Bell's Vireo (Vireo bellii)
Associated Species: Other species that may use habitat in a similar way and/or respond similarly
to threats, management, and conservation activities include Brown Thrasher, Common
Yellowthroat, Yellow-breasted Chat, and Blue Grosbeak.
Distribution: Bell's Vireos breed in Northern Mexico and Central and Southwest U.S. In
Colorado they nest on the northeastern plains along the South Platte, Arikaree, and Republican
rivers, and very rarely in riparian areas elsewhere on the eastern plains.
Habitat Requirements: Bell's Vireos require dense lowland riparian shrubs for nesting and
Ecology: In April and early May, these vireos return to their breeding grounds from wintering in
western Mexico and Central America. The breeding season begins in late May. Nesting is
completed by early August, and fall migration begins in early September. Their diet consists
almost exclusively of insects, but they eat some berries late in the summer.
Management Issues and Conservation Recommendations: Bell's Vireos have experienced
significant declines in the central U.S. in recent years, for reasons not clearly understood. They
were probably never abundant in Colorado because of a paucity of suitable habitat. Protect
shrubby riparian zones from activities that degrade them, such as channelization, aggregate
mining, grazing, urbanization, and development of roads and recreational facilities.
Brown-headed Cowbird nest parasitism causes substantial declines in nesting success in some
areas. Reduce or eliminate livestock grazing in areas with Bell's Vireo breeding populations.
Plant suitable shrubby vegetation in riparian zones in northeastern Colorado to expand habitat
patches for vireos and to deter cowbirds.
Status and Reasons for Concern: This species has a high conservation need locally and
throughout its range, and is on the national Watch List. Bell's Vireos are not adequately
monitored by BBS in the shortgrass physiographic area, and the data are too sparse for
meaningful analysis of trends. However, BBS data from 1966 to 1996 reveal a statistically
significant, survey-wide annual rate of decline (-3.4%; P < 0.01; n = 248). This species is
monitored by MCB with statewide census.
Biological Objectives: Increase the species' distribution and abundance, with progress toward
meeting this objective measured by returns from BBS and MCB monitoring programs.
Selected References: Andrews and Righter 1992, Brown 1993, Kingery 1998.