Summary
The ability to discriminate among signallers and to respond to them on an individual basis provides receivers with substantial benefits. For example, discriminating among signallers allows receivers to ignore unreliable individuals or to focus their territorial defence on unfamiliar intruders. Such discrimination requires signals to be individually distinctive; that is, signals must vary more among than within individuals. Furthermore, receivers must be able to discriminate among the signals of different individuals. In this study, we used fine structural analysis to show that the simple songs of male black-capped chickadees are individually distinctive, but that substantial variation exists both within and among recordings of the same individual. This finding emphasizes the need for multiple recordings of each individual in studies of individual distinctiveness, since failing to measure variation across recordings of the same individual can make it difficult to determine whether signals vary among individuals or whether they simply vary among different recording sessions. To test whether chickadees discriminate among the signals of different individuals, we used a playback experiment in which we broadcast priming and discrimination stimuli to 45 territorial males. When individuals heard the playback of two different males, they produced more songs and remained near the loudspeaker for a longer period than when they heard two different exemplars from the same male. Chickadees can therefore discriminate among singers based exclusively on their songs, which may help to explain how chickadees eavesdrop on singing contests and subsequently select extrapair mates on the basis of song contest performance.
Methodology
General
We studied a free-living population of black-capped chickadees at the Queen’s University Biological Station (44°34′N, 76°19′W) between 10 January and 21 May 2009. During January, we captured 200 chickadees in Potter traps baited with sunflower seeds. We attached an aluminium Canadian Wildlife Service band and a unique combination of three coloured leg bands to their legs for identification. We estimated sex using the formula in Desrochers (1990), which incorporates measures of body mass, wing length and outer rectrix length (males are slightly larger than females). We confirmed the sex of birds in spring by observing reproductive behaviour. All research complied with the ASAB/ABS Guidelines for the Use of Animals in Research and was approved by the Animal Care Committee at the University of Windsor (AUPP 09-06).
Individual Distinctiveness
Male black-capped chickadees produce a simple two-note song that is referred to onomatopoetically as a fee-bee. The fee note has descending frequency modulation, whereas the bee note has a nearly constant frequency that is lower than the minimum frequency of the fee note (see Figure 1 in Mennill & Otter 2007). Although the song is simple, individual males can vary their songs by transposing the two-note phrase along a continuous frequency range of approximately 860 Hz. The song is approximately 1 s in duration and is repeated many times during the dawn chorus and throughout the day.
We recorded songs from chickadees during naturally occurring song bouts during the breeding season between 22 April and 13 May 2009 on mornings (0530–0915 hours) when wind speed did not exceed 5 km/h. When a male was heard singing, we approached him to within 5.7 ± 3.3 m (X̄±SE), identified him, and remained as still as possible. Singing was recorded with a Marantz recorder (model PMD660; sampling rate 44 100 Hz; accuracy 16 bits; format WAVE) and a shotgun microphone that was pointed directly at the singing male (Audio-Technica AT8015; frequency response 40–20 000 Hz). We ended recording when the subject stopped singing or flew away, or when we had recorded a minimum of 30 songs. We noted the time, described the recording location, and measured the approximate distance between the microphone and subject.