Breeding Birds of the Platte River Valley
Bobolink -- (Dolichonyx oryzivorous)
Nebraska Status: A fairly common migrant in central areas, less common east and west; summer resident nearly statewide (Johnsgard 1980). Peak migrations occur during mid May in spring and during 29 July to 20 August in fall.
Platte River Status: An abundant migrant and common nesting species. Occurrence dates at the Mormon Island Crane Meadows, Hall County, range from 4 May to 27 August. Tout (1947) found bobolink in Lincoln County during 5 May to 27 September, and Rosche (1979) reported the earliest arrival in the lower North Platte River Valley as 6 May. National Geographic Society (1983) indicated that the Platte River system is at about the southern limit of this species breeding range in North America.
Breeding Range: The bobolink is a locally abundant nesting species in the Platte River Valley west to North Platte, and in adjacent areas of the Eastern Plain and Dissected Plain physiographic regions west to the Lincoln-Dawson county lines (principally in alfalfa fields). Locally common in the Sandhills region and in the Platte River Valley of Keith, Deuel, and Garden Counties. Uncommon and highly local in the Western Plain physiographic region.
Breeding Population: The estimated population of 43,500 breeding pairs in 1979-80 made up 1.5% of the total breeding bird population in the study area. Bobolink ranked 21st in abundance among all species present.
Habitat: We found the highest mean breeding densities in wet prairie (9.2 pairs/km2), followed by alfalfa (3.2 pairs/km2), upland native prairie (2.7 pairs/km2), domestic hayland (0.5 pairs/km2), and wheat (0.5 pairs/km2). Graber and Graber (1963) reported 155 birds/km2 from northern Illinois mixed hayfields. Stewart (1975) reported that in North Dakota, bobolink occupied ungrazed and lightly grazed mixed grass prairie, tall grass prairie, wet meadow zones of natural basin wetlands, and domestic hayland during the nesting season. Faanes (1982) found a mean breeding density of 34 pairs/km2 in extensive reaches of wet meadow vegetation in central North Dakota. Kantrud and Kologiski (1982) found lightly grazed native grasslands on high-moisture soils in southeastern South Dakota to support the largest breeding densities of bobolink in the northern Great Plains. Plant species in Kantrud and Kologiski's study area showing above average plant cover in bobolink territories were western wheatgrass, Canada bluegrass, and green needlegrass.
Effect of Habitat Alteration: Because of the close association of bobolink to wet meadow vegetation, this species has been negatively impacted by de-watering of the Platte River system and the resultant encroachment of wooded vegetation. Kantrud and Kologiski's (1982) finding that highest breeding densities of bobolink occurred on ungrazed and lightly grazed native grasslands points to the negative relationship between this species and grazing by livestock. The extensive conversion of native grasslands to cropland (especially corn) is suggestive of a negative impact on bobolink. The second highest mean breeding density of bobolink in our study area occurred in alfalfa fields. Bent (1958) believed that bobolink expansion in North America followed the advancement of civilization westward because of favorable habitat created by the cultivation of forage crops. Our data suggest that alfalfa is an important nesting habitat for bobolink. The peak of nesting activity by this species, especially in alfalfa fields, coincides with the peak harvest period of alfalfa. Although specific data on reproductive success in alfalfa are lacking, we suspect that bobolink are at best only marginally successful in producing broods in agricultural habitats that are continually being harvested during their nesting period.
Nesting Data: Egg dates at the Mormon Island Crane Meadows, Hall County, range from 4 June to 21 June; mean clutch size is 7.0 eggs. Among 10 bobolink nests found on Mormon Island, all but two were parasitized by brown-headed cowbirds. North Dakota egg dates range from 4 June to 27 June (Stewart 1975).
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