Animal signalling contests are used by males to advertise to choosy females and to repel male competitors. During countersinging interactions in songbirds, males vary the type and timing of songs with respect to their opponent's behaviour. In black-capped chickadees, Poecile atricapillus, frequency matching and song overlapping appear to be important in territory defence and mate attraction. We studied frequency matching and overlapping behaviour during 100 naturally occurring diurnal song contests among male chickadees using an Acoustic Location System (ALS). The ALS consisted of 16 microphones that recorded countersinging interactions across multiple territories simultaneously, allowing us to triangulate the position of individuals based on delays in sound arrival at each microphone. We used the ALS to record 10 neighbourhoods of chickadees whose relative dominance status had been tabulated during the preceding winter. In 80% of contests there was at least one instance of overlapping between the contestants. In 37% of contests, the contestants were frequency matched within 50 Hz. Neither overlapping nor matching occurred at levels different from those expected by chance. However, contests that contained frequency matching had significantly more instances of overlapping than nonmatched contests. There were no rank-related differences in the proportion of opponents' songs that were frequency matched or overlapped. In using an Acoustic Location System to record entire neighbourhoods of territorial songbirds, this study is the first to quantify song matching and overlapping by free-living animals in the context of natural countersinging exchanges between familiar territorial neighbours.
We studied black-capped chickadees at the Queen's University Biological Station near Kingston, Ontario, Canada (44°34′N, 76°19′W) between January and July 2005 and 2006. We captured adults in the winter using treadle traps baited with seeds. We banded each individual with an aluminium Canadian Wildlife Services band as well as a unique combination of coloured leg bands (N = 149 birds in 2005, N = 236 birds in 2006). We determined the dominance ranks of birds in winter flocks by observing pairwise interactions at feeding stations (N = 2811 interactions in 2005, N = 8423 interactions in 2006). A bird was scored as dominant if it supplanted or chased an opponent, resisted a supplanting attack by an opponent, elicited a submissive posture in an opponent, or fed while an opponent waited to approach a feeder (see Ratcliffe et al. 2007 for details). We classified ‘high-ranking males’ as the top-ranking male in flocks with two or three males, or the top two males in flocks with four or five males. We classified ‘low-ranking males’ as the bottom-ranking male in flocks with two or three males or the bottom two males in flocks with four or five males. We classified ‘mid-ranking males’ only in flocks with three or five males.
During late April and early May of each year, as birds began defending all-purpose territories, we recorded the dawn chorus of all territorial males using directional microphones (Sennheiser MKH-70) and solid-state digital recorders (Marantz PMD660 or PMD670). Fine structural characteristics of black-capped chickadee song are individually distinctive (Christie et al. 2004a) and the focal recordings were used to verify male identity in the passive ALS recordings.
Our ALS consisted of an array of 16 omnidirectional microphones connected to a central computer by 2200 m of microphone cable. The microphones were housed in rain guards made of PVC tubing mounted on top of 3 m wooden poles. Microphone poles were elevated off the ground and attached to trees with bungee cords. Input from all microphones was digitized using a multichannel data acquisition card (National Instruments DAQ-6260) and stored as 16-channel AIF files (16-bit sampling, 22 050 Hz sampling rate). This design was an extension of the eight microphone system used by Mennill et al. (2006).