Sounds

Development

In captive settings first-year males can learn both Type I and Type II Songs (described below) and contexts in which to use songs from recordings of songs (Spector et al. 1989). Some males show apparent changes in song repertoires within and between seasons (Cosens and Sealy 1986, Spector 1989), suggesting that song learning continues into adulthood. Hypothesis that males are influenced in learning Type I Songs by the male parent and in learning Type II Songs by neighbors after dispersal requires testing in the field (Spector 1992).

Vocal Array

Call Notes. Call notes subject of little formal study (but see Gill 1995). These include notes given by begging nestlings, by birds responding to the presence of predators, by nocturnal migrants, and by birds in diverse social encounters. Many of these notes are short with a rapid frequency sweep and can be characterized as Chip notes. Number of categories of such notes and behavioral correlates are little known. Two such Chip notes are often associated with songs: during the dawn bout of Type II Song, singing males often intersperse Chip notes with songs (see Fig. 2), and females often give simple, high frequency Chip during or at end of mate’s Type I Song. A Chip note is used by migrant male and female Yellow Warblers as part of their nonbreeding territorial behavior, with prominent dawn and dusk bouts (Neudorf and Tarof 1998). A “hiss” call has been described as being used in territorial defense (Gill 1995).

Several calls used in the context of nest defense include Seet or Zeet, Chip, “metallic chip, ” and “warble” (Hobson and Sealy 1989a, Gill 1995, Gill and Sealy 1996). Gill (1995) provided sonagrams of these calls. Of these calls used in response to intrusions near the nest, the Seet call may be somewhat specialized for use in response to threats from parasitic cowbirds. Of 22 calls given in response to cowbird intrusions, 20 were Seet call, whereas only 18 of 75 calls given in response to other intruders were Seet call (Gill 1995). Similarly, birds confronted with a mounted Brown-headed Cowbird were more likely to give Seet call than were those birds confronted with a grackle or sparrow mount; Yellow Warblers allopatric to breeding cowbirds used fewer Seet calls and did not use them differentially in response to cowbird or other mounts (Gill 1995).

Song. Songs delivered primarily by males; female song recorded rarely (Hobson and Sealy 1990). Discussion here refers to male song. Songs consist of 2–5 clusters (“phrases”) of usually repeated short units (“syllables”; see illustration in Spector 1991) for a total of about 6–10 syllables/song. Song about 1 s in length, highly variable in details. For males with >1,000 recorded songs, 10–17 song types identified (n = 8); song types as rare as 1 of 5,628 recorded songs (Spector 1992). Frequency range typically from 3–8 kHz with greatest concentra-tion of power in range 4.5–5.5 kHz (DAS). Some songs fit the mnemonic “sweet sweet sweet I’m so sweet” or “sweet sweet sweet sweeter than sweet,” but songs can be quite variable within and between populations.

Songs classified as Type I or Type II based on delivery (described below), sometimes referred to as “repeat mode” and “serial mode” songs (Weary et al. 1992). Characterization as “accented ending” (AE) or “unaccented ending” (UE) songs not generally applicable. In many populations, song endings vary among song types independently of song use; nevertheless, in some ne. U.S. populations, most Type I Songs and few Type II Songs have sharply upsweeping terminal syllables; thus the AE/UE distinction may correspond locally with Type I and Type II categorization (Morse 1966, Spector 1991).

Geographic Variation

Little information on geographic variation of Yellow Warbler songs in North America. The frequency of occurrence and details of song types can vary over a few kilometers, but some song types persist over scales of ≥100 km (DAS). The “accented ending” songs with a terminal upsweep common in e. North America are rare at Delta Marsh, Manitoba (S. E. Cosens pers. comm.). Songs of resident forms in Costa Rica and Belize recognizable as this species but subjectively different from North American populations (NKK). Some Mangrove songs are longer in duration and have lower frequency distributions (thus sounding lower pitched) than migratory Yellow Warbler songs (D. Mennill pers. comm.; DAS).

Phenology

Males may sing within 1 d of arrival on breeding grounds, but singing often erratic and reduced for several days (phenology and daily pattern discussions based on Massachusetts observations by DAS). Within a week of arrival, males have normal daytime singing behavior (see below for dependence on pairing status). Dawn bout of Type II singing develops over several weeks: for the first 2 wk the dawn bout may be omitted or extremely short (<10 min); during the incubation and nestling periods the dawn bout can reach 30–45 min; during the fledgling period the dawn bout may shorten slightly; near end of fledgling period, both the dawn bout and daytime singing rapidly decline. During Prebasic molt, song is rare; during this time male hatch-year (HY) birds give some subsong and plastic song, and some adult males may engage in plastic song (Spector 1989).

Daily Pattern

During nesting, song may start as early as 1 h before sunrise. Dawn bout consists of Type II singing interspersed with male Chip notes; Chip notes are rare with song during daylight (see figure 24.4 in Staicer et al. 1996 for graph of Yellow Warbler dawn bout). Dawn bout ends typically before sunrise. Daytime singing includes Type I and Type II Songs. Pattern, if any, after dawn not studied.

Places Of Vocalizing

Most song is delivered by males perched on shrubs and trees. No detailed data on locations of birds in vegetation during singing. Type I Songs are often given by males approaching nests to feed nestlings and sometimes given by males perched on the nest (Spector 1989). Song is occasionally given in flight; at these times the flying bird sometimes stalls slightly and song sometimes differs slightly from same song type delivered from a perch; there is no evidence, though, for a specialized flight song display (Spector 1992; but see Burtt 1986 for suggestion of such a display).

Repertoire And Delivery Of Songs

Song repertoire of male divided into 2 groups of songs based on use in singing behaviors, not on song structure: Type I and Type II. This section is based on Spector 1991 .

Each male has 1 (perhaps occasionally 2) Type I Songs used at a relatively slow rate (5.2 songs/min) in long strings of the same song type. Type I Songs are rarely used in dawn bout. Each male also has several (range 5–16 in a sample of 17 Massachusetts males) Type II Songs used at a relatively fast rate (10.6 songs/min at dawn; 6.6 songs/min daytime) with much immediate variety (95% of songs at dawn and 48% of daytime Type II Songs followed by different song type). Type II singing makes up almost all of the dawn bout. Type I and Type II Songs and singing behaviors are probably homologous to the similar categories for other species in Dendroica and related genera; thus they are “First Category” and “Second Category” songs (as used by Spector 1992).

Many song types can be used as either Type I or Type II Songs. There are no absolute structural differences between Type I and Type II Songs, but there are average structural differences. Type I Songs have higher frequency distributions, especially in the first syllable, and have a more gradual amplitude increase in the first few syllables than do Type II Songs. It is not known whether male or female Yellow Warblers can use these average structural differences to judge whether a song type of an unfamiliar bird is a Type I or a Type II Song without hearing the song type used in Type I or Type II singing behaviors.

It is difficult to produce meaningful estimates of average song repertoire size. Recording at dawn is necessary for sampling Type II repertoire, but even large samples can miss song types delivered rarely (Spector 1991). Maximum reported repertoire sizes are similar for several studies (17, 15, 14, and 17 songs, respectively, from Bankwitz and Thompson 1979, Cosens and Sealy 1986, Lemon et al. 1983, Spector 1991) suggesting that most male Yellow Warblers have song repertoires of under 20 song types.

Any song type may be sung at reduced amplitude; such muting of songs is especially likely when a singing male is close to another individual (DAS). Variant Yellow Warbler songs are produced in several ways: number of repetitions of syllable within song type often varies; incomplete songs are produced by omitting one or more syllable types from end of song; and different song types are sometimes run together to produce combination songs (see sonagrams in Bankwitz and Thompson 1979). Counting variants as separate song types produces much higher estimates of song reper-toire size (e.g., up to 43 song types/male; Cassidy 1987).

Preliminary data suggest that Golden and Mangrove warbler song systems are similar to those of migrant Yellow Warblers, although the song types are quite different (DAS).

Social Context And Presumed Functions

Type I Songs sung copiously by unmated males during daylight; Type I singing declines after pairing. This pattern is observed in normal pairing and in both experimental and natural mate loss (Spector 1991, Weary et al. 1994). Mate guarding males are normally silent, but when they lose sight of their mates they sing Type I Songs to which female may respond with call notes (Spector 1991). Type I Songs sung as male approaches nest to feed nestlings and some-times sung from nest itself (Spector 1989). Type II Songs used in dawn bout, with apparent matched countersinging of similar song types by neighboring males, in encounters between males, and in spon-taneous daytime singing, especially after pairing (Spector 1991). Use of Type I and Type II Songs also reported to vary with location in male’s territory (Morse 1966, Weary et al. 1994).

Pattern of differential song use indicates that Type I Song and singing behavior used for male-female communication, both for mate attraction and for interactions within pair, and that Type II Song and singing behavior are used for male-male communication (Ficken and Ficken 1965, Morse 1966, Spector 1991, Weary et al. 1994). These conclusions are subject to the limitation that all songs can be heard by both males and females, and each listener may extract useful information from all songs of a given male.

Both Type I and Type II Songs can be used by to discriminate between neighbors and strangers (Weary et al. 1992). Yellow Warblers respond less strongly to song degraded by transmission through 100 m of habitat than to relatively undegraded song, suggesting an ability to use song degradation to judge distance to singer (Fotheringham et al. 1997).

None known to have a communicative function.