Bell's Vireo (Vireo belli)

Status: State Threatened (1989).

Occurrence: Uncommon migrant southwest and mid-state, rare migrant southeast, uncommon summer resident southwest. Vireos arrive in southern Wisconsin between April-May and leave in July-August. Summer sightings have occurred in Buffalo, Jackson, Monroe, Vernon, Crawford, Sauk, Iowa, Lafayette, Green, Rock, Columbia, Green Lake, Grant, Marquette, Dodge, La Crosse (and east in the forest preserve), Dane (Madison, Mazomanie, and down to Thomson Prairie area), and southwestern Trempealeau counties. Migrants may be seen in surrounding counties. A map outlining Pre-1977 and 1997 to Present Distribution is available.

Aid to ID: Plumage is drab gray to green above and white to yellow below. Distinguishing markings include a faint white eye ring and 2 pale wing bars (lower bar more prominent).

Habitat: Prefer dense vegetation characteristic of mid-successional habitats, or any successional stage exhibiting dense understory vegetation, particularly upland and lowland carr, riparian areas, brushy fields, and young second-growth forest or woodland. This species is largely absent in intensively cultivated areas, forests, pure grasslands, and arid areas. Vireo habitat often includes black locusts, hawthorn and lilac clumps, alder thickets, hazelnut, rose, staghorn sumac, grape, brambles, box elder, black cherry, honeysuckle, and mulberry.

Food Habits: Insectivorous. Diet includes caterpillars (Lepidoptera), stinkbugs (Pentatomidae), bees and wasps (Hymenoptera), weevils (Rhynchophora), and small spiders. They feed in the lower strata of shrubs and trees.

Natural History:

Breeding: Clutch size: 3 - 5 whitish eggs; laid from April to July. Incubation: 14 days by both parents. Young fledge 10-12 days after hatching.
Nest: A small basket-like cup made of leaves, grasses, bark, and spider webs that are lined with fine grasses, coarse hairs, feathers, and wool. Suspended from small, lateral, or terminal forks of low, pendant branches in dense bushes, small trees, and occasionally herbaceous vegetation about 2-5 feet above the ground, rarely higher.

Management Considerations: Land use patterns, particularly along riparian habitats, strongly influence vireo abundance. Habitat modifications, including agriculture, urbanization, grazing, flood control projects, and reservoir construction, have reduced habitat for this species. Large water releases from dams and reservoirs in April-June can inundate low-lying vireo nests in downstream areas, resulting in high nest loss and egg/nestling mortality. Modifications that promote habitat patchiness apparently increase rates of cowbird parasitism, resulting in reduced nesting success. Increased habitat patchiness also acts to segregate remaining breeding vireos into disjunct subpopulations that are more susceptible to extirpation. Additionally, human interference at nest site may cause premature fledging of young. Conservation measures include: preserving shrubby vegetation along roadsides, fences, and powerlines. To improve shrub habitat, remove exotic vegetation and occasionally trees and shrubs that no longer provide dense vegetation cover below 7 feet. Habitat manipulation should occur before or after migrations. Pesticides should not be used on or near vireo habitat for vireos rely on invertebrates for food. Revegetate riparian and shrub corridors to increase extent of nesting habitat and to deter cowbirds.