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Article Excerpt
Although it is generally agreed that birdsong serves two basic functions, mate attraction and territory maintenance (Catchpole and Slater 1995), there are striking differences in how various songbirds acquire the songs needed for these functions. In many species, young males imitate only conspecific songs heard during a sensitive period of song acquisition (Marler 1981, Catchpole and Slater 1995). In contrast, several species mimic heterospecific songs (e.g., Northern Mockingbird, Mimus polyglottos; Howard 1974, Owen-Ashley et al. 2002). Others not only mimic, but also create new versions of song through progressive modification of previously memorized song, known as improvisation, and/or through invention of entirely new songs unlike anything heard by the young bird (Marler and Peters 1982) (e.g., Gray Catbird, Dumetella carolinensis, Kroodsma et al. 1997). There are also species that rely almost entirely on improvisation or invention to develop songs (e.g., Sedge Wren, Cistothorus platensis, Kroodsma et al. 1999a). While imitation and mimicry are widespread among all taxa with vocal learning (e.g., dolphins, Tyack 1986; hummingbirds, Baptista and Schuchmann 1990; songbirds, Nelson et al. 1995; parrots, Hile et al. 2000), improvisation or invention has been documented in only a few songbird species (e.g., Nightingale, Luscinia megarhynchos, Hultsch and Kopp 1989; Indigo Bunting, Passerina cyanea, Payne 1996; Sedge Wren, Kroodsma et al. 1999a, Hughes et al. 2002) and possibly the signature whistles of dolphins (Sayigh 1990).
It is not understood why some species improvise or invent (Kroodsma 1996), nor is it known how extensive these tendencies are among songbirds or how many times they have evolved. A better understanding of the selective forces for improvising and inventing will emerge only after additional species are studied and only after life history traits are correlated to particular styles of song development. A challenge to such studies is that distinguishing between songs generated by improvisation, invention, or inaccurate imitation is difficult and often rather subjective. To distinguish improvisation from invention, the researcher must be able to document song elements changing over time, from something closely resembling tutor song to songs that may not resemble the tutor song at all. If, however, this period of improvisation is occurring during the winter months when a bird may be only mentally rehearsing song, it would be impossible to distinguish between these two types of song learning.
It has been suspected that American Robins (Turdus migratorius) improvise or invent when acquiring song. An early study of robin song found no shared song elements between any of the wild robins studied, even among neighbors (Konishi 1965). Konishi proposed two possible reasons for this lack of shared elements: (1) young robins improvise or invent the elements of their repertoires during the song acquisition phase, or (2) robins learn through imitation, but then disperse to breeding grounds where their song elements are unique (Konishi 1965). Later studies revealed that robins shared one to five elements with neighboring robins (Dziadosz 1977, Thomas 1979, Tsipoura 1985, Sousa 1999), whereas most elements were unique (Tsipoura 1985). The fact that robins share a few elements with close neighbors but not with males from more distant locations (Dziadosz 1977, Sousa 1999) suggests that the shared elements are imitated, but that the unique elements are either improvised, invented, or learned elsewhere. Because of the difficulties in distinguishing between improvisation and invention, I refer to the song learning processes of robins in terms of imitation and invention, but with the understanding that robins may actually be improvising some song elements. Here I provide evidence that robins both imitate and invent/improvise song elements, based on research with both wild populations of robins and hand-reared nestlings.
METHODS
Description of robin song.--The song of the American Robin is composed of sequences of "song elements" that are made up of one or more "notes" shown as continuous markings on a spectrogram (Fig. 1). Male robins sing two song element types (Konishi 1965, Dziadosz 1977, Hsu 1991). The more common is the familiar whistle-like song usually described as some variation of cheerily, cheer up, cheer up, cheerily, cheer up (Sallabanks and James 1999). These elements generally sound like clear whistles, but can blend into buzzes or trills. Male robins typically have between 6 and 25 whistle elements in their repertoires (Sallabanks and James 1999; SLJ unpubl, data). The second type of element, described as the hisselly, or whisper, song (W. M. Tyler, as quoted in Bent 1949, and Young 1955, respectively), is generally sung very softly and has a much more complex structure. Robins tend to combine both whistle and hisselly elements to form groups typically consisting of 3-8 elements (Fig. 2). Although robins have a larger repertoire of hisselly than whistle elements, they typically sing whistle elements 5 to 10 times more frequently than hisselly elements (Konishi 1965; SLJ unpubl, data). Therefore, I chose to look for evidence of imitation and invention in the whistle elements of both wild and hand-reared robins.
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Recording and analyzing songs of wild robins.--I recorded the pre-dawn song of 42 male robins throughout the 2002 breeding season at three locations in Hampshire County, western Massachusetts: 16 birds at the Quabbin Cemetery (42[degrees] 16' 48" N, 72[degrees] 18' 32" W), 16 birds at Mt. Pollux Conservation Area (42[degrees] 19' 39" N, 72[degrees] 30' 06" W), and 11 birds at Wildwood Cemetery (42[degrees] 23' 23" N, 72[degrees] 30' 44" W). The three sites were between 6 and 21 km apart and consisted of open, mowed grassy areas with trees, shrubs,...
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