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Black-throated Blue Warbler

Setophaga caerulescens

Order:
Passeriformes
Family:
Parulidae
Sections

Breeding

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Figure 2. Annual cycle of breeding, molt, and migration of the Black-throated Blue Warbler.

Figure depicts phenology for the northeastern United States. Thick lines show peak activity, thin lines off-peak activity.

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Black-throated Blue Warbler nest.

Nest usually located in dense shrub layer of either deciduous or mixed coniferous-deciduous habitat. Typically hidden by foliage, usually with one or more sprays of leaves directly overhead, and placed in a fork of a low shrub or sapling; often incorporating a leaning dead stick or branch, which serves as a tripod-like support for the nest.

© Sara Kaiser, New Hampshire, United States, 20 June 2011
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Typical nest of the Black-throated Blue Warbler.

Nest constructed of thin strips of bark, often obtained from white or yellow birch, and occasionally pieces of rotten wood, glued together by using silk from spider webs and apparently saliva. Nest collected near S. Sterling, Wayne Co., PA. 16 June 1917. Ruler is 8 cm.; photographer Rene Corado.

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Female Black-throated Blue Warbler on nest.

Photo by Alisa Muniz.

© Sara Kaiser, New Hampshire, United States, 20 June 2011
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Black-throated Blue Warbler clutch, NY.

Stephentown, Rensselaer Co., NY. 4 June.; photographer Rene Corado

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Black-throated Blue Warbler nest with eggs.

The inner wall of the nest is composed of shredded bark fibers, and lined with fine black rootlets, pine needles, bits of moss, and strands of mammal hair. Eggs are ivory or creamy white, speckled, blotched or clouded with tones of brown, chestnut, or gray. Markings variable, usually concentrated at the large end.

© Danielle Aube, New Hampshire, United States, 18 June 2017
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Black-throated Blue Warbler eggs and hatchlings.

Photo by Patrick Roberts.

© Sara Kaiser, New Hampshire, United States, 20 June 2011
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Black-throated Blue Warbler hatchlings.

Young birds are altricial, naked, except for downy tufts (gray) on head, neck, and dorsum, eyes closed.

© Sara Kaiser, New Hampshire, United States, 20 June 2011
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Black-throated Blue Warbler nestlings near fledging.
© Sara Kaiser, New Hampshire, United States, 20 June 2011
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Hatchling Black-throated Blue Warbler.

Gray tufts of down on capital, spinal, crural, caudal, and alar tracts. Sex of fledglings can be distinguished by the edging to the remiges and rectrices, blue-green in males and olive in females.

© Sara Kaiser, New Hampshire, United States, 20 June 2011
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Adult male Black-throated Blue Warbler with fledgling.

Either one or both adults, usually both, remain in attendance with the fledglings, providing food and giving alarm calls. In many cases, the adults split the brood, one or two young going with each parent.

© William Leigh, Virginia, United States, 10 July 2016

Phenology

Pair Formation

Males actively follow females when females first appear on territory, and sometimes pursue them in fast-flying, erratic chases in the understory (See Behavior: Sexual Behavior). These chases can involve two or even three males, as well as the female. Females eventually settle within an area and are associated (i.e., paired) with the local territorial male. Returning females often settle near their nesting area of the previous year (see Demography and Populations: Breeding Dispersal and Fidelity to Breeding Site). Whether females breeding for the first time or dispersing females visit multiple territories before settling is unknown.

Nest Building

When weather is favorable, nest building begins within 3–7 d of female’s arrival, but may be delayed by as much as 14 d if weather is cold or wet (RTH). Earliest nest building recorded at Hubbard Brook in New Hampshire on 12 May (RTH). First nests of the season are built in about 4–5 d, whereas nests following nest failure or second broods later in the season are constructed more rapidly, usually in 3–4 d (SAK, NLR).

In New Hampshire, breeding is initiated earlier in warmer springs, and early breeders are more likely to attempt a second brood than those starting later in the season, leading to selection favoring early breeding (Townsend et al. 2013). Individuals adjust mean lay date appropriately to substantial inter-annual variation in spring temperature (2.46–8.31º C), and pairs with yearling males bred later than pairs with older males (1986–2010; Townsend et al. 2013).

Nests lost to predation or weather events are usually replaced. At Hubbard Brook in New Hampshire, some females build as many as 5 nests in a season owing to heavy predation, extreme weather, and/or multiple brooding (Holmes et al. 1992, SAK). In North Carolina, females made as many as 6 nesting attempts (Stodola et al. 2013). Building up to 6 nests per season must be an energy demanding process for a female, especially if none is successful (see First/Later Broods, below). Time to the start of a new nest averaged 6.5 d following nest failure and 10.5 days following a successful fledging in North Carolina (Stodola et al. 2013).

First/Later Broods

Figure 2. One, sometimes 2, and rarely 3 broods attempted per season in New Hampshire (Holmes et al. 1992, Nagy and Holmes 2005b, RTH). First clutches initiated between 16 May-5 June; second clutches (following fledging of first) between 25 June and late July. Third clutches (following fledging of second) in late July or early August, but rare (documented on 3 occasions in 30 yr of intensive study at Hubbard Brook; only one of these third clutches was observed to fledge young (RTH, TSS). Between 1986 and 2010, mean initiation date of first clutch at Hubbard Brook was 30–31 May (range 18–19 May to 23–24 June) (Townsend et al. 2013). The frequency of those females successfully completing a first clutch and then laying a second ranged from 0 to 87% (n = 7 years, Nagy and Holmes 2005b). Double-brooding is common at Hubbard Brook; e.g., 97 of 309 females (31%) laid second clutches after successfully fledging young from the first clutch (1986–2010; Townsend et al. 2013). In the southern Allegheny Mountains, "nesting begins in May and continues until the end of June" (Cairns, in Bent 1953b). Clutches were initiated in mountains of western North and South Carolina in late May to early June (Potter et al. 1980). Near the southern end of the breeding range in the Nantahala National Forest, North Carolina, double-brooding (but no triple-brooding) occurred, but data on frequency was not given (Stodola et al. 2009, Stodola et al. 2010). At this site, mean date of clutch initiation for first broods was 26 May and for second broods, 25 June (Stodola et al. 2009).

Nest Site

Selection

Apparently chosen solely by the female. Male sometimes accompanies female while she is searching for nest sites and for nest material (RTH), and sometimes seems to show the female nest sites by perching and settling or turning in likely nest locations (see below).

Microhabitat

Usually located in dense shrub layer of either deciduous or mixed coniferous-deciduous habitat. Frequently in broad-leaved evergreen shrubs (e.g., laurel, rhododendron), conifer saplings (e.g., spruce, yew), deciduous shrubs (viburnums), or saplings (beech, maple). In a northern hardwood forest at Hubbard Brook in north-central New Hampshire, nests (n = 182) were situated in hobblebush, Viburnum lantanoides (51%), beech saplings (21%), and sugar maple saplings (9%), with the remainder in red spruce, balsam fir, striped and mountain (Acer spicatum) maple, fern clumps, and other shrub layer vegetation (RTH). Nest sometimes placed in a cluster of dead leaves, for instance among the dry foliage of a large branch or tree that has recently fallen to the ground (Nice 1930a, Walkinshaw and Dyer 1953, RTH). In other parts of New England, nests in mountain laurel (Harding 1931), alder (Nice 1930a), and raspberry thickets (Bent 1953b). In southern part of its range, Cairns (in Bent 1953b) reported that "nests were often placed in laurel, gooseberry, and chestnut, and blue cohosh (Caulophyllum thalictroides)." Of 562 nests found in Nantahala National Forest, North Carolina, 71% were placed in rhododendron (Rhododendron maximum), 6% in eastern hemlock (Tsuga canadensis), 6% in huckleberry (Gaylussacia spp.), 4% in blackberry (Rubus), 2% in mountain laurel (Kalmia latifolia), with the remaining in 28 other plant species (Stodola et al. 2013).

Site Characteristics

Typically in a fork of a low shrub or sapling; often incorporating a leaning dead stick or branch, which serves as a tripod-like support for the nest. Nest heights generally low, usually within 1–1.5 m of the ground. In North Carolina, heights of early nests averaged 0.48 m (n = 54), late nests at 1.81 m (n = 48) (Guzy 1995). In central New Hampshire, nests averaged 0.44 m ± 0.18 (SD, n = 182) above ground level (RTH), with occasional nests above 2 m (e.g., two nests above 10 m, one at 11 m in a yellow birch tree, next to the bole and partially supported by thin branch sprouts (K. E. Petit and RTH, unpublished data), and another at 17 m in an eastern hemlock (J. J. Barg and RTH, unpublished data). In the Nantahala National Forest in North Carolina, 95% (n = 563) of nests were placed between 0.95 and 1.3 m in height (Stodola et al. 2009).

Nest

Construction

Built by the female typically in 3–5 d (Harding 1931, see Nest Building). Harding reported that the male helps by bringing some nest materials, and may shape the nest occasionally. However, from intensive observations over several decades in central New Hampshire, males never seen to take part in nest building (RTH, NLR). Construction occurs at frequent intervals throughout the day.

Structure and Composition

Nest constructed of thin strips of bark, often obtained from white or yellow birch, and occasionally pieces of rotten wood, glued together by using silk from spider webs and apparently saliva (Harding 1931, Bent 1953b, RTH). The inner wall is composed of shredded bark fibers, and lined with fine black rootlets, pine needles, bits of moss, and strands of mammal hair (Harding 1931, RTH). The latter from horses, skunks, and humans (Harding 1931), moose and porcupine (RTH), and deer (Walkinshaw and Dyer 1953). Some of these fine, hair-like black strands may actually be rhizomorphs of horsehair fungus (Marasimius androsaceus), which has been identified to be a frequent component of nest lining materials used by forest birds in New England (McFarland and Rimmer 1996). It has been hypothesized that this fungus may provide some benefit to nestlings as it is reported to have antibiotic and anticarcinogenic properties (Aubrecht et al. 2013).

In the southern part of its range, Cairns (Bent 1953b) reported that nests were "composed of rhododendron or grape-vine bark, interwoven with birch-bark, moss, spider-webs, and occasionally bits of rotten wood. The interior is neatly lined with hair-like moss, resembling fine black roots, mixed with a few sprays of bright red moss...".

Dimensions

Mean outside diameter 8–9.6 cm; nest height 5–7.8 cm; inside diameter 4.5–5 cm; and depth 2.5–4 cm (n = 10; Peck and James 1987). At Hubbard Brook (2010–2013), mean maximum nest depth 6.6 cm ± 1.4 SD; maximum cup depth 3.5 cm ± 0.9 SD; maximum outer diameter 8.4 cm ± 1.1 SD; maximum inner diameter 5.7 cm ± 0.8 SD (n = 837 nests; SAK). Nest wall thickness (difference between outer and inner nest diameter) increased with elevation at Hubbard Brook (2010–2013): low elevation 2.39 cm ± 0.09 SE, mid elevation 2.69 cm ± 0.07 SE, and high elevation 2.85 cm ± 0.10 SE (n = 388 nests; M. G. Smith, SAK, and MSW, unpublished data). Nest wall thickness is not associated with temperatures during nest building (M. G. Smith, SAK, and MSW, unpublished data).

Microclimate

Nest hidden by foliage, usually with one or more sprays of leaves directly overhead (Nice 1930a, Walkinshaw and Dyer 1953, Holway 1991). Mean stem density (stems per m2 within a 1-m radius of the nest) increases with elevation at Hubbard Brook (2010–2013): low elevation 10.15 ± 0.86 SE, mid elevation 12.68 ± 0.51 SE, and high elevation 13.60 ± 0.89 SE (n = 490 nests; M. G. Smith, SAK, and MSW, unpublished data).

A Hubbard Brook (2010–2013), mean nest height 53.1 cm ± 52.1 SD; distance from main stem 42.1 cm ± 61.2 SD; % vegetation obscured overhead 71.9 % ± 32.5 SD; % vegetation obscured from 1 m north 47.9 % ± 36.0 SD; % vegetation obscured from 1 m east 47.3 % ± 36.8 SD; % vegetation obscured from 1 m south = 47.6 % ± 36.6 SD; % vegetation obscured from 1 m west = 46.4 % ± 36.5 SD; aspect bearing downslope = 138.7 ± 80.0º (n = 837 nests; SAK).

Maintenance or Reuse of Nests

Nests not reused, although in one case an abandoned platform from earlier in the season was built upon successfully (P. P. Marra, personal communication). Material from old nests, their own and other species, sometimes used in construction of new nests (P. P. Marra, personal communication).

Eggs

Shape

Ovate to short ovate, rarely tending to elongate ovate (Bent 1953b).

Size

From a sample of 50 eggs (Bent 1953b), mean length 16.9 mm (range 15.2–18.9), breadth 12.8 mm (range 11.8–13.5). From 76 eggs (20 clutches) measured by Western Foundation for Vertebrate Zoology, mean length 17.1 mm (range 16.2–18.2), breadth 12.7 mm (range 11.1–13.4). At Hubbard Brook (2013–2016), mean egg mass 1.42 g, mean yolk mass 0.31 g, mean shell mass 0.08 g, and mean albumen mass 1.02 g (n = 95 eggs; N. Krauss, unpublished data). Yolk is generally about 20–25% of the eggs, albumen is 70–75%, and shell is 5–6% (N. Krauss, unpublished data).

Eggshell Thickness

Shell thickness from surface to inner membrane averaged 95.66 mm ± 4.51 SE, and eggs contained an average of 19.43 mg ± 1.27 SE of calcium (Hubbard Brook, n = 16; Taliaferro et al. 2001). Based on analysis of stable isotopes, calcium in eggshells at Hubbard Brook in New Hampshire derives almost exclusively from local food sources, and was not imported from overwintering or stopover areas (Blum et al. 2001).

Color

Ivory or creamy white, speckled, blotched or clouded with tones of brown, chestnut, or gray (Harding 1931, Bent 1953b). Markings variable, usually concentrated at the large end, often forming a loose wreath; sometimes a solid cap of brown (Bent 1953b) or indistinct (Harrison 1978a).

Clutch Size

Clutches typically contain 4 eggs, but range from 2 to 5 (Bent 1953b, Griscom and Sprunt 1957, RTH, NLR). In a sample of 768 nests at Hubbard Brook in New Hampshire, clutch size averaged 3.6 eggs ± 0.9 SD, with a range of 2 to 5 and both a median and mode of 4 (TSS, RTH). Mean clutch size in the Nantahala National Forest, North Carolina was 3.7 eggs (n = 562; Stodola et al. 2013). For more details, see Demography and Populations: Measures of Breeding Activity.

Egg Laying

A single egg is laid each day, usually early morning (Harding 1931, RTH). Otherwise, female rarely at the nest during the egg-laying period. A new nest is built and clutch replaced if lost to weather or predators. No evidence for intraspecific egg dumping (Chuang et al. 1999, Webster et al. 2001, Kaiser et al. 2015, Kaiser et al. 2017a).

Incubation

Onset of Broodiness and Incubation in Relation to Laying

Incubation often starts on the day before the last egg is laid (RTH) or on the morning following the laying of the last egg (Harding 1931).

Incubation Patches

Single medial abdominal patch in females only.

Incubation Period

Incubation period 12–13 d from laying of the last egg (Harding 1931, Harrison 1978a, Peck and James 1987). In New Hampshire, averaged 13.0 d ± 0.5 SD (n = 47 pairs), measured from the day before the last egg is laid, when incubation usually begins (Holmes et al. 1992). In North Carolina, reported to be 12.4 d ± 0.5 SD (Guzy 1995), or 12 d (Stodola et al. 2009).

Parental Behavior

Only the female incubates. Sits tightly and flushes only when danger is near. When disturbed from the nest, female drops to the ground, flutters off in a distraction display, feigning broken wing (RTH). Female may abandon the nest up to 6 days into incubation when disturbed (SAK). While female is incubating, male often sings nearby, and occasionally feeds incubating female (Harding 1931, SAK). Female spends 75% of the daylight hours (and all night) on the nest, and incubates for bouts of 20–50 min (Black 1975). Females spend an average of 64% of daylight hours incubating in bouts lasting 20.5 min ± 1.5 SE, and make 2.4 ± 0.1 SE departures from the nest/h on trips that last 10.6 min ± 0.7 SE (Joyce et al. 2001). Incubation bouts are longer and females spend more time incubating per hour in the mornings and late afternoons than at midday (Joyce et al. 2001). Females regularly observed poking the bottom of the nest with her bill, consistent with egg-rolling or examination for parasites such as protocaliphord fly larvae (SAK, NLR). Males occasionally bring food to incubating females, including just prior to hatch (e.g., 7 of 191 nests (4%) videotaped during incubation, 2009–2012 at Hubbard Brook; SAK). In some of these cases the female was not present on the nest and male feeding may have been “anticipatory food-bringing”, in which the male comes to the nest with food, anticipating hatching of the eggs.

Hardiness of Eggs Against Temperature Stress: Effect of Egg Neglect

No information.

Hatching

Preliminary Events and Vocalizations

Not recorded.

Shell Breaking and Emergence

Eggs usually hatch within a single day, but hatching occasionally extends until the next day (RTH).

Parental Assistance and Disposal of Eggshells

Female disposes of the eggshells by "crushing and eating them without leaving the nest" (Harding 1931), or more typically by carrying them away (Harding 1931, RTH). Male typically begins feeding nestlings within hours of hatching (Harding 1931), but are less attentive at nests late in the season (RTH, NLR).

Young Birds

Condition at Hatching

Altricial; eyes closed and naked, except for downy gray tufts on head, neck, and dorsum.

Growth and Development

Nestling period averages 8.6 d ± 0.6 SD (n = 77) in New Hampshire (Holmes et al. 1992) and 11.4 d ± 0.5 SD in North Carolina (Guzy 1995). Mass increases from 1.3–1.6 g on day 1 (day 0 = hatching) to 5–5.5 g on day 4, and to 7.5–8.2 g on day 6 (Rodenhouse 1986), which is the last day nestlings can be handled without causing premature nest departure. Linear growth slope ranges from 0.48 to 1.69 g/d (n = 295 nestlings), varying within and among broods (Rodenhouse 1986). By the time of fledging on day 8 or day 9, young approximately equal to adults in weight. Mass of recently fledged young ranges from 9.6 to 9.9 g (RTH). In North Carolina, mean nestling mass on day 7 was 7.43 g + 0.05 SE (n = 182; Stodola et al. 2010).

Eyes open by about day 4 (Harding 1931); gape yellow, mouth lining pink (Harrison 1978a); according to Harding (Harding 1931), primaries break sheaths on day 4, and are 10 mm long on day 5. At Hubbard Brook, data indicate primaries usually break sheaths on day 5 or day 6 (NLR, TSS). Young can fly weakly when they leave the nest on day 8 or 9 (RTH). At fledging, sexes cannot be identified by plumage, but male and female plumage is evident within two weeks post-fledging (RTH). Young give a low buzzing call when 7–8 d of age, and a chittering sound when hungry (Harding 1931); see Sounds and Vocal Behavior.

Parental Care

Brooding

Female frequently broods the nestlings on the day of hatching and at times during the nestling period when the weather is cool or rainy (RTH). Nestlings brooded significantly more often early in the nestling period (RTH). Males not recorded brooding nestlings.

At Hubbard Brook in New Hampshire, brooding patterns differ by elevation (M. G. Smith, SAK, and MSW, unpublished data), with females having longer off-bouts and spending more time off their nests at high elevation compared to mid and low elevations (n = 193 nests); mean duration for off-bouts at low elevation 390.9 s ± 7.2 SE, mid elevation 378.7 s ± 4.4 SE, and high elevation 402.2 s ± 5.7 SE; proportion of time off nest at low elevation 0.34 ± 0.01 SE, mid elevation 0.33 ± 0.01 SE, and high elevation 0.35 ± 0.01 SE. Mean off-bout frequency (h-1) does not differ by elevation (n = 193 nests); low elevation 3.16 ± 0.03 SE, mid elevation 3.16 ± 0.02 SE, and high elevation 3.17 ± 0.03 SE.

Daily brooding patterns at Hubbard Brook vary over the early nestling stage (M. G. Smith, SAK, and MSW, unpublished data). Mean off-bout duration and proportion of time off the nest decrease from dawn to dusk and from nestling day 3 to day 5. Mean off-bout frequency remain relatively constant on day 3, but peak between 0900 and 1100 on day 4 and day 5. Weather during the nestling stage was related to female brooding patterns (M. G. Smith, SAK, and MSW, unpublished data). At lower temperatures and during periods with higher rainfall, females spent more time off the nest, resulting in nestlings with lower than average mass near fledging.

Feeding

Early studies report both males and females feeding nestlings at about equal rates, and both appeared to increase their feeding rate as fledging approaches (Nice 1930a, Walkinshaw and Dyer 1953, Goodbred and Holmes 1996). In New Hampshire, adults brought food to the nest at rate of 3–4 trips/h when nestlings were 2 d old, to about 10/h when 8 d of age; provisioning rates did not vary with time of day (Goodbred and Holmes 1996). In more detailed studies in North Carolina, males’ provisioning rate per nestling increased with number of young in the nest, while that of females decreased (Stodola et al. 2009). Furthermore, at this study site, nestling mass on day 6 of the nestling period was directly related to provisioning rate by the male, but not to that by the female (Stodola et al. 2010). Although females made more visits to the nest than males, males provided more biomass (larger prey) per hour than did females (Stodola et al. 2010). Also, females provisioned young of first broods at a rate lower than in second broods, while males’ provisioning rate did not differ between first and second broods (Stodola et al. 2009).

Parental feeding rates vary with food availability. Food supplementation experiments at Hubbard Brook in New Hampshire showed that food-supplemented females on low quality territories provisioned at a significantly higher rate, delivered more prey biomass, and spent more time at the nest than did control females; males did not adjust their provisioning to supplemental feeding (Kaiser et al. 2014, see also Nagy et al. 2007). Older (after-second-year) males adjust provisioning rate in response to the presence of extra-pair young in the nest, making fewer provisioning trips when extra-pair young are present than when they are absent, but younger (second-year) males do not show any such adjustment (Chuang-Dobbs et al. 2001b). Provisioning rates also vary with population density (Sillett et al. 2004).

At Hubbard Brook, males and females respond similarly to environmental variability (Kaiser 2013). Both sexes provision their nestlings more frequently with smaller food loads when food resources are low, which is associated with cooler conditions. Lower rates of male provisioning are correlated with lighter nestlings near fledging, whereas female provisioning rate does not have the same effect on offspring condition. Male provisioning rates are lower and more variable than female provisioning rates between first and second broods. This pattern in male parental behavior likely accounts for the relationship between male provisioning rate and offspring condition, and potentially observed declines in offspring condition with time of season (Kaiser 2013).

Adults carry food in their beaks; no regurgitation documented (Harding 1931, RTH). Large prey items, such as caterpillars or large moths, are usually brought one at a time, but several items may be brought at the same time, especially if they are flies or other winged insects. Based on photographic records made at nests, equally as many trips to the nest are made with multiple prey as with single prey items (Rodenhouse 1986, NLR). Most common food brought to young nestlings are small flying insects (mostly Diptera and Hymenoptera), spiders, and small usually smooth-skinned caterpillars (Harding 1931, RTH). For larger nestlings, crane flies, moths, and both large and small caterpillars are major foods (Rodenhouse 1986, Rodenhouse and Holmes 1992). Caterpillars comprised 60–87% of the estimated prey biomass brought to nestlings in New Hampshire (Goodbred and Holmes 1996).

Nest Sanitation

Parents either eat fecal sacs or carry them away from the nest (Nice 1930a, Harding 1931). Both male and female ate the fecal sacs during the first 5 days, and after that carried them away, sometimes placing them on dead branches (Harding 1931). Similar patterns have been observed at Hubbard Brook (RTH, TSS, SAK).

Parental Carrying of Young

Not known.

Cooperative Breeding

Not known.

Brood Parasitism

Identify of Parasitic Species

The only known brood parasite is the Brown-headed Cowbird (Molothrus ater).

Frequency of Occurrence, Seasonal or Geographic Variation

Nests are infrequently parasitized (Friedmann et al. 1977, Friedmann and Kiff 1985). In Ontario, 3 of 23 (13%) nests were parasitized (Peck and James 1987). From over 5,300 nests examined in White Mountain National Forest in New Hampshire between 1982–2016, none was parasitized (RTH), although one female was seen feeding a cowbird fledgling in the Hubbard Brook valley in July 1975 (S. K. Robinson, personal communication). In a relatively unfragmented managed hardwood forest in northern Michigan, only one of 160 clutches was parasitized by a cowbird (K. R. Hall, unpublished data). In small forest fragments of west-central Vermont, 16% of nests monitored (sample size not indicated) failed due to brood parasitism (Cornell and Donovan 2010a).

Timing of Laying in Relation to Host's Laying

Usually occurs during the normal laying period of host, beginning after the first host egg appears in the nest (K. Cornell, personal communication). In Vermont, number of cowbird eggs per host nest range from 1 to 4 (K. Cornell, unpublished data).

Response to Parasitic Mother, Eggs, or Nestlings

Host female will incubate cowbird eggs and feed cowbird nestlings and fledglings as if they are her own. Host males have also been observed feeding cowbird nestlings and fledglings. The number of cowbird fledglings range from 1–2 (K. L. Cornell, unpublished data). Response to parasitic mother is unknown.

Effects of Parasitism on Host

Not known. This species may be less subject to cowbird parasitism because of its preference for nesting in large tracts of relatively undisturbed forest. In most cases, a single cowbird nestling can cause complete reproductive failure for this host. In Vermont, of 25 nests that were parasitized, the host was able to produce a single host fledgling on only 5 (20%) occasions (K. L. Cornell, unpublished data).

Success of Parasite with This Host

In Vermont, 15 out of 25 (60%) parasitized nests successfully fledged at least one cowbird (K. L Cornell, unpublished data).

Fledgling Stage

Departure from the Nest

All young typically leave nest within a period of 30–60 min (Harding 1931, RTH). If this happens late in the day, they may return to the nest for that night, to be brooded by the female (RTH). When hatching is asynchronous, young may leave the nest 1 or even 2 days apart.

Period from Hatching to Departure

Departure from nest ranges from day 8 to day 10 (day 0 = hatching day; Harding 1931, RTH). At Hubbard Brook in New Hampshire, the mean nestling period averaged 8.6 d ± 0.5 SD (n = 77; Holmes et al. 1992).

Condition of Development at Departure

At departure, young are close to or at adult weight. Tail and wing feathers are about half grown; many down feathers about head and dorsum. Sexes indistinguishable at this stage. Can fly weakly, usually fluttering from one understory branch to another.

Association with Parents or Other Young

Both males and females give distraction display involving broken-wing act when newly fledged young are disturbed (L. R. Nagy, personal communication). Either one or both adults, usually both, remain in attendance with the fledglings, providing food and giving alarm calls. In many cases, the adults split the brood, 1 or 2 young going with each parent (Black 1975, Davis 2001a). When females are attempting a second clutch after a successful early clutch, the male becomes the main and perhaps the sole overseer of the young (RTH, Davis 2001a). Females, though, have been seen feeding fledglings while building subsequent nests (L. R. Nagy, personal communication). At nests that produce young late in the season (mid-July to mid-August), the female seems to be the main (sometimes the only) provider for and protector of the fledglings (RTH).

Ability to Get Around, Feed, and Care for Self

Upon leaving the nest, fledglings stay low in the understory for a week or two, usually remaining within the territory, but sometimes moving 200–300 m or more away during the first 2 wk. Attending adult(s) probably provide most food in the first week, although young birds do attack and capture prey within 2–3 d of fledging (RTH). Normal period of dependency is probably about 2–3 wk post fledging, although adults feed young up to 4 wk in some cases (RTH). Young are flying well and foraging actively by 10–14 d after fledging. Family groups are present in breeding habitat in July and August, occasionally into September (RTH).

Recommended Citation

Holmes, R. T., S. A. Kaiser, N. L. Rodenhouse, T. S. Sillett, M. S. Webster, P. Pyle, and M. A. Patten (2017). Black-throated Blue Warbler (Setophaga caerulescens), version 3.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.btbwar.03