Requires large tracts of mature hardwood forests with closed canopies and sub-canopies and an open understory, often near water (Dunn and Garrett 1997). Cerulean Warblers in North Carolina are generally found in cove hardwood forests with an abundance of tulip poplar (Simpson 1992).

Nest placed 15-50 feet above ground, away from the trunk on a horizontal limb of a deciduous tree (Ehrlich et al. 1988). Forage high in the canopy, in the upper half of trees that may be of greater diameter than average (Robbins, Fitzpatrick, and Hamel 1992).

NATURE SERVE GLOBAL HABITAT COMMENTS:

BREEDING: A tentative description of the characteristics of breeding habitat is a structurally mature hardwood forest in a mesic or wetter situation, with a closed canopy. The size of the trees is of primary importance and their species identity secondary. Landscape situation and context has a strong bearing on whether otherwise suitable breeding habitat will actually contain warblers (Hamel 1992). Habitat is frequently described as mature deciduous forest, particularly in floodplains or other mesic conditions (Robbins et al. 1992 in Tennessee and Maryland; Kahl et al. 1985 in Missouri). Territories in central and western Tennessee are found in forest stands with numerous large trees. Within the stands, territories are located in the areas with more large trees than is typical for the stand and are absent from the portions with small trees (Hamel 1988, Robbins et al. 1992). Observations of the utilization of forest vegetation found birds in the tree canopy, almost always above the midpoint of the tree, at an average height of 17 m in a 22-m-tall tree. These results somewhat contradict the statements of Morse (1989, citing Anderson and Shugart 1974), that this warbler utilizes habitat components in proportion to their availability.

Robbins et al. (1989) found occurrences to be associated with large tracts of mature, semi-open deciduous forest in Maryland and adjacent states. Distribution of breeding birds was positively correlated with the natural log of forest area (P < 0.01) and the square root of tree basal area (P < 0.05), and negatively correlated with the arcsine of percent canopy cover by coniferous trees (P < 0.05). In Missouri, Kahl et al. (1985, cited in Robbins et al. 1992), found that habitat around song perches was most consistently characterized by a large number of live stems > 30 cm dbh (range = 50-150 per ha), and a high (always > 18 m), closed canopy (> 85%, never < 65%). Other important features included an intermediate number of woody stems < 2.5 cm dbh (1,030-2,800 per ha, never < 1,030), and few dead stems 2.5-9.9 cm dbh (always < 175 per ha).

In North Carolina, a disjunct population occurs in the old-growth, mature floodplain forest communities of well-drained natural levees within 330 m of the Roanoke River (Lynch 1981, cited in Robbins et al. 1992). The dominant canopy species are sycamore (PLATANUS OCCIDENTALIS), green ash (FRAXINUS PENNSYLVANICA), and sugarberry (CELTIS LAEVIGATA). These communities are characterized by a closed canopy ranging in height from 24-30 m, a distinct shrub layer, and complete ground cover. In the Cumberland Mountains of eastern Tennessee, a dense population occurs at Frozen Head State Natural Area (C.P. Nicholson, pers. comm.). This area's second-growth forest is in advanced growth, with a high, closed canopy of large trees similar to those described above, in mesic cove and slope topography. Forest composition is of a diverse assemblage of hardwood trees. In North Carolina, also found in mature cove hardwood forests on relatively steep slopes with little understory (H. LeGrand, pers. comm.).

Placement of the nest has been described differently by various authors. Nests in tall tree, about 4.5-27 m up (typically high in tree), well out on large branch, often [apparently] near forest opening. More information is needed on nest site preferences, especially the relationship (if any) to canopy gaps (Hamel 1992). Previous summaries of breeding habitat from studies in the 1980s indicate that the forest in which the birds breed is one with a closed canopy. Bent (1953) states that '...the nest is usually placed...over an opening [sic].' A nest discovered in central Tennessee in 1950 was built in an elm at the edge of an opening beside a farmhouse (K.A. Goodpasture, pers. comm.). Bent (1953) describes a nest site that was five meters out on a limb of a tulip-tree (LIRIODENDRON TULIPIFERA), and 14.8 m up '....with no other limb between it and the ground.' Harrison (1984) presents a photograph of breeding habitat which shows a discontinuous canopy edge at a forest road.

Several important questions arise from these conflicting indications of nesting habitat. Do cerulean warblers usually nest in continuous, unbroken forest? Do they build their nests in the canopy of the forest where little other vegetation occurs between the nest and the groundcover? Do they typically build their nests in association with canopy gaps in otherwise unbroken forest? Are they indifferent to nest situation, using closed canopy forest, canopy gaps, and the edge between forest and other land uses in proportion to their availability? The literature is insufficiently clear to distinguish these alternatives. Observer bias is certainly possible in reporting nest locations when nest searches were haphazard, as was almost certainly the case in the existing literature. In such a case, nests built in locations that are easy to find would appear with greater relative frequency in the literature than in the field. One further possibility is that current observations may not reflect the preferences of the birds as they existed in the past. Instead, the current situation may be only a remnant of the actual capability of the birds; a remnant produced by the action of limiting factors (Hamel 1992).