Minnesota Point during both the spring and fall migration seasons between 2006 and 2009 (Bracey 2011).

Despite a stable and perhaps even increasing population trend, the challenges ahead for the Brown Creeper may be formidable. In the interim, Minnesota provides suitable habitat for a robust population of these little forest inhabitants. The next time you think you see a piece of bark moving on a tree trunk, take a closer look and enjoy the antics of a bird that is well adapted to a unique niche within the forest environment.