Setophaga cerulea (Wilson, 1810)
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Basis for Listing
Cerulean warblers inhabit mature, mesic deciduous forests with large trees, and a closed, or semi-closed canopy. This canopy-dwelling species is associated with mature floodplain forests, as well as upland forest types. In many portions of its range, including Minnesota, mature forests are giving way to urban and agricultural development or are being subjected to even-age management regimes that preclude the growth of large, old canopy trees. Field studies have further demonstrated the strong preference of the species for large, unfragmented forest tracts (Robbins et al. 1992). The cerulean warbler's preferred habitat on the wintering grounds at high elevations in Peru is also threatened with conversion to other land uses.
The cerulean warbler is a small bird that primarily remains in the upper forest canopy; thus it is difficult to see and is best identified by its vocalizations. Its song consists of a series of buzzy notes on the same pitch, followed by a slightly higher, drawn-out buzz. This song is similar to that of the northern parula (Parula americana) and the black-throated blue warbler (Setophaga caerulescens), but the breeding range of the latter two species rarely overlap that of the cerulean warbler. Adult males have sky (cerulean) blue upperparts with two white wing-bars; white underparts with streaked flanks; and a narrow, blue-black throat band. The adult female is a much duller bluish-green above, whitish-yellow below, and has a distinct whitish eyebrow.
The cerulean warbler requires large tracts of deciduous forest with mature to old-growth trees and a structurally diverse canopy (Hamel 2000). Minimum forest tract size estimates vary widely and by region, but there is general agreement that the cerulean warbler needs large unfragmented tracts (Robbins et al. 1992, Hamel 2000). In Minnesota, the species is found in both lowland forests (such as floodplain and lowland hardwoods) and mesic upland deciduous forests. In southeastern Minnesota, cerulean warblers are most commonly found on forested slopes, typically adjacent to streams or lowland forests, and in extensive floodplain forests along major rivers. Cerulean warblers in central Minnesota typically occur in upland oak, maple, and/or basswood dominated forests, usually in tracts with numerous wooded potholes or wet meadow openings within the forest.
Biology / Life History
The cerulean warbler is typically present in breeding areas from early to mid-May until August. Wintering grounds are in the Andes of South America. Little is known about its behavior on the wintering grounds, however, foraging flocks have been observed in the canopy of mixed species. Breeding territory size is typically 1-2 ha (2.5-5 ac.) Nests are built well away from the bole of the tree on a lateral limb in mid-story or overstory canopy, often near gaps in the canopy. Cerulean warblers usually raise one brood per season, and their clutch size is typically 3-5 eggs (Hamel 2000).
Conservation / Management
On the breeding grounds, loss and fragmentation of mature deciduous forest, especially along stream valleys, is the most serious threat facing the cerulean warbler. Management of forest tracts to reduce fragmentation is needed. In addition, management to promote uneven aged stands with mature trees increases habitat quality (Hamel 2000) and should be encouraged. Brown-headed cowbird (Molothrus ater) brood parasitism, human disturbance, and chemical contamination are also factors in the decline of this species. In southeastern Minnesota, many valley bottoms where lowland or floodplain forest once occurred have been disturbed by grazing, logging, and cultivation. In central Minnesota, forest tracts, particularly those near lakes, are being negatively impacted by residential development.
Conservation Efforts in Minnesota
Breeding bird inventories have been conducted by the Minnesota Biological Survey in most of the counties within the potential distribution of cerulean warblers in the state. Our knowledge of the breeding distribution of this species has greatly increased as a result of this and other survey efforts. To date, more than 160 records of cerulean warblers have been documented in 33 of Minnesota's 87 counties. Forest management prescriptions that would create or maintain optimal cerulean warbler habitat are needed (Hamel 2000).