Song learning is suggested by: existence of multiple song variations within populations; fairly common occurrence of individuals with atypical songs (ML61402711, ML79567871), and occasional incidence of certain individuals that sing songs more similar to songs of other species. However, there is no information on sensitive periods of learning. Nestlings rarely vocalize or give soft vocalizations; fledglings frequently call when accompanying adult birds.
Because of the species' secretive behavior, drab coloration, and use of dense habitat, vocalizations may be of greater social importance in Swainson's Warbler than in many other wood-warbler species. This is supported by very aggressive responses of male Swainson's Warblers to conspecific songs, coupled with their apparent disregard for model birds (TJB). Eight types of vocalizations are described: Primary Advertising Song, Incomplete Song, Whisper Song, Complex Song, Call Note, Agitated Call Note, Distress Call, and Flight Call.
Primary Advertising Song (examples: ML73908, ML62210, ML59719, ML27391721, ML55973451): somewhat variable among individuals and typically expressed as whee whee whee whip-poor-will (see Figure 3 and Audio Gallery) (149). Other phonetic renditions include tee-o tee-o (tee) whit-sut-say bee-o, or tee-o tee toot-sut-say bee-u, or whee-u whee whit-sut-say bee-o (138).
This song is very loud, with a ringing quality. Wright (160) detected individuals up to 120 m away and estimated a 65% detection probability within this radius (given the bird is singing). Under ideal atmospheric conditions, trained observers can hear songs up to 200 m away in some cases (TJB, NMA). The song varies widely in number of notes, pitch of specific notes, and speed of song delivery within populations throughout the breeding range (58, TJB, NMA; G. Graves, personal communication).
The Primary Advertising Song has 2 main parts, the first typically comprising 2 to 4 high, clear introductory notes (whee), all well separated, the first 2 or 3 often on an even pitch, and the last one a half-tone lower (149). The second part often comprises 3 to 5 rapid, slurred syllables (whip-poor-will), with will separated into 2 syllables and emphasis on whip and wi part of will.
This song, or part of the song, has been described as sounding like that of Louisiana Waterthrush, Hooded Warbler, or Yellow-throated Warbler (Setophaga dominica). The Primary Advertising Song apparently functions to establish and maintain a breeding territory and to attract one or more females to that territory (183, 1). This song may also inform a female of the male's location, as she may fly to him to feed in close proximity during the prebreeding or incubation periods (1).
Incomplete Song (examples: ML79565721, ML79561381): Individuals occasionally sing incomplete songs that consist of the first part (1 to 4 notes) of the Primary Advertising Song given at full volume; this may occur during territorial disputes or in response to song playbacks, but some individuals frequently deliver incomplete songs under normal circumstances (e.g., while foraging or perched and singing). Meanley (139) reported that males sometimes sang Incomplete Songs near territory borders after a territorial dispute, but that they would sing Primary Advertising Songs as they moved deeper into their territories. Individuals often switch between singing Primary Advertising and Incomplete Songs, although Primary Advertising Songs are generally much more common.
Whisper Song (example: ML79564541): The Whisper song is defined as “the soft inward rendering of the primary advertising song, with or without slight variations” (183). Whisper Songs generally sound exactly like Primary Advertising Songs but at a greatly reduced volume such that the detection distance is likely reduced to 10 to 15 m. Occasionally only the first 3 or 4 notes of the song are sung quietly. The Whisper Song is occasionally given while the male is foraging on the ground, but more often appears to occur in territorial disputes or in response to song playbacks. Often, Whisper Songs are interspersed with Primary Advertising Songs. Legg (in 149) reported hearing a longer than usual song given as a Whisper Song on several occasions.
Complex Song (examples: ML78559341, ML61177651): We presume what has been termed the Flight Song (1) and the soft continuous twittering described under the Whisper Song (1) are the same song, and will use the terminology of Pieplow (184): the Complex Song. Complex Songs are apparently either given either at a low volume while foraging or more loudly in flight. Complex songs are delivered quite rarely relative to Primary Advertising Songs (and their variants, Whisper and Incomplete Songs). Meanley (1) described it as musical twittering or constant chatter resembling that of American Goldfinch (Spinus tristis), kinglets (Regulus spp.), or Blue-gray Gnatcatcher (Polioptila caerulea). While this song does sound like a jumble of notes, the 2 available recordings suggest this song is stereotyped (184). On the ground it is seldom audible beyond about 10 m and can be repeated continuously for as long as 3 minutes. If given in flight, this song can be similar in volume to Primary Advertising Songs, continuous without pause and lasting as long as the flight. One bird delivered a Complex Song after a territorial dispute, flying from the ground in a spiraling flight to a height of about 11 m (185). In hundreds of hours of observation on the breeding grounds, NMA heard the Complex Song in flight only once. In that instance, NMA surprised the foraging male, which then sang an Incomplete Song, gave several Call Notes, and flew away singing the Complex Song. It is possible that instances of the complex song are more common at twilight or nocturnally, such as in the Ovenbird (186).
Call Note (examples: ML27395121, ML79558321, ML79555801). Relatively harsh, long, and loud chip (184); often sounds quite similar to the chip note that makes up the first or last note of White-eyed Vireo songs (1).
Call Notes are generally given by males during territorial disputes and by both sexes in response to playbacks of songs or calls. Males appear to respond differently to Agitated Call Notes often given by males (see below), and seemingly softer and less harsh versions delivered by females. The frequent chipping of the female during the pre-nesting period appears to attract the male and facilitate the close contact observed between males and females during this period (139). The female also occasionally gives Call Notes near the nest during the incubation or nestling periods. Call Notes may be used by both sexes when potential predators are in their vicinity or close to their nest (1).
Agitated Call Note (examples: ML55972601, ML49216921). The regular chip Call Note grades into an Agitated Call Note (cheep in 184) which differs spectrographically (the Call Note is a single slanted line, whereas the Agitated Call Note resembles an upside-down V). May recall House Sparrow (Passer domesticus). Agitated Call Notes are often given in territorial skirmishes, and in response to audio playback.
Distress Call (example: ML79569381). An extremely strident call was given by an individual being removed from a mist-net (TJB and NMA). This call is probably given rarely.
Flight Call (examples: ML76801961, ML80718711; Figure 4). Presumably given in flight by nocturnal migrants. Evans and O'Brien (187) described the flight call as “a high, clear, rising seeth with a piercing quality.” Also represented as zeep (1) or sreet (184). Given by both sexes at various times year-round, presumably to remain in contact with other birds. On the breeding grounds, these calls appear to be given most often during the pre-nesting period when males and females are in close contact. In contrast to Call Notes, these calls are generally given when individuals are in close proximity, often < 5 m. Meanley (1) noted paired females sometimes give this note when the male in an adjacent territory sings. In response to conspecific song playbacks, females appear to increase the rate at which these calls are given and males appear to often respond more strongly to these calls than to the conspecific song (TJB).
At the population level, singing is most vigorous immediately after arrival in late April and early May, decreases somewhat in late May, June and July, and becomes very sporadic in August and September. The seasonal song cycle is strongly related to the breeding cycle. Individual males appear to sing most after arriving on breeding grounds until a pair bond is formed, then decrease singing dramatically, with songs delivered rarely during the prenesting period (1). Males sing sporadically during nest-building, increase song output during incubation, and again sing sporadically during the nestling period. Although singing is most noticeable in a population early in the breeding season when breeding cycles are synchronized, nest failures cause later breeding attempts to be asynchronous and lead to variability among individuals in singing behavior. Swainson's Warbler is not reported to sing during migration and is reported singing rarely on the overwintering grounds (S. Glowinski Matamoros, personal communication).
Daily Pattern of Vocalizing
Individuals usually start singing before dawn and sing most vigorously until midmorning or early afternoon, when singing becomes more sporadic. However, individuals may sing at any hour of the day, and there is variability among individuals in daily patterns. Males usually do not sing in the late evening. In Virginia, one male sang 1,168 songs from 04:27 to 17:00 in a single day (185). Meanley (185) reported that songs are given in series, with males singing steadily for several minutes, and then pausing for a short time before beginning a new song series.
Males generally sing more consistently when perched, and more sporadically while foraging on the ground. The average singing rate for 53 males in Illinois was 5.1 songs/min (58). Based on 107 different 5-min observations, the song rate of males in Arkansas was 2.9 songs/min (TJB). Males may sing during all hours of the day and in most weather conditions, including hot summer afternoons and during rain showers (25, 1). However, the extreme variability in both daily and seasonal patterns within and among individuals as well as the large areas used by Swainson's Warblers (see Behavior: Spacing) lead to lower detection probability than for many other species.
Places of Vocalizing
In southern Illinois, 85% of vocalizing sites had giant cane in the understory and a relatively low percentage of herbaceous ground cover (164). Anich (153) found that habitat surrounding perch locations was generally similar to habitat used for other purposes (see Distribution, Migration, and Habitat: Habitat), although perch sites had less subcanopy and vine cover and greater cane-stem density than foraging locations. Song perches may be on the ground, on logs, or in trees, shrubs, or vines, often lower than 10 m, occasionally up to 15 m or higher (1; TJB, NMA). In Illinois, the mean minimum perch height of 19 males was 1.3 m, the mean maximum perch height was 5.8 m, and song perches ranged from the ground level to 10.7 m (58). Song perches in trees are usually in the subcanopy stratum, often in dead or bare branches. Males often remain stationary on singing perches in trees or shrubs until they are ready to fly to the next song perch (25), but they occasionally move to adjacent perches. Singing from the ground is generally sporadic since the male alternates between singing and foraging (1).
Repertoire and Delivery of Songs
Each male generally has only one Primary Advertising Song, although there is variation in song structure between males within and among populations (1, 58). In Arkansas, we observed one male that consistently sang 2 Primary Advertising Songs when provoked by conspecific song playbacks (TJB, NMA), and a color-banded male that sang different Primary Advertising Songs in successive years, changing from a typical to an atypical song (TJB). However, the general consistency of Primary Advertising Songs and variability within populations makes slight differences among males useful for identifying individuals. Males put considerable effort into singing, as described by Eddleman (58): “Birds held the body nearly horizontal, with the head perpendicular to the body. The wings were slightly flexed and the tail depressed slightly. Upon delivering a note, the bill was opened wide and the entire body trembled.”