Summary
The dawn chorus of songbirds provides an ideal opportunity to study communication networks because multiple singers are within signalling range of each other, permitting eavesdropping by both males and females. Using an Acoustic Location System, we examined the dawn chorus singing behaviour of male black-capped chickadees (Poecile atricapillus) in 15 neighbourhoods to determine whether singing behaviour is consistent with the communication network model. We calculated levels of frequency matching for 19 focal males and all of their neighbours. The observed level of frequency matching was greater than expected by chance. All males were involved in multi-way matching at dawn and often matched two or three neighbours simultaneously. The identity of individuals involved in three-way matches was related to both previous winter-flock membership and the relative dominance rank of the interacting males. We show that male black-capped chickadee dawn choruses are interactive communication networks where males are involved in high levels of matching with neighbours, and they match multiple individuals both simultaneously and sequentially. Additionally, the existence of multi-way matching and the identities of individuals involved suggest that individual males may eavesdrop at dawn. This is the first study to quantify network communication during the dawn chorus in multiple neighbourhoods.
Methodology
We studied a banded population of black-capped chickadees at Queen's Uni versity Biology Station (44°34'N, 76°19'W), near Kingston, ON, Canada from January to July, 2005-2007. Adult birds were captured in winter using treadle-traps baited with sunflower seeds and banded with a unique combination of three coloured bands and a numbered aluminum Canadian Wildlife Service band (N = 149 birds in 2005, = 236 in 2006, = 61 in 2007). We determined the dominance hierarchies in winter flocks by observing pair wise aggressive interactions at feeding stations (N = 2811 interactions in 2005, = 8423 in 2006, = 1100 in 2007). Behaviours of dominant birds included supplanting or chasing subordinates; behaviours of subordinate birds included waiting to feed and displaying submissive postures (see Ratcliffe et al., 2007 for details). We classified males into three rank categories: (1) high-ranking males were the top two males in flocks of four or five males and the top male in flocks of two or three males; (2) mid-ranking males were the middle male in flocks of three or five males; and (3) low ranking males were the bottom two males in flocks of four or five males and bottom male in flocks of two and three males (Mennill et al., 2004).
We collected focal recordings of 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., 2004b; Wilson & Mennill, 2010). We used these focal recording to confirm male identities in the Acoustic Location System recordings when identification on the basis of relative territorial position or field observations was in doubt.
From April 27 to May 15, 2005 to 2007, we recorded neighbourhoods en compassing 6-10 territorial male black-capped chickadees (average 7.2 ± 0.3 territories, iV = 15) using a 16-microphone Acoustic Location System. A neighbourhood consisted of a cluster of breeding territories with males who were familiar with each other either from sharing membership in the same winter flock or by interacting with individuals from nearby flocks. We recorded 15 different neighbourhoods of up to 160000 m2, five in each of 2005, 2006 and 2007. The Acoustic Location System consisted of 16 omni-directional microphones housed in polyvinyl chloride (PVC) 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 multi-channel data acquisition card (National Instruments DAQ-6260) and recorded as 16-channel AIFF files using Chickadee v1.9 recording software (J. Burt, Seattle, WA, USA). This setup was an extension of the 8-microphone system described by (Mennill et al., 2006). We recorded from 0425 to 1130 EST on two to four consecutive days in each neighbourhood. During all recording sessions, three or four observers noted the activities of individual birds within the recording area, including details of male singing locations and identities.