- University of Alberta
Many species form social groups with dominance hierarchies. Often, individuals possess a status signal that indicates dominance rank. Songbirds produce songs that are used to attract mates or repel rivals, and acoustic features within songs can also indicate an individual's quality, including dominance rank. Acoustic status signals have been reported in the songs of male black-capped chickadees, Poecile atricapillus, a nonmigratory North American songbird. Here we used two operant conditioning tasks to examine acoustic preference for and discrimination of conspecific songs produced by males varying in dominance rank. We used a choice preference task to examine birds' preferences for listening to dominant or subordinate songs and conducted an instrumental learning task to determine whether chickadees considered dominant and subordinate songs as belonging to separate signal categories based on acoustic features. Overall, our results provide little evidence that birds used open-ended categorization when discriminating, but there is evidence that songs from different geographical regions may contain acoustic similarity based on dominance rank. Consistent with previous song discrimination studies with black-capped chickadees, we found sex differences in discrimination abilities, with females learning the discrimination faster than males. We also found evidence that performance accuracy during the instrumental learning task correlates with acoustic song preference. Overall, these results suggest that when biologically relevant signals (e.g. male songs) are used as stimuli during a perceptual task, the birds' responses may be differentially affected based on individual differences among the subjects performing the task (including sex and underlying preference) and the salience associated with the stimuli (e.g. dominance rank of the singer).
Nineteen black-capped chickadees (10 males and 9 females, sex determined by DNA analysis of blood samples; Griffiths, Double, Orr, & Dawson, 1998) were tested between March and May 2012. Birds were captured in Edmonton (North Saskatchewan River Valley, 53.53°N, 113.53°W; Mill Creek Ravine, 53.52°N, 113.47°W), or Stony Plain (53.46°N, 114.01°W), Alberta, Canada between January and March 2012 and were at least 1 year of age at capture (verified by examining colour and shape of the outer tail retrices; Pyle, 1997).
When not in the experimental apparatus, birds were housed singly in cages (30 × 40 cm and 40 cm high, Rolf C. Hagen, Inc., Montreal, QB) in colony rooms within the visual and auditory range of conspecifics. Birds had ad libitum access to food (Mazuri Small Bird Maintenance Diet; Mazuri, St Louis, MO, U.S.A.), water (vitamin supplemented on alternating days; Prime vitamin supplement; Hagen, Inc.), grit and cuttlebone. Birds received nutritional supplements, including three to five sunflower seeds daily, one superworm (Zophobas morio) three times a week, and a mixture of eggs and greens (spinach or parsley) twice a week. The colony rooms were kept on a light:dark cycle that mimicked the natural light cycle for Edmonton, Alberta, Canada. Birds were naïve to all experimental procedures.
Choice preference task
During the choice preference task, birds were individually tested in a sound-attenuating chamber (117 × 120 cm and 200 cm high; Industrial Acoustics Company, Bronx, NY). The testing space measured 67 × 116 cm and 116 cm high. In the chamber, there were three perches (1.75 cm in diameter and 10 cm in length) each in front of a Fostex FE108E Σ full-range speaker (Fostex Corp., Tokyo, Japan; frequency response range 80–18 000 Hz). During testing, landing on two of the perches resulted in stimulus playback, while landing on the third perch resulted in no auditory playback (i.e. a neutral perch). The back wall and each side wall had one speaker mounted to it. Each perch was located 100 cm from the floor of the testing area. Each perch contained an infrared beam used to monitor when a bird landed on the perch. A single-board computer (Palya & Walter, 2001) and personal computer controlled which stimuli played and recorded responses. Stimuli were played from a CD through a Cambridge Azur 640A Integrated Amplifier (Cambridge Audio, London, U.K.) to the speakers within the chamber. During the testing session, birds had ad libitum access to Mazuri and water located on the floor of the testing space.