Males of many songbird species participate in a distinct chorus beginning before sunrise. Despite its ubiquity, the function of dawn chorusing remains poorly understood. We tested the social dynamics hypothesis, which states that males sing at dawn to mediate their social relationships with neighbors through interactive communication. Using a 16-microphone acoustic location system, we recorded 29 entire dawn choruses in 10 neighborhoods of 6–10 territorial male black-capped chickadees (Poecile atricapillus) of known dominance rank. We analyzed song frequency matching and overlapping between neighboring males in 10 choruses and compared the intensity of these behaviors with social factors. Chickadees matched the frequency of their neighbor's songs more often than expected by chance. The level of matching was higher between neighbors who belonged to different flocks during the previous winter than between neighbors who had been flockmates. Males of the same dominance rank matched each other more than males of disparate ranks. There was no relationship between matching and pairing status or distance between opponents. Overlapping was used less than expected by chance. No measures of song overlapping were related to measured social factors. Our results show that neighboring male chickadees interact vocally at dawn by frequency matching. This is the first study to show that the intensity of songbird vocal interactions at dawn varies with social factors, supporting the social dynamics hypothesis.
We studied a banded population of black-capped chickadees at Queen's University Biology Station, near Kingston, Ontario, Canada (44°34′N, 76°19′W) from January to July, 2005 and 2006. Adult birds were captured in winter using treadle traps baited with sunflower seeds and banded with a unique combination of 3-colored bands and a numbered aluminum Canadian Wildlife Service band (N = 149 in 2005, N = 236 birds in 2006). We determined the dominance hierarchy in winter flocks by observing pairwise interactions at feeding stations (N = 2811 interactions in 2005, N = 8423 interactions in 2006). Behaviors of dominant birds included supplanting or chasing subordinates, whereas behaviors of subordinate birds included waiting to feed and displaying submissive postures (Ratcliffe et al. 2007). We classified males into 3 rank categories: 1) high-ranking males were the top 2 males in flocks of 4 or 5 males and top male in flocks of 2 or 3 males; 2) mid-ranking males were the middle male in flocks of 3 or 5 males; and 3) low-ranking males were the bottom 2 males in flocks of 4 or 5 males and bottom male in flocks of 2 and 3 males (Mennill et al. 2004).
We collected focal recordings of all males using directional microphones (Sennheiser MKH-70) and solid-state digital recorders (Marantz PMD660 or PMD670) between April 25 and May 24, 2005 and 2006. Chickadee songs differ between males in temporal, frequency, and relative amplitude characteristics, making male songs individually distinctive (Christie et al. 2004a). We used these focal recording to confirm male identities in the ALS recordings when identification was in doubt.
From April 27 to May 15, 2005 and 2006, we recorded neighborhoods encompassing 6–10 black-capped chickadee territories using a 16-microphone ALS. We recorded in 10 different areas of approximately 160 000 m2, 5 in each of 2005 and 2006. The ALS consisted of 16 omni-directional microphones housed in polyvinyl chloride tube rain covers and mounted on 3-m long wooden poles that were elevated and attached to small trees using bungee cords. Microphones were connected to a central computer using 2200 m of cable. Input from all microphones was digitized using a multichannel data acquisition card (National Instruments DAQ-6260) and recorded as 16-channel AIFF files using Chickadee v1.9 recording software (John Burt, Seattle, WA). This setup was an extension of the 8-microphone system described by Mennill et al. (2006). We recorded from 0425 to 1130 eastern standard time (EST) on 2 or 3 consecutive days in each ALS configuration. During recording sessions, 2 or 3 observers transcribed the activities of individual birds within the recording area, including details of male singing locations and identities.