Although the vocalizations of many Setophaga warblers have been studied intensively (see Spector 1992), those of the Black-throated Blue Warbler have, until recently, received less attention. The information that follows is based mostly on qualitative, long-term observations of this species by the research team at the Hubbard Brook Experimental Forest in New Hampshire (RTH, MSW, and colleagues), as well as some recent studies examining vocalizations in the context of mate attraction and male-male competition.
Calls. Both males and females utter a flat sounding ctuk, which is somewhat sharper than the calls of most other wood warblers, and in fact is reminiscent of the contact call of the Dark-eyed Junco (Junco hyemalis). This call note is given frequently throughout the breeding period by both sexes, but especially by females. In winter, it is essentially the only vocalization given by both sexes. Females in distraction display near the nest give a high-pitched twittering or chittering, with call notes interspersed (RTH). Both males and females sometimes give a high-pitched seep call. This may be a warning call to the young, as it is given when a human observer is near fledglings or a nest with nestlings (D. Hof, personal communication). Begging calls of nestlings occur at frequencies of 6–7 kHz, lower than that of most ground-nesting parulids, and appear to reduce predation by ground predators (Haskell 1999). Males also utter a fast, guttural, "machine-gun"-like trilled or sputtered series of notes (Figure 4d). This "aggressive trill" is given during close encounters between two males, usually preceding or during a chase or fight (Hof 2014). It is also given occasionally by a male when chasing a female during pair formation and nest building (RTH).
Song. Singing is almost exclusively by the male, although females have been reported to sing occasionally (TSS, SAK). There is considerable song variation within and between individuals, but two primary song categories, often referred to as ‘song types’, predominate. The most frequent song, which is given over and over again by a male from the center of his territory, consists of 3 to 7 notes (though can be more in some individuals), with the last commonly slurred upward (Bent 1953b, RTH). This song, which corresponds to the ‘type 1 song’ for other Setophaga warblers (see Beebee 2002, Beebee 2004, Beebee 2004), is sung at a repeatable tempo within individuals, but the tempo varies substantially across males, with some males singing up to three times faster than others (G. Colbeck, unpublished data). This type 1 song is most often pure-toned, but is buzzy in some individuals or populations, and is rendered as 'zee- zee-zee-zreeeeee' (see Figure 4a)
The second main song type, referred to as the ‘type 2 song’, consists of 2 to 6 notes and can be given as 'zree- zree- zhrurrrr.' This song consists of buzzy notes for all individuals, and its tempo is more or less uniform across individuals (G. Colbeck, unpublished data). However, type 2 songs are much more variable than are type 1 songs, both across individuals in a neighborhood and over larger geographic areas (Hof 2014). The type 2 song is characterized by either an unslurred ending or a downward slur (Figure 4b) (Bent 1953b; G. Colbeck, unpublished data), and is somewhat lower in pitch than the type 1 song (Hof 2014).
Soft versions of both song types 1 and 2, often almost whispered, are given by males when associating with females and as a prelude to aggressive encounters among males (Hof and Hazlett 2010, Hof and Podos 2013). This "soft song" is sometimes given without the upward or downward slurred ending (RTH; D. Hof, unpublished data).
A third, less-frequently given song, referred to here as song type 3, consists of 3–5 notes, almost whistle-like in quality, given more slowly than the standard song types, with pauses between notes and each note slurred upward, the last being the loudest and most emphatic: zhrurrrr.....zhrurrrr........ zhrurrrrrrr (Figure 4c; RTH). This song appears to be used in similar contexts to those of the type 2 song (D. Hof, unpublished data), and so may be a slower and less buzzy version of that song type.
Song types do not vary strongly across geographic regions. However, detailed spectrographic comparisons of male song recordings obtained from one breeding population in the north (Hubbard Brook Experimental Forest, New Hampshire, 43.9°N, 71.6°W) and one breeding population in the south (Nantahala National Forest, Macon County, North Carolina, 35.0°N, 83.4°W) revealed subtle yet significant differences in a number of acoustic parameters; for example songs of southern birds had greater frequency band width and higher entropy, and also tended to have shorter trills for some song types, than did songs of northern birds (Colbeck et al. 2010). Although these differences are not easily discerned by the human ear, playback experiments indicate that males in some populations do discriminate by responding more strongly to local song playbacks than to nonlocal song playbacks (Colbeck et al. 2010). However, this discrimination may be variable across populations: males in the northern study population showed differential responses to regional song playbacks, but males in the southern study population did not (Colbeck et al. 2010).
Most singing occurs during the breeding period, from arrival in early May through July or early August. In some years, individuals sing well into August and early September, later than most other species in these forests (RTH). Rarely sing on winter grounds, but have been heard in Jamaica to sing occasionally shortly after arrival and just prior to northward departure (RTH); will sometimes sing in winter in response to audio playback of conspecific song (RTH). Also sing occasionally during spring migration, before arrival on breeding grounds.
Places of Vocalizing
Most songs are delivered by males when perched in mid to lower levels in the forest canopy. In New Hampshire, song perches average 8.1 m ± 3.8 SD (n = 47) in height, similar to that of the sympatric Least Flycatcher (Empidonax minimus) but lower than those of most other co-occurring passerines (Holmes 1986). Males occasionally sing when flying to attack an intruding male or when courting a female (see below).
Repertoire and Delivery of Songs
Individual males have several songs in its repertoire (see above), with an apparent repertoire size that varies from two to seven songs and an average of 3.5 songs per male (Hof 2014). In general, each male sings at least one type 1 and one type 2 song, but repertoires tend to be larger for type 2 songs (Hof 2014).
Proportion of time spent singing varies with stage of the nesting cycle, with males singing most when their social mates are incubating, presumably to attract additional social mates or extra-pair mates. Song rate does not appear to vary with population density (Sillett et al. 2004).
Social Context and Presumed Function of Vocalizations
Type 1 songs are often (but not exclusively) given from well within the territory and during the least confrontational contexts when other males are not present. These songs also predominate during the early breeding season before nest initiation, and are often given in the vicinity of the nest (Hof 2014). Thus, type 1 songs may function to broadcast territory ownership and/or attract a mate. In contrast, song-types 2 and 3 are often given by counter-singing males near their territorial boundaries, and appear to function in inter-male communication. Males signal intent to defend their territory or escalate an aggressive encounter by switching from type 1 to type 2 songs during counter-singing (Hof and Podos 2013). When a male encounters an intruder on his territory, and also sometimes when encountering a neighbor at a shared boundary, it will often give aggressive trills and/or sing soft songs (MSW). Soft song, of either type 1 or type 2 song, is a strong signal of aggressive intent, as a male that gives a soft song is highly likely to attack an intruder (Hof and Hazlett 2010, Hof and Podos 2013).
Although soft song signals aggressive motivation, it also appears to be used for communication between the male and female. Males will often give soft songs, particularly soft versions of the type 1 song (Hof 2014), when in close proximity to the female and/or the nest, often doing so when the female is in the understory and the male is on a branch above her. Such female-directed soft songs appear to be most common when the female is nest building, and likely plays a role in courtship and/or mate guarding (see Hof 2014).