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 foraging habitat.

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.