Bobolink Dolichonyx oryzivorus

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Life History Groupings:

Breeding Habitat:Grassland

Nest Type:Open-cup nesting passerine

Migration Status:Neotropical migrant

Nest Location:Ground-low nesting

Species Account:

Bobolinks are conspicuous summer residents of hayfields, lightly grazed pastures, tall- and mixed-grass prairies, reclaimed strip mines, and similar habitats dominated by tall grasses. As is true for many other grassland birds, this species underwent a noticeable population increase and range expansion in eastern North America during the nineteenth century when agricultural fields dominated the deforested landscape (Andrle and Carroll 1988, Brewer et al. 1991, Peterjohn and Rice 1991). Bobolink population trends were reversed during the twentieth century, especially after 1940 when changing agricultural practices greatly reduced the amount of suitable grassland habitats for this species.

Along BBS routes, breeding Bobolinks are most numerous in a band stretching across the northern U.S. and southern Canada from the Maritime provinces and New England to North Dakota and Manitoba (Relative Abundance Map). While they are locally common in portions of Montana, they are generally recorded in small numbers throughout the western portion of their range.

BBS data indicate that Bobolink populations have generally declined throughout their breeding range (Trend Map). Areas with increasing populations tend to be small and very locally distributed, most notably from eastern North Dakota to western Ontario and in eastern Ontario and adjacent Quebec. The only significant increase during 1966-1994 occurs in the Ohio Hills (S22) stratum; significant declines are evident in 10 states/provinces, 8 physiographic strata, the Eastern and Central BBS regions, U.S., Canada, and survey-wide (Trend List). These long-term trend estimates are strongly influenced by the population trends after 1980, when all significant trends are in a negative direction. During 1966-1979, however, there are nearly equal numbers of significant increases and declines in states/provinces and strata while regional trends include increases in the Eastern BBS Region, Canada, and survey-wide, and declines in the Central BBS Region and U.S. Trends in the Western BBS Region have remained fairly stable.

The survey-wide indices are fairly stable through the late 1970s, followed by a consistent decline (Survey-wide Annual Indices). However, a variety of temporal patterns exist in trends within states/provinces and strata. In the midwestern U.S., two temporal patterns prevail. These patterns are fairly stable trends through the mid-1970s, followed by substantial declines as exemplified in Iowa, and noticeable declines through the late 1970s followed by only slight declines as shown in Illinois (Illinois Annual Indices) (Iowa Annual Indices). In the Maritime provinces and eastern Canada, the prevailing temporal pattern is for an increasing tendency through the early 1980s, followed by a precipitous decline. Population trends in New Brunswick are indicative of this pattern (New Brunswick Annual Indices). Bobolinks in the Great Lakes Plains stratum (S16) have consistently declined throughout the survey period (Great Lakes Plain Annual Indices). Not all populations are declining, such as Ontario and Pennsylvania which have experienced periods of slight increases and decreases but have remained fairly stable overall (Ontario Annual Indices). (Pennsylvania Annual Indices). In the Ohio Hills stratum (S22), population increases after 1980 may be the result of the occupation of newly created habitats on reclaimed strip mines (Whitmore and Hall 1978) (Ohio Hills Annual Indices).

In addition to habitat loss, the factor most frequently cited for declines in Bobolink populations is the more frequent mowing of hayfields (Brauning 1992, Brewer et al. 1991). Many hayfields are being cut in late May and at more frequent intervals throughout the summer, which does not provide late migrants such as Bobolinks an opportunity to successfully raise a brood before they have to start their fall migration. Additionally, the Bobolink is one of very few North American passerines whose entire winter range is south of the equator in South America. Its winter biology is poorly known, and factors on its South American winter range could also be contributing to the recent population declines.

Literature Cited

Andrle, R.F., and J.R. Carroll, eds.  1988.  The atlas of breeding                    
     birds in New York state.  Cornell Univ. Press, Ithaca, NY.                       
     551 pp.                                                                          
Brauning, D.W., ed.  1992. Atlas of breeding birds in Pennsylvania.                   
     Univ. of Pittsburgh Press, Pittsburgh, PA.  484 pp.                              
Brewer, R., G.A. McPeek, and R.J. Adams, Jr., eds.  1991.  The                        
     atlas of breeding birds of Michigan.  Michigan State Univ.                       
     Press, East Lansing, MI.  594 pp.                                                
Peterjohn, B.G., and D.L. Rice.  1991.  The Ohio breeding bird                        
     atlas.  Ohio Dept. Natur. Resour., Columbus, OH.  416 pp.                        
Whitmore, R.C., and G.A. Hall.  1978.  The response of passerine                      
     species to a new resource: reclaimed strip mines in West                         
     Virginia.  Am. Birds 32:6-9.