Yellow-rumped warblers use two song categories: each male tends to use a unique song in each category in Southern Oregon.
Song systems of species with 2 song categories also vary. Individuals of some species tend to have a single song within each category, including Golden-cheeked Warblers (Setophaga chrysoparia; Bolsinger 2000) and Black-throated Gray Warblers (S. nigrescens; Janes and Ryker 2011), whereas others have multiple songs in each category, such as Chestnut-sided Warblers (S. pensylvanica; Byers and Kroodsma 1992).
Among species with 2 song categories, first- and second-category songs of some can be distinguished by song structure alone, including those of Black-throated Green Warblers (S. virens; Morse 1967) and Hermit Warblers (S. occidentalis; Janes and Ryker 2006). These are termed "form-encoded" songs (Byers 1995).
Song categories of other species (among them American Redstarts (S. ruticilla; Lemon and others 1975), Grace's Warblers (S. graciae; Staicer 1989), Yellow Warblers (S. petechia; Spector 1991), and Pine Warblers (S. pinus; Price and Crawford 2013)] can only be determined by the context in which they are used (early season prior to pairing or at dawn after pairing), and are termed "performance-encoded" songs. Performance-encoded first-category singing is typically delivered as a repeated series of the same song, referred to as "repeat mode," while second-category singing typically involves a medley of different songs, referred to as "mixed mode" (Price and Crawford 2013). First-category songs of a male may differ from that of neighboring individuals within a population. Among American Redstarts, males in a population tend to converge on the same first-category song (Lemon and others 1975).
The terminology used to describe the singing of warblers with 2 song categories is confused. First-category singing has been identified variously as accented-ending, repeated, Type B, and Type I, whereas the terms for second-category singing include unaccented-ending, serial, Type A, and Type II (Ficken and Ficken 1962; Morse 1967; Lemon and others 1975; Morrison and Hardy 1983). When comparing the songs of different species with different terminology, Spector (1992) recommends using the terms "first category" and "second category" singing.
Although the song systems of many woodwarblers have been well described, some have not. Relatively little is known of the singing of Yellow-rumped Warblers (S. coronata). Morse (1989) identified the songs used at dawn and dusk as different from songs used at other times, suggesting 2 categories of song.
If Yellow-rumped Warblers possess 2 song categories, we expect individuals to deliver songs early in the breeding season prior to the appearance of females that differ from songs delivered in the dawn chorus after pairing. If Yellow-rumped Warblers use 2 song categories, we ask if the songs are form-encoded or performance-encoded and whether they have multiple songs in one or both categories. If the songs are form-encoded, we expect that a single song from different individuals can be unambiguously classified as first- or second-category songs based on shared features.
The 2.0-[km.sup.2] study area was located on Mount Ashland, 13.5 km south of Ashland, Oregon, and consisted of White Fir (Abies concolor) and Shasta Fir (A. magnifica var. shastensis) forest interspersed with sub-alpine meadows. On 16 April 2014, we marked the locations (sampling points) of 14 singing male Yellow-rumped Warblers. In the ensuing 2 wk, an additional 1 to 3 males established territories in the vicinity of most sampling points, and locations of these individuals were also noted relative to each sampling point with the intent of associating songs with a specific individual on subsequent visits. All songs recorded from each of the 14 sampling points were included in the analyses. Sampling points were separated by [??] = 441 m (s = 156 m), and no male could be heard from more than 1 sampling point.
We visited each sampling point beginning before the initiation of singing in the mornings and continuing until 09:00, on a weekly schedule until 8 July. On this date, fledged young from many pairs were observed following parents and little singing was heard. Yellow-rumped Warblers rear a single brood annually in our study area.
Each day of sampling began prior to the initiation of singing in the morning. We attempted to record each singing male heard from a sampling point for a period of at least 1 min. Because the dawn chorus was brief, we were unable to record the dawn singing at more than 10 sampling points on a given day. Each sampling day began with the next sampling point in the sequence. In this way sampling began with a different sampling point on each day.
Singing ceased for about an hour following the dawn chorus. When singing resumed, we visited each of the 14 sampling points and attempted to record each singing male for a period of at least 1 min.
We use the term "song type" for songs used in different contexts: early season prior to the appearance of females, or dawn after the appearance of females. We use the term "variant" to identify a unique song that differed in structure from other songs. We did not consider songs differing only in the number of syllables in a phrase to be different variants. A phrase is composed of [greater than or equal to] 2 repeated syllables or a single unique syllable. A syllable consists of one, or more often a group of notes typically repeated in a series. A note is a continuous trace on a spectrogram.
We recorded each singing male using a Marantz PMD660 digital recorder coupled with a Sennheiser ME-62 microphone placed in a 62cm parabolic reflector, and generated spectrograms using Raven (ver. 1.2.1, Cornell University, Ithaca NY). Song rate was calculated for each bout of [greater than or equal to] 3 songs by dividing the number of songs within the interval between the initiation of the first and last songs by the duration of the interval. Differences in the singing rate of first- and second-category songs were evaluated with a T-test.
A total of 1412 songs in 283 bouts were recorded at each of 14 sampling points between 16 April and 8 July 2014. One to 4 birds were recorded in the immediate vicinity of each of the sampling points ([??] = 2.5, s = 1.1) for a total of 35 males. These are minimum numbers based on the maximum number of individuals singing simultaneously at the various sampling points. A total of 89 song variants were identified ([??] = 15.9, s = 15.0 songs recorded per variant).
Twenty-six variants were recorded early in the breeding season before the commencement of dawn singing on 19 May. Each bout at this time consisted of a single variant, and songs differed markedly among individuals (Fig. 1). At 2 sampling points, 2 adjacent males used the same variant, whereas all other variants appeared to be unique to a given male.
Forty-four variants were recorded in dawn singing on and subsequent to 19 May (Fig. 2). Females were first observed on this date. Song bouts consisted of a single repeated variant with 1 exception. On this occasion, a 2-min dawn song bout (n = 18 songs) included 2 variants. Chip-like call notes did not accompany dawn singing. Songs delivered from the same tree early in the breeding season before the appearance of the females and again in the dawn chorus after the appearance of the females differed (Fig. 3).
Dawn singing typically ceased each day as sunlight shone directly on the treetops in the immediate vicinity of the male. Singing prior to sunrise by some individuals tended to be delivered from the same tree each morning, but more often individuals delivered dawn songs from different perches separated by >50 m on different mornings. On a given morning an individual remained at a single perch for the duration of the dawn chorus, or at most, moved to a neighboring tree, usually <10 m away, near the conclusion of singing.
Fifty-one variants were recorded later in the morning following the dawn chorus; singing typically resumed approximately 45 min to 1 h following the conclusion of the dawn chorus. Post-dawn song bouts were often short (1-5 min in duration) followed by periods of silence of [greater than or equal to] 10 min.
Fifteen early-season variants at the various sampling points were also recorded in post-dawn singing. Eighteen dawn variants were also recorded in post-dawn singing. Only 2 variants were recorded both early in the breeding season before the arrival of females and in dawn singing.
Songs in different contexts were delivered at different rates (f = 8.53, df = 145, P < 0.001). Dawn songs were delivered at a faster rate ([bar.x] = 8.1, s = 2.7 songs min-1, n = 101) than early season songs ([bar.x] = 4.8, s = 1.4 songs [min.sup.-1], n = 46). Songs later in the morning, after the dawn chorus, were delivered at a rate of [bar.x] = 5.0, s = 1.5 songs [min.sup.-1], n = 116. Singing later in the morning, after the dawn chorus, included bouts of either the early-season song or the dawn song.
Since bouts consisted of a single variant (with only 1 exception) and individual birds were not marked, we could not determine with confidence whether individual males sang multiple early-season or multiple dawn variants. However, at one sampling point, there were 3 widely separated individuals that could be readily distinguished by location and distinctiveness of the songs throughout the spring. Only 3 early-season variants and 3 dawn variants were recorded in 27 bouts at this location.
Thirty-six variants were recorded only once on the study area and these may represent additional variants used by males. They may also represent songs delivered by peripheral males that rarely approached the sampling points.
These included 7 early-season variants, 13 dawn variants, and 16 variants only recorded after the dawn chorus.
Yellow-rumped Warbler songs delivered early in the breeding season prior to the arrival of the females differed from songs delivered before dawn later in the season after the arrival of the females, with rare exception. Additionally, 2 different songs were recorded from individuals in the same tree at each of several sampling points in the 2 contexts, presumably representing the same individual in each case. These observations support the prediction that Yellow-rumped Warblers have 2 categories of song and are consistent with the singing of many other Setophaga warblers (for example Lemon and others 1975; Kroodsma and others 1989; Staicer 1989; Spector 1991; Bolsinger 2000; Janes and Ryker 2006, 2011; Price and Crawford 2013).
The song system of Yellow-rumped Warblers differed from other closely related warblers, including Yellow-throated (S. dominica), Olive-capped (S. pityophila), Pine (S. pinus), and Palm Warblers (S. palmarum; Lovette and others 2010). Singing of Olive-capped Warblers has not been studied, but Yellow-throated Warblers tend to use a single song (McKay and Hall 2012). A 2nd song is heard only on rare occasions (McKay 2008). Palm Warblers use 2 categories of songs, although the context and nature of the 2 song types has not been described (Wilson 2013). The singing of Pine Warblers has been studied in greater detail (Price and Crawford 2013). Pine Warblers have 2 singing modes, a "repeat mode" where males deliver bouts consisting of 1 song and a "mixed mode" that consists of a medley of different songs. Mixed-mode singing is used at dawn, whereas repeat-mode singing is used later in the day intermixed with mixed-mode singing. Yellow-rumped Warblers also have 2 song categories, but appear to use a single song in each category.
The singing of warblers with 2 song categories may be either performance-encoded or formen-coded. Performance-encoded songs are identified by the context in which they are delivered and can differ in structure among individuals and populations, whereas a single song from a species with form-encoded songs can be classified as either first- or second-category songs based on the structure of the song (Byers 1995). The singing behavior of Yellow-rumped Warblers is performance-encoded. First-category (Type I) variants differed markedly in structure among individuals. Second-category (Type II) songs also varied among individuals. No obvious song features united the various songs of different individuals in the 2 contexts into discrete categories either with respect to frequency, duration, or syllable structure.
However, unlike other species with performance-encoded songs (for example Yellow Warbler, Grace's Warbler, Pine Warbler, and American Redstarts), song bouts delivered in the context of Type II singing by Yellow-rumped Warblers were not composed of a medley of different songs (Lemon and others 1975; Staicer 1989; Spector 1991; Price and Crawford 2013). The songs of Yellow-rumped Warblers in each bout of both song categories consisted of a single variant, with rare exception.
The existence of 2 song categories is further supported by singing rates. Second-category singing is typically delivered at a faster rate than first-category singing (Spector 1992). The singing rate in Type II song bouts of Yellow-rumped Warblers was nearly double that of Type I singing.
Among many species of wood-warblers, neighboring males tend to share songs. Neighboring American Redstarts tend to converge on a single first-category song for the breeding season (Lemon and others 1975). Neighboring Chestnut-sided Warblers tend to converge on the use of similar second-category songs (Byers 1996). We observed little tendency for male Yellow-rumped Warblers to share songs, either first or second category.
We found little evidence for multiple songs in either category, but because bouts consisted of a single variant, with rare exception, and we did not mark birds, we were unable to determine with certainty whether males sang multiple variants in either song category. Unlike several other Setophaga warblers, individual Yellow-rumped Warblers tended to use different perches on different mornings for dawn singing. Species such as Golden-cheeked, Black-throated Gray, and Hermit Warblers tend to use the same song perch throughout the breeding season (Bolsinger 2000; Janes and Ryker 2006, 2011).
We thank T Tippin for her assistance in the field. We also thank the reviewers for their helpful comments. We thank Southern Oregon University for the personal development funds to pursue this project.
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Submitted 5 January 2016, accepted 13 July 2016.
Corresponding Editor: Joan Hagar.
Stewart W Janes, Lee Ryker, Robert M Ryan
Biology Department, Southern Oregon University, Ashland, OR 97520 USA
Caption: FIGURE 1. Examples of Yellow-rumped Warbler songs delivered prior to the arrival of females (Type I) in southern Oregon, April and early May 2014. Each song represents a different individual.
Caption: FIGURE 2. Examples of Yellow-rumped Warbler songs delivered at dawn after the arrival of females (Type II) in southern Oregon, May and June 2014. Each song represents a different individual.
Caption: FIGURE 3. Examples of Type I songs recorded early in the breeding season before the appearance of females, and Type II songs recorded from the same tree before sunrise after the arrival of females in southern Oregon, 2014. The matched songs are presumed to be from the same individuals. Numbers associated with each spectrogram identify the sampling point and individual.
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|Author:||Janes, Stewart W.; Ryker, Lee; Ryan, Robert M.|
|Publication:||Northwestern Naturalist: A Journal of Vertebrate Biology|
|Date:||Mar 22, 2017|
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