Frequency matching, overlapping and movement behaviour in diurnal countersinging interactions of black-capped chickadees

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.