Song plasticity over time and vocal learning in clay-colored thrushes

Abstract

Songbirds have been traditionally classified into close-ended or open-ended learning species according to the length of the sensitive period during which birds are able to memorize new vocalizations. Closed-ended learners are generally not capable of changing their song after the first year of life, while open-ended learners show song plasticity as adults. A few Turdus species have been suggested to be open-ended learners, but no long-term study has been conducted to investigate their song plasticity over time. We analyzed the songs of clay-colored thrushes, T. grayi, over four successive breeding seasons to assess song plasticity in their syllable repertoires within and between breeding seasons. A total of 16,262 syllables were classified through visual inspection of spectrograms and multidimensional scaling analysis based on spectrogram correlations. On average, 563 ± 153 (SD) syllables per male per breeding season were analyzed. Male repertoire size was 9-20 syllable types. Males were capable of modifying their syllable repertoire between the initial and final periods of the breeding season. Song plasticity within breeding seasons may be associated with imitation between neighboring males, suggesting song learning in males that were ≥2 years old. This short-term plasticity is not enough, however, to explain the high proportion of change (mean = 65 % syllable types) in repertoire composition between breeding seasons in adult males. Song plasticity resulting from annual changes in repertoire composition could be explained by open-ended learning, but another mechanism, extended memory and re-expression, could also explain long-term plasticity. Experimental studies controlling the acoustic environment are needed to determine which mechanism is responsible for such a high level of song plasticity.

Are northern mockingbirds classic open‐ended song learners?

“…Despite these caveats, documentation of open‐ended song‐learning remains important, and the primary methods of my study (longitudinal sampling, adult tutoring) represent our best tools for doing so. Evidence for open‐ended learning remains mixed in several well‐studied songbird species, including nightingales ( Luscinia megarhynchos , Brehm; Todt & Geberzahn, 2003; Kipper et al., 2004; Kiefer, Sommer, Scharff, Kipper, & Mundry, 2009), domesticated canaries ( Serinus canaria , Linneaus; Nottebohm & Nottebohm, 1978; Nottebohm, Nottebohm, & Crane, 1986; Belzner, Voigt, Catchpole, & Leitner, 2009) and clay‐colour thrushes ( Turdus grayi , Bonaparte; Vargas‐Castro et al., 2015). Our best examples of open‐ended learners might be parrots, as evidenced by several undisputed studies (Hile, Plummer, & Striedter, 2000; Manabe et al., 2008; Pepperberg, 1992), plus the evidence of thousands of pet owners.…”

“…Derrickson, 1987), which can lead to misleading conclusions about which song types are added or dropped from the repertoire. It might also indicate the emergence of a “silent” song type memorized early in life, but not expressed until later in life (Geberzahn & Hultsch, 2003; Vargas‐Castro, Sánchez, & Barrantes, 2015).…”

Is black plumage an adaptation to high elevations in a cosmopolitan bird genus?

“…At these elevations low temperatures, low atmospheric pressure and oxygen levels, and high solar radiation often function as barriers, limiting the number of species that adapt to these conditions (Körner , Keller et al ). A fair amount of studies have focused on breathing rates (Ramirez et al , Storz et al 2010, Ivy and Scott ), body morphology (Price , Landmann and Winding , Bears et al 2008), cold tolerance (Swanson and Liknes , Swanson and Garland ), mass change (Bears et al 2008) or change in plumage color (Friedman and Remeŝ , Delhey , Galván et al , Medina et al ), to understand the adaptive response of birds to highland environments.…”

“…In many other species, there are striking differences in plumage coloration between sexes, in which females are usually cryptic and males have bright and sometimes extravagant plumages as a result of sexual selection (e.g. Paradisaeidae, Pipridae, Cotingidae; Andersson , Zahavi and Zahavi , Bortolotti , Dale et al ). Despite that evolutionary causes of plumage coloration have received much attention (Darwin , Wallace , Huxley , Mayr , Cowless et al ), the effect of high mountain environmental conditions on the evolution of plumage coloration has been overlooked (McNaught and Owens , Galeotti et al 2003, Badyaev and Young ).…”

“…In cold highland environments, many bird species have also black plumage contradicting the Gloger's Rule that expect more species with dark plumage in warmer regions (Serventy , Ward et al ), but agreeing with the Bogert's rule (Bogert , Clusella Trullas et al , Gaston et al 2008), which proposes that darker color improves thermoregulation efficacy in cold conditions. The energetic cost of thermoregulation increases in birds as the difference between body temperature and ambient temperature increases (Calder and King , Keller et al , Stager et al ), and its effect is expected to be more severe at high elevations. Two conditions reduce the thermoregulation cost in birds: 1) hypothermia and torpor in which birds by lowering their body temperature reduce their thermal gradient, and 2) the use of an external heat source (e.g.…”