Abstract
The overlap of home ranges, copulation and mate guarding behaviour were studied in socially monogamous Black-faced Buntings Emberiza spodocephala from April to July in 1992at Sugadaira (36°32′N, 138°20′E, 1, 250 m alt). Each male bunting had a stable home range and a song area within that. Since males frequently intruded into neighbouring home ranges to seek extra-pair copulations, the home ranges overlapped greatly. Song areas also overlapped with each other. Once males acquired mates, they did not sing frequently. These suggest that males did not maintain the song area as a territory after pair formation. Males guarded their mates by spending much time near their mates and by following them closely. The shortness of intra-pair distance and the percentage of flights in which the female was followed by her mate peaked during the presumed fertile period. Males started mate guarding soon after pair formation. All 29intra-pair copulations were observed during the fertile period. Aggressive behaviors between males due to attempts of extra-pair copulation were observed between pair formation and the fertile period. The half of 10extra-pair copulations were observed during the fertile period. It means that males could not defend their mates completely. These results suggest that male Blackfaced Buntings guard their mates rather than exclusive areas, while they seek opportunities to attempt extra-pair copulations. Black-faced Buntings live in the brushy and visually occluded habitats. These habitats would prevent males from performing mate guarding and territorial defence simultaneously. Males responded aggressively to extra-pair male intrusions which started and continued after pair formation and took part in incubation. Male-biased sex ratio in this area also makes males to give priority to mate guarding over territorial defence to assure their paternity.
