Bobolink

A regular breeding species and migrant. The Bobolink was common in abundance during the Minnesota Breeding Bird Atlas (MNBBA).

Distributed throughout the north-central and northeastern United States and southern Canada, from southern British Columbia and eastern Washington and Oregon, east to southern Quebec and Newfoundland in the north and western Virginia in the south. The Bobolink is most abundant in the northern Great Plains (Figure 1).

Assigned a Continental Concern Score of 14/20 by Partners in Flight and added to their 2016 Watch List; designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources and a Stewardship Species by Audubon Minnesota.

A long-distance, Neotropical migrant undertaking one of the longest migratory trips of any New World songbird to its wintering grounds in South America.

A variety of terrestrial insects, weed seeds, and small grains.

A ground nest usually nestled at the base of grassland forbs.

This stunning blackbird, with its unique plumage and bubbly aerial song is near the northern periphery of its breeding range in Minnesota. Roberts (1932) considered the Bobolink a common summer resident throughout the state “wherever there are suitable lowlands.” Most common in the western grasslands, it also could be found further east in small hayfields and pastures that were embedded across the forested landscape. Confirmed nesting records (nests with eggs or young) were reported from 8 counties including such widely scattered locations as Pipestone County in southwestern Minnesota, north to Marshall County in northwestern Minnesota, and east to Hennepin, Isanti, Mille Lacs, and Scott Counties. At the time, Roberts expressed concern that the Bobolink was declining in abundance and was being replaced by the more aggressive Brewer’s Blackbird. The latter species was rapidly expanding its breeding distribution from the northern Great Plains east across the Great Lakes, New England, and the Maritime Provinces. Both are open habitat species.

Both Green and Janssen (1975) and Janssen (1987) described the Bobolink’s distribution in a manner similar to that of Roberts, pointing out that it was most abundant in the western and northwestern regions of the state and least abundant in the north-central and northeastern regions. Janssen identified 15 counties where nesting had been confirmed since 1970. Hertzel and Janssen (1998) later added six more counties to the list.

As of 2014, the Minnesota Biological Survey (MBS) reported a total of 1,653 breeding season locations. Widespread throughout the Prairie Parklands and Tallgrass Aspen Parklands, a significant number of records also were reported in north-central Minnesota, from Clearwater County east to central St. Louis County, with a particularly high number of reports in Aitkin County (Minnesota Department of Natural Resources 2016).

The MNBBA reported 3,376 breeding records in 37.6% (1,804/4,801) of the surveyed atlas blocks and 50.6% (1,182/2,337) of the priority blocks. Breeding evidence was documented in 148 blocks (Figures 2 and 3; Table 1). The birds were observed in all 87 Minnesota counties, including one record at a farmstead near the North Shore of Lake Superior in Cook County. Breeding evidence was documented in 61 of Minnesota’s 87 counties, including 44 counties that were additions to those delineated by Hertzel and Janssen (1998).

As indicated by earlier accounts, MNBBA records indicate the species is most abundant in the Prairie Parklands and Tallgrass Aspen Parklands Ecological Provinces. It is widely dispersed through the Eastern Broadleaf Forest Province as well, with the exception of the Twin Cities and Rochester metropolitan areas; it remains least abundant in the Laurentian Mixed Forest, especially in Cook and Lake Counties. Despite Roberts’s concern about its status one hundred years ago, the Bobolink remains a common and widely distributed species in Minnesota.

Atlas data were used to develop a model to predict the probability of encountering Bobolinks statewide (Figure 4). The result predicted the core of the species’ breeding distribution is in the Red River valley and the Aspen Parklands of northwestern Minnesota. A narrow band of high densities also stretches across central Minnesota from west-central Minnesota east to Kanabec and western Pine County and south from west-central Minnesota to the Iowa border. Elsewhere outside of the Laurentian Mixed Forest Province the species is common but at moderate breeding densities. Although there are small pockets of abundance predicted throughout the northern forest, Bobolinks are predicted to be rare to absent across the region.

Historically, Bobolinks expanded east as forests were cleared and expanded further west as more arid lands were irrigated and cultivated (Renfrew et al. 2015). Today, as some of these previously cleared areas in the east are abandoned and once again succeed to forest, the Bobolink’s range is receding locally, such as in the Southern Shield of Ontario (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.

Prior to European settlement, the Bobolink was found in the mesic tallgrass prairies of the Midwest. As these areas came under intensive cultivation, new opportunities were created to the east when large areas of the eastern deciduous forest were cleared for agriculture. Although the species is still found in those few native prairie sites that remain, it has adapted to make extensive use of hayfields, lightly grazed pastures, small grain fields, and planted cover, including Conservation Reserve Program (CRP) fields. Indeed, the species is a frequent inhabitant of planted fields embedded in Minnesota’s northern forest region (Figure 5; Renfrew et al. 2015; Dechant et al. [1999] 2001). A study of grassland habitats in North Dakota linked the species’ presence to hayfields and sites planted in nonnative, exotic grasses (Madden et al. 2000). In this study, the authors suggest that tall, exotic grasses, such as smooth brome and quack grass, are structurally similar to the native grasses found in mesic, tallgrass prairies, thus readily attracting Bobolinks to the common, nonnative grassland habitats. The primary habitats within 200 m of MNBBA point counts where Bobolinks were detected were upland grassland followed by cropland and marsh/wet meadow (Figure 6).

Regardless of their origin, suitable sites are characterized by moderate to tall vegetation, including both grasses and forbs; moderate vegetation densities; moderately developed litter layers; and the absence of woody vegetation. Forbs, in particular, are important for providing nesting cover. Selected sites are often characterized as “low lying” (e.g., Roberts 1932; Bock et al. 1999), which often describes more mesic grasslands with greater vegetation density compared with drier upland sites, where the vegetation tends to be more sparsely distributed.

Bobolinks demonstrate some area sensitivity, with higher nesting densities and higher reproductive success reported in larger habitat patches (e.g., Renfrew et al. 2015; Johnson and Igl 2001). In Minnesota, nest depredation was lowest on large grasslands that ranged from 130 to 486 ha in size and highest on small grasslands that were 32 ha and smaller (Johnson and Temple 1990). Reproductive success also was higher for nests located more than 45 m from a forest edge (Johnson and Temple 1986).

Data gathered by the federal Breeding Bird Survey (BBS) have helped biologists estimate the North American population of Bobolinks at 9.7 million breeding birds (Rosenberg et al. 2016). In 2013, Minnesota was estimated to support nearly 13% of the North American population (Partners in Flight Science Committee 2013), which would place the current number of birds in Minnesota at approximately 1.3 million breeding adults. The estimate derived using MNBBA data is significantly higher at 6.5 million birds (95% confidence interval of 5.8 – 7.6 million).

Although the Bobolink is common and widely dispersed throughout Minnesota, the core of its breeding range is in western North Dakota (Figure 1; Sauer et al. 2017). The average number of birds detected on federal BBS routes in Minnesota each year is 23. The only state with a higher average is North Dakota, where an average of 36 birds is detected per BBS route each year (Sauer et al. 2017). Site-level breeding densities can vary considerably. In at least one study conducted in Minnesota, breeding densities ranged from 15 birds per 40 ha on CRP tracts to 18 birds per 40 ha on Waterfowl Production Areas (Koford 1999). Another study of 30 CRP fields in western Minnesota documented an average 20 pairs per 40 ha (Hanowski 1995).

Since the BBS began in the mid-1960s, data collected throughout the species’ breeding range showed that Bobolinks have experienced a significant average annual population decline of 2.06%, resulting in a cumulative loss of over 60% since 1966 (Rosenberg et al. 2016). In Minnesota, the population also has declined significantly at the rate of 1.86% per year since 1967 (Sauer et al. 2017; Figure 7). Populations are declining in nearly every region across North America except for a few small pockets, including the Badlands of South Dakota and northern Missouri.

The Bobolink encounters challenges on its breeding grounds as well as during migration and on its wintering grounds. On its breeding grounds, reproductive success is affected by a number of factors, including the loss and degradation of grassland habitats, high predation on ground nests, and numerous land use changes. Renfrew and her colleagues (2015) point to three major changes in land use that threaten Bobolink populations: 1) a significant decline in the number of acres maintained as hayfields in the northeastern United States, 2) an increase in the number of acres planted to alfalfa, a less-preferred grassland cover for Bobolinks, and 3) progressively earlier dates in the summer for mowing hayfields. The latter occurs nearly two to three weeks earlier than occurred in the mid-1950s and now coincides with dates when young are still in the nest and unable to escape the mower.

The Bobolink’s impressively long migratory journey is also rife with challenges. In the late 1800s and early 1900s, thousands of birds were captured during the fall migration in southern states and sent to food markets in Philadelphia, Boston, and New York. Even greater numbers were slaughtered as they stopped in the rice fields of the southeastern United States (Bent 1958). Although U.S. rice cultivation has moved west of the species’ major migratory route, the Bobolink is still considered a pest in the rice fields of South America, where it is known as the “Rice Bird.” Wintering birds also are trapped and sold in Argentina as pets and in Jamaica for food (Renfrew et al. 2015).

With a Continental Concern Score of 14/20, the Bobolink is on the Partners in Flight’s 2016 Watch List, largely because of increasing concern regarding its population decline (Rosenberg et al. 2016). The species is not state listed in Minnesota but has been designated a Species in Greatest Conservation Need (Minnesota Department of Natural Resources 2015). It is recognized as a conservation priority by several states, provinces, and bird conservation planning initiatives. Because nearly 13% of the global population occurs in Minnesota, it also has been designated a State Stewardship Species by Audubon Minnesota (Pfannmuller 2012).

The management needs of the species have received a fair amount of attention as well. Dechant and her colleagues ([1999] 2001) recommend that the keys to successfully managing Bobolinks are to provide large tracts of suitable habitat, to control the succession of woody shrubs and trees, and to protect habitat from disturbance during the nesting season. More specific recommendations developed for the Upper Midwest include: 1) postponing haying until late June, 2) mowing or burning only 30% of each site every two to three years to create patches of different successional stages, and 3) grazing at moderate levels on a rotational system (Dechant et al. [1999] 2001).

Sites planted and protected under the Conservation Reserve Program (CRP) provide important breeding habitat and should be managed accordingly. Unfortunately many of these acres are not permanently protected and are subject to the vagaries of the commodity markets. High crop prices provide few incentives for the individual farmer to forego converting CRP acres to productive crops.

Because Bobolinks prefer more mesic conditions, warming temperatures may pose a significant threat to the species. Indeed, a recent modeling analysis conducted by scientists with the National Audubon Society (2016) classified the Bobolink as a “climate threatened” species that could lose 80% of its current breeding habitat by the year 2080. However, considerable habitat may be available north of the species’ current range, perhaps providing new opportunities for colonization (Langham et al. 2015).

Minnesota has a special responsibility to provide for the future of this Stewardship Species. Current programs underway to improve efforts to protect and restore grassland habitats under the umbrella of the Minnesota Prairie Landscape Initiative (Minnesota Prairie Plan Working Group 2011) should help ensure that this unique grassland inhabitant, lauded by many early naturalists for its beautiful and joyful prairie song, remains a part of Minnesota’s conservation future.