Setophaga cerulea

Plumage of adult male is bright blue above and white below with a dark blue or black line across the throat. On the wing are two white wingbars. The feathers of the back have distinct black centers and the flanks are streaked with black. Flight feathers are black edged with blue and the alula and primary coverts are black (Pyle et al. 1987).

Plumage of adult female is greenish above and washed with blue, especially on the rump; the underparts are washed with yellow, the flanks have indistinct black streaking, the flight feathers are blackish edged with bluish-green, and the alula and primary coverts are blackish.

Juvenal plumage is brownish-gray above, with a pale median crown stripe and entirely white underparts. Young females (hatching year/second year) have brownish flight feathers edged with greenish-yellow and brownish alula and primary coverts. Young males have brownish-black flight feathers edged grayish-green, with brownish-black alula and primary coverts.

Full adult plumage is acquired at second prebasic molt. Prebasic molt, partial in first summer, and complete in subsequent years, takes place on the breeding grounds. Partial pre-alternate molt apparently takes place on the wintering grounds (Pyle et al. 1987).

This is a small, arboreal, typical DENDROICA warbler. The mean weight of 36 individuals including male and female adults and young was 9.1 gm (s.d. = 0.60 gm, range = 8.1-10.2 gm) (Clench and Leberman 1978). Wing length ranged from 62-70 mm in males (n = 30), and 58-64 mm in females (n = 26); tail length ranged from 38-43 mm (Pyle et al. 1987). Exposed culmen length was 10 mm (Chapman 1917).

VOCALIZATIONS: The buzzy, frequently two-parted song is similar to that of the northern parula (PARULA AMERICANA). The initial portion of the song is measured, with three or four notes, followed by a second part on a higher pitch of a similar number of notes but sung twice as fast, viz., "ZHEE ZHEE ZHEE ZHEE-zhizhizhizhi." The song is distinguished from that of the northern parula in that cerulean warblers do not terminate their song with a rising trill, or with the "tipping over" note with which the northern parulas typically end their songs. However, songs of northern parulas that terminate in neither of these features are very similar to those of cerulean warblers and considerable practice in the field is necessary to distinguish the two quickly and confidently. Saunders (cited in Bent 1953) describes the song as similar to that of the blackburnian warbler (DENDROICA FUSCA), "...but the loudness, different quality, and lower pitch distinguish it." Allen (cited in Bent 1953) renders the song "WEE WEE WEE WEE bzzz". Others note the similarity of the pattern of the song to that of the black-throated blue warbler (DENDROICA CAERULESCENS).

NEST: made of brown bark covered with gray plant material such as lichens and mosses and lined with mosses (Bent 1953, Harrison 1984).

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). In eastern Ontario, breeding territories were characterized by well-spaced large trees, with high canopies and dense foliage cover at heights between 12-18 meters (Jones and Robertson 2001). 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).

NON-BREEDING: Concentrated on the eastern slopes of the Andes Mountains in western South America. Elevational range is limited to the lower slopes between 500-1,500 m, in precisely the elevation at which human habitat encroachment is proceeding most rapidly (Skutch, cited in Bent 1953; J. Fitzpatrick and S. Robinson, pers. comms.). The winter habitat is mature deciduous forest, also with large trees, although quantitative measurements are so far lacking. This recent information is different than that quoted by Bent (1953, referring to Taczanowski) that the birds range between 10,000-13,000 ft (3,000-4,000 m) in the Peruvian Andes. Terborgh (1989) associates cerulean warblers with montane forests of middle elevations in the Northern Andes. He further indicates that this warbler does not accept disturbed habitats. In migration, occurs in various forest, woodland, second growth, and scrub habitats; forest canopy, gaps and edges, semi-open areas, usually high in trees (Stiles and Skutch 1989). In winter, occurs in forest and woodland borders on mountain slopes, primarily in tall, primary, evergreen forest (Robbins et al. 1992) (deciduous, according to Hamel 1992).

In various areas, the highest breeding density was 82-290 pairs per sq km (Robbins et al. 1992). Little work has been done on wintering ecology. J. Fitzpatrick and S. Robinson (pers. comm.) have observed that birds participate in canopy-foraging flocks of forest insectivores, with only one or two warblers in a particular flock. This implies that the foraging flock is a resource that is actively defended by the individual warbler against conspecifics. Though empirical data to support this hypothesis are lacking, this behavioral tendency could be of critical importance. Single flock membership implies a finite limit to the total population; a limit determined by available space. However, these recent findings are in contrast to the earlier reports of movements of large flocks of warblers (Bent 1953).

Apparently a monogamous, single brooded species. The compact nest is built by the female on the lateral limbs of a tree and placed at a considerable distance (e.g., 2-7 m: Harrison 1984) from the bole of the tree, usually saddled on a large, lateral branch, attached perhaps to a small protruding twig. The nest is rather shallow for a warbler (Bent 1953). Variation in site selection is considerable, particularly with respect to the distance from the bole. All authors agree that nests are not built near the ground. Heights from 5-20 m are reported by Bent (1953), Harrison (1984), and Hands et al. (1989), and the typical height is probably above the middle of this range.

The female lays three to five, usually four, eggs. Incubation is believed to be about 12 days (Harrison 1975), and nest life of the young is nine to ten days (n = 1: Southern 1962). The young are fed by both parents, and subsequent to fledging, they move first to lower-level vegetation where their parents feed them, and then gradually farther afield as they approach independence.