Walking, Hopping, etc.
Hops on ground (Burtt and Hailman 1978).
Flits rapidly among perches and foliage, or makes short or long flights from tree to tree, or from understory to canopy and return.
Swimming and Diving
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Hops on ground (Burtt and Hailman 1978).
Flits rapidly among perches and foliage, or makes short or long flights from tree to tree, or from understory to canopy and return.
Little specific information. Head-scratching performed over the drooped wing, as is typical for adult Setophaga (Burtt and Hailman 1978).
During the breeding season at Hubbard Brook in New Hampshire, males spent 30–32% of daylight hours foraging, 19–22% foraging combined with singing, 17–40% singing from perches, 3–7% preening and in self maintenance, 0.4–0.9% in aggressive interactions, and 1–9% sitting or "resting," all varying with stage of nesting cycle (Black 1975). Proportion of time spent counter-singing and foraging by males varies with number of neighboring conspecifics (Sillett et al. 2004). Females in breeding season spent an average of 75.1% of daylight hours incubating and 21.5% foraging; after eggs hatch, both sexes forage for about 70–75% of daylight hours (Black 1975). During the overwintering period, both sexes spend a large portion of their time foraging (RTH), although this has not been quantified.
Upon arrival on the breeding grounds, males move around the forest, often covering large areas (5–10 ha or more). Territorial disputes occur frequently, with prolonged chases and conflict bouts, lasting hours or sometimes days. In these chases, 2 (or occasionally 3) males swirl around the forest, chasing from high in the canopy to the shrub layer, sometimes landing on the ground for brief periods of time.
When an intruder is first discovered, the aggressor flies in rapidly, often on a gliding flutter, giving the machine gun-like aggressive trill (see Sounds and Vocal Behavior), a series of rapidly-delivered chip notes, and occasionally a song. Perching near the intruder, the aggressor assumes a head-forward posture, wings slightly flexed and drooped (RTH). In most cases, these encounters lead to chases, which are often prolonged (e.g., 0.5 to several min). Chases and attacks are sometimes preceded by soft song given by the aggressor (see Sounds and Vocal Behavior). When 2 neighbors are contesting territorial boundaries, the chases often follow a circular pattern covering distances of 50 to 100 m or more. These boundary contests are similar to, but less stereotyped than, the circle chases of the American Redstart (Setophaga ruticilla) (Ficken 1962a).
Sometimes the aggressor lands on the back of its opponent, forcing it to the ground, pecking the other strongly with its beak and striking it with its wings (RTH). Once on the ground, the pursuit continues, with wings flailing; sometimes the two birds lock their feet together, and flutter into the air a meter or two before falling back to the ground. These combat sequences are usually followed by aerial chases, which may terminate in another bout of physical combat. Such chase-combat conflicts usually last several minutes, but may continue between the same individuals for hours or even periodically over several days (RTH). These aggressive encounters sometimes start off with one individual giving a quiet version of the song, the aggressive trill, or a high-pitched chipping, but during the periods of actual physical combat the birds are usually silent. After separating at the end of a combat sequence, one or both birds may move to a nearby perch and sing. Sometimes the two males will remain perched within 1–2 m of each other, and remain motionless for minutes, often staring in opposite directions (RTH).
The Black-throated Blue Warbler may convey increasingly pronounced levels of aggressive motivation using vocal signaling. In a study using sequential playback (Hof and Podos 2013), males responded to low-level threat (simulated interaction using audio playback at the territory boundary) by switching from the type 1 song (used in mate attraction) to the type 2 song (used in male-male competition; see Sounds and Vocal Behavior). When the level of threat was experimentally escalated (audio playback within the male’s territory accompanied by a taxidermy mount), males used a low-amplitude ‘soft song’, which predicted later attack of the mount (Hof and Hazlett 2010, Hof and Podos 2013). Males that retreated switched back to a type 1 song.
The white wing-patch of the male may signal social status and age/experience, and mediate aggressive interactions. In a plumage manipulation study, males with experimentally blackened wing spots were involved in more agonistic interactions (mostly vocal) with males both within their territories and at territory boundaries than control males (Cline et al. 2016).
Shortly after arrival, females also chase one other, probably a territorial or "dispersing" mechanism. These chases involve both short and long flights, with one bird trying to drive the other away. These encounters may last only minutes, although they have been recorded persisting for hours and even for several days (RTH). In some cases, pairs appear go up against each other, male against male and female against female (P. P. Marra, personal communication; RTH).
Most birds in summer and overwintering periods are solitary and generally intolerant of the approach of a conspecific of the same sex (but see below).
In summer, males defend and advertise territories and, along with their social mate, do most if not all feeding within these defended areas (RTH). Males respond strongly to the presence and/or song of conspecifics, as well as to audio broadcast of their own species' song. Individuals respond to broadcast of mobbing calls only within their territory (Betts et al. 2005). Chases and fighting are frequent during the early establishment period, and late-returning older males have been recorded displacing earlier-arriving younger males (RTH). Territory size ranges from about 1–4 ha, depending on habitat, being smallest where the shrub layer is dense and heterogeneous (Steele 1992, RTH).
Little evidence for floaters. Experiments have indicated that when males were experimentally removed from their territories, they were replaced by other males, most of which were neighbors moving in from adjacent areas (Marra and Holmes 1997). If both a male and his female mate were removed from a territory, the territory remained vacant (in 9 of 10 cases), suggesting that the presence of new males in male-only removals was due primarily to the presence of a female in that area (similar results reported by Sillett et al. 2004). Thus, the availability of females seems important in attracting and sustaining male territorial activity.
No evidence for interspecific territoriality, although some interspecific aggression occurs. These warblers have been seen to supplant Ovenbirds (Seiurus aurocapilla) and American Redstarts, and to be chased by Least Flycatchers (Empidonax minimus) (RTH), Black-throated Green Warblers, and Red-eyed Vireos (Vireo olivaceus) (NLR). Among the co-occurring forest birds at Hubbard Brook, they overlap most in foraging methods and locations with Least Flycatchers and American Redstarts (Holmes et al. 1979b, Holmes et al. 1986), suggesting the opportunity for interspecific competition.
During the overwintering period, the Black-throated Blue Warbler is mostly solitary, occupying more or less exclusive home ranges or territories (Holmes et al. 1989, Wunderle 1992), which vary in size from 0.2–0.3 ha for males and 0.15–0.26 ha for females (Wunderle 1995). Winter territoriality in both males and females is indicated by (1) their overdispersion through available habitats (Sliwa 1991, Sliwa and Sherry 1992, RTH), (2) the occurrence of agonistic interactions both within and between sexes, including occasional prolonged combat (Holmes et al. 1989), (3) strong site attachment within and between winters (Holmes and Sherry 1992, Wunderle 1995), and (4) their aggressive-like responses to audio broadcasts that simulate territorial intrusions (Holmes et al. 1989). Within sexes, winter territories seem to be relatively exclusive within a sex, but some spatial overlap occurs between those of males and females (Holmes et al. 1989). In some cases, young (hatch-year) individuals arrive in winter habitats prior to returning adults (RTH, Wunderle 1995), resulting in frequent aggression once the older, site-faithful individuals return, which usually results in the eventual displacement of the younger birds (RTH). This was documented in Puerto Rico where 6 young males were displaced by 4 returning older males, 2 returning older females, and 3 early-arriving young females were displaced by 1 older male and 2 older females (Wunderle 1995). Thus, considerable aggression and/or spatial avoidance occur between the sexes and different age classes, suggesting competition for suitable overwintering habitat (Holmes et al. 1989). In contrast to the above, some individuals occupying shrubby high elevation sites in Puerto Rico were less localized in their activities, and have been characterized as 'wanderers' (Wunderle 1995). Birds in these habitats, predominately females, feed mostly on berries and fruits, and this different spacing pattern may be a response to the abundance and seasonality of this food source. Relatively high proportions of wanderers (~40%) were also found in coffee plantations in the Dominican Republic (Wunderle and Latta 2000). In shade-coffee plantations in Jamaica, aggression varied with sex, home-range size, proximity to home-range center, habitat, food supply, and the presence of another aggressive bird nearby (Smith et al. 2012). The latter study also documented that individuals responded more aggressively to simulated intrusions (audio broadcasts with male taxonomic mount) in overstory shade trees where insects were more abundant, suggesting a link between aggression and food abundance.
At times, however, it is not uncommon in winter to see a male and a female foraging near each other (RTH), and even occasionally 2 males will forage close together, with no aggression, but it is not clear in what context these occur. Additional studies of social interactions and spacing patterns in overwintering individuals are needed to clarify such situations.
Because this species rarely sings during the overwintering period (see Sounds and Vocal Behavior), territorial spacing in that season appears to be achieved through visual contact (e.g., avoidance), chasing, and probably chip notes (Holmes et al. 1989).
Most breeding males are socially monogamous, with one female nesting within each territory. Polygyny does occur, however, but at low and variable levels, ranging from 0% to 15% at Hubbard Brook in New Hampshire (Petit et al. 1988, Holmes et al. 1992, RTH) and 0% to 7% in northern Michigan (K. R. Hall, unpublished data). Yearling males have significantly smaller testes than older males, as well as less asymmetry in testis size, suggesting that age class should be considered in the assessment of sperm competition and mating systems in this species (Graves 2004b). Extra-pair copulations are frequent (see below).
Offspring sex ratio at Hubbard Brook in New Hampshire (2009–2012) was significantly male-biased for the population (0.57), driven by overproduction of sons in habitat with high food availability (0.59) (K. Grabenstein and MSW, unpublished data). However, sex allocation was not associated with food availability during the fertile stage and did not differ between food-supplemented and control nests in a supplemental feeding experiment. Maternal condition also did not influence sex ratio and no sex-specific rearing costs of offspring were found (K. Grabenstein and MSW, unpublished data).
Most pairs remain together during a breeding season, which often involves several renesting attempts or second broods. Switching of mates within a season between breeding attempts also occurs, but very infrequently (RTH). The number of pairs reuniting between seasons is relatively high: of 20 instances in which both members of a pair returned to the breeding area in a subsequent year, 16 (80%) reunited (P. P. Marra and RTH, unpublished data).
Once in breeding habitat, females move around in the low understory, mostly foraging but also apparently scouting for potential nest sites. At these times, males follow closely, often soft singing overhead. Frequently, a male suddenly will dive at the female and pursue her in erratic chases through the understory. These chases sometimes last several minutes, and may occur several times per hour over one to several days. At times a male will approach the female in a fluttery flight, giving the soft song. When the two birds are perched, the male often assumes a posture in which the wings are slightly drooped, head forward and slightly up, with bill open, as he faces the female (RTH). Males are sometimes observed to quickly and briefly face the female with raised and spread wings, making the wing spots apparent to her (MSW), which may be part of precopulatory courtship. This behavior is very brief and infrequent, however, and so is rarely observed.
Mate guarding is frequent in this species, with the male remaining close to the female, often singing softly from perches 3–4 m overhead and following her as she forages and moves about the territory. The timing of this behavior has not been quantified rigorously, but appears to occur most intensively during nest building and before incubation commences, i.e., during the fertile period (Chuang-Dobbs et al. 2001a). Soft singing by a male is generally a good clue that a female is nearby, and assists an observer in locating a nest-building or egg-laying female (RTH). Males that guard their mates closely are less likely to have extra-pair young in their nests, and experimental detention of males for 1 hr during the fertile period increased the probability that a brood would contain extra-pair young (Chuang-Dobbs et al. 2001a). Thus, mate guarding seems to be relatively effective in ensuring paternity. Males on territories with higher food availability (natural and experimentally enhanced) invest more effort into mate guarding during their social mate’s fertile stage and are less likely to be cuckolded (Kaiser et al. 2014, Kaiser et al. 2015).
During incubation, the male occasionally sings high over the nest, but he usually remains in other parts of the territory during this time. Males sire extra-pair young more frequently during the incubation stage than during any other nest stage, especially males on high-quality territories (i.e., more food resources; Kaiser et al. 2017a). Males often follow incubating females when they leave the nest to forage.
Copulation has only been observed occasionally in this species. The female solicits copulation by quivering her wings, the male mounts for 2–3 s and then flies off, and the female preens (Marra 1993). This takes place on horizontal branches 3–8 m above the ground. One case of reverse mounting has been recorded, in which the female mounted the male, who was giving the submissive wing-quivering behavior (Marra 1993).
Extra-pair copulations are frequent in this species. At the Hubbard Brook Experimental Forest, 23% of all nestlings (n = 413) were sired by extra-pair males (usually males on neighboring territories) and 34% of all broods (n = 117) contained at least one extra-pair young (1995–1998; Chuang et al. 1999, Webster et al. 2001). In a more recent study at Hubbard Brook (2009–2012), 43% of all nestlings (n = 900) were sired by extra-pair males and 56% of all broods (n = 152) contained at least one extra-pair young (Kaiser et al. 2015, Kaiser et al. 2017a, Kaiser et al. 2017b). Cumulatively, from 1995–2015, 41% of all nestlings (n = 4,413) were sired by extra-pair males and 57% of all broods (n = 1,346) contained at least one extra-pair young (SAK, MSW). Other populations appear to show similar, but somewhat variable, levels of extra-pair paternity. First, a 3-yr study of a population in Vermont (D. Hof, unpublished data) found that 33.2% of nestlings were sired by extra-pair males (n = 208 nestlings) and 52.9% of broods contained extra-pair young (n = 68 broods). Similarly, at a study site in the Southern Appalachians in North Carolina (M. Cline, unpublished data), 32% of nestlings were sired by an extra-pair male (n = 257 nestlings) and 57% of broods contained at least one nestling sired by an extra-pair male (n = 67 broods).
Opportunities for extra-pair copulations appear to be constrained by social and ecological conditions. The probability of a male siring extra-pair young decreases with increasing distance to females and breeding density (Kaiser et al. 2017a). The frequency of extra-pair copulations is positively associated with local breeding synchrony, indicating that the temporal and spatial occurrence of fertile females is important in influencing the prevalence of extra-pair copulations (Chuang et al. 1999). Most males sire extra-pair young while their social mate incubates, especially males on food-abundant territories (Kaiser et al. 2017a). Males on food-poor territories are more likely to sire extra-pair young, and these offspring are produced farther from the male’s territory relative to males on food-abundant territories (Kaiser et al. 2015, Kaiser et al. 2017a).
Extra-pair sires have significantly higher annual return rates than do males that were cuckolded (67% versus 42%, MSW), suggesting that females may select high quality males as extra-pair mates. The basis of female choice of males for copulation, however, remains elusive. Recent evidence, based on microsatellites, suggests that male heterozygosity may not influence female extra-pair mate choice, that females do not chose genetically dissimilar extra-pair mates, nor that extra-pair offspring were more heterozygous than within-pair offspring (Smith et al. 2005c). No male characteristic (body size, wing spot size) has been found to be associated with extra-pair mating success (Webster et al. 2001; M. Cline, unpublished data), possibly due to spatial and temporal constraints on reproductive interactions (Webster et al. 2001, Kaiser et al. 2017a). However, older males (ASY) are more likely to sire extra-pair young than younger males (SY), which is likely associated with territory quality (Kaiser et al. 2015, Kaiser et al. 2017a; M. Cline, unpublished data). Finally, extra-pair paternity also influences parental care. For example, older (ASY) males that sired all young in their broods fed nestlings at higher rates than did ASY males that sired only some of the young in their broods, suggesting that males may be able to assess their paternity (Chuang-Dobbs et al. 2001b).
In breeding areas, occurs in pairs, with strong aggressive interactions between neighboring males and between neighboring females (see Agonistic Behavior). In migration, found in loose flocks, more often with a variety of species other than its own (Brewster 1906). In overwintering areas, largely territorial, with some overlap between males and females (Holmes et al. 1989, see above). Reported to occur as single individuals within mixed-species flocks in Cuba (Eaton 1953) and in an extralimital occurrence in Ecuador (n = 1 male, Martin et al. 2004d).
Raptorial birds prey primarily on adults, whereas Blue Jay (Cyanocitta cristata) and a variety of mammals take eggs and nestlings. Sharp-shinned Hawk (Accipiter striatus) have been seen to attack, but not kill, nesting females (J. Koslowski, personal communication), and have been video recorded removing nestlings from a nest (C. O'Neill and RTH, unpublished data). Red squirrel (Tamiasciurus hudsonicus), eastern chipmunk (Tamias striatus), pine marten (Martes americana), fisher (Pekania pennanti), and in one case, a black bear (Ursus americanus) were observed and/or video-recorded taking eggs and nestlings in New Hampshire (Rodenhouse 1986, Reitsma et al. 1990, RTH, SAK). At Hubbard Brook in New Hampshire, eastern chipmunk and Blue Jay abundance were most associated with nest mortality at low elevations, whereas red squirrel abundance was associated with nest mortality at both low and high elevations (A. Muniz, unpublished data). Cameras at artificial nests placed in shrubs to simulate Black-throated Blue Warbler nests and baited with quail eggs documented predation by red squirrel 55%, fisher 19%, eastern chipmunk 10%, black bear 7%, flying squirrel (Glaucomys spp.) 5%, and Blue Jay 5% (n = 42; Sloan et al. 1998). Raccoon (Procyon lotor) have also been recorded taking eggs from artificial nests in the shrub layer (Reitsma et al. 1990). The only reported case of snake predation on nests of this species is of a garter snake (Thamnophis sirtalis) taking eggs in northern Michigan (K. R. Hall, unpublished data); no depredation by snakes at Hubbard Brook in over 40 years during which the fate of thousands of nests have been monitored (RTH, NLR). On two occasions at Hubbard Brook, slugs were observed feeding on 6–7 d old, living but apparently weakened, nestlings (Biasiolli 2009). On the first of these occasions, nestlings were found on the ground underneath their nest after heavy rains damaged the nest cup. The nestlings were able to beg for food and their parents were attempting to feed the 3 nestlings, which each had a slug attached to the upper or underside of their wing and fresh lesions from the slugs. On the second occasion, one slug was found feeding on 4 nestlings in the nest, all of which had lesions and suffered injuries to their eyes and bills. All of the nestlings were later found dead.
Both sexes mob predators near nest, giving frequent call notes (RTH). When flushed from the nest, the female performs a distraction display, flapping wings wildly or dragging a wing and giving high-pitched chittering/twittering notes. In one case, a female was recorded on videotape attacking a chipmunk by diving at it repeatedly as it climbed toward a nest in a low shrub; the chipmunk retreated and the nestlings fledged successfully (RTH). Both sexes give a high-pitched, difficult-to-locate ‘seet’ vocalization when a raptor is seen nearby. Respond strongly to broadcast of mobbing calls of Black-capped Chickadeee, Poecile atricapilla (Gunn et al. 2000), although the usefulness of this technique for estimating reproductive success appears limited (Betts et al. 2005, Doran et al. 2005). In a study that broadcasted vocalizations of eastern chipmunks and red squirrels to incubating females (Hellmich 2015), responses were stronger to eastern chipmunk vocalization than to red squirrel vocalizations alone or both of these mammalian predator vocalizations combined. Adults appear to adjust their anti-predator response depending on the nest predator (Hellmich 2015).