Two cases of interspecific aggression by overwintering black-throated divers directed at great crested grebes Podiceps cristatus, are for the first time described from Georgia and the South Caucasus. In one case, the attack on a pair of great crested grebes had a fatal outcome for one grebe. The reasons for interspecific aggression outside the breeding season and the biological basis of this behaviour is discussed and observations compared with available literature. Due to the anecdotal nature of current knowledge of this puzzling issue no explanation for interspecific aggression per se is provided. The previously published hypothesis that competition for fish prey might play a role is supported. Observed interspecific aggression included adopting the aggressive display postures and performing actions used in territorial defence typical for the black-throated diver and the four larger Gavia species, such as fencing, crouching, bill-dipping, rushing, bow-jumping, and wing-flapping. Calls uttered during attack are a characteristic shortened form of the territorial yodel. A previously unpublished behaviour possibly linked to interspecific aggression of black-throated divers is described. Observation data obtained from 2021-2025 show that the black-throated diver is a regular passage migrant and winter guest at Tbilisi reservoir, emphasising the importance of the reservoir as a migration stop-over for the species in inland Georgia.
