The study, based on more than 1,000 hours of observation spread over 165 days between 2021 and 2025 at Tbilisi reservoir in Georgia, documented twelve cases of interspecific aggression by black-throated divers. In one of two events described in full detail, the outcome was fatal for the target. On 25 December 2024, a diver approached a cove where five black-necked grebes and three great crested grebes were resting and foraging. The diver accelerated, pushed a strong bow wave before itself, and began swimming in circles around the group. It then adopted the crouch posture and slowly advanced toward two adult great crested grebes, a territorial pair known to breed regularly in the cove. The pair responded aggressively, uttering harsh throaty calls. The diver lifted its wings, adopted an upright fencing posture with its bill turned toward its breast and neck arched upward, performed a series of bow-jumps, and gave a loud piercing call before plunging underwater to attack.

What followed was a series of rapid splash dives, underwater chases, and fights just below the surface. One adult grebe emerged mid-fight and frantically flew off, pursued briefly by the diver, which surfaced half-submerged with its bill wide open and gave a second, harsher call. After another round of fierce underwater combat, the remaining adult grebe surfaced and dragged itself into flooded willow scrub, clearly injured, with its left wing hanging limp. The diver, having cleared the area, settled down after three rounds of wing-flapping alternating with repeated bill-dipping, then returned at a leisurely pace to the centre of the reservoir. The entire event lasted approximately four minutes. The following day, the observer found a dead adult great crested grebe in exposed bankside tree roots about fifty metres away, presumably the defeated bird. The carcass showed large haematomas under the upper rib cage on the left side, a deep ventral wound where the diver’s bill had slipped along the sternum and incised the skin for roughly two centimetres, and a toe lobe on the right leg nearly detached at its base.

Black-throated divers are medium-sized piscivorous waterbirds of the order Gaviiformes, breeding on freshwater lakes across northern Eurasia from the British Isles to the Bering Sea. They are famous for their loud, haunting, far-carrying yodelling calls, which have made them icons of northern wilderness. On their breeding grounds, divers are highly territorial and aggressively defend nesting sites and chicks against conspecifics and other waterbirds alike. Territorial fights between males can become violent and occasionally fatal. Spectacular cases involving larger diver species have been documented, including a reported incident from Maine in which a bald eagle was found dead near a nesting pair of great northern divers, with radiographic examination revealing a puncture wound the size of a diver’s bill extending straight into the heart. The black-throated diver, the Palaearctic counterpart of the North Pacific diver, shares many ecological and behavioural similarities with its larger congeners, including the propensity for violent territorial defence.

What makes the Tbilisi observations remarkable is that they occurred entirely outside the breeding season, on the overwintering grounds, far from any nesting territory. Established hypotheses for interspecific aggression, such as defence of young against predators or defence of food resources, struggle to account for such behaviour. Predominantly non-piscivorous waterfowl have been attacked despite posing no threat to diver offspring. Interspecific aggression has regularly been observed in single, unmated divers, undermining sexual selection explanations. Moreover, in at least eight of twelve observations at Tbilisi reservoir where aging was possible with reasonable certainty, juveniles or immatures were involved, dispelling any direct link to mating-associated aggression. The theoretical frameworks that exist for interspecific aggression, notably Character Displacement Theory and Optimal Aggression Theory, depend on assumptions about territoriality, competition, and reproduction among closely related sympatric species, prerequisites that simply do not apply to an immature, sexually inactive diver attacking grebes on a wintering lake thousands of kilometres from its breeding range.

Despite these limitations, the study’s author notes parallels with recent work on great northern divers in Norway, where interspecific aggression during winter was concluded to be most likely related to fish capture. In that study, aggression toward non-piscivorous birds was explained by their potential to disturb the divers’ fish prey. At Tbilisi reservoir, the main target in all observed cases of interspecific aggression was the great crested grebe, in terms of body dimensions, preferred prey size, and abundance surely the strongest potential competitor for fish present. Ducks and smaller grebe species retreated from the fighting arena as apparent collateral damage, while the only observed attack directed at a duck, a common goldeneye swimming beside several grebes, was evidently triggered by the grebes’ presence. The author therefore cautiously shares the conclusion that interspecific competition for fish and concentrations of prey might play a role in triggering this behaviour.

Ten of the twelve observed attacks took place in late autumn and the first half of the winter half-year, and all occurred close to shore in relatively shallow areas of the reservoir. This spatial and seasonal pattern suggests that the critical factor behind clashes may be found underwater. The seasonal behaviour and spatial distribution of preferred fish prey, such as swarming and foraging gatherings in shallower areas, might determine the spatial use of the reservoir by piscivorous diving birds and thus influence the likelihood of encounters between competitors. Fish species present in the reservoir include several prey items taken by piscivorous birds, and the reservoir itself, built in 1953 as an artificial lake covering approximately 11.5 square kilometres, offers excellent opportunities to study wintering divers along its largely treeless, accessible rocky banks.

The aggressive displays observed match the territorial defence repertoire typical of the black-throated diver and the four larger Gavia species: fencing, crouching, bill-dipping, rushing, bow-jumping, and wing-flapping. The calls uttered during attack were a characteristic shortened form of the territorial yodel, retaining the general character and timbre of the typical breeding-season yodel but condensed into a brief, sharp utterance. All diver species tend to be silent outside the breeding season, and the observer reports never having heard calls of swimming black-throated divers in decades of field experience along European coasts and inland lakes, suggesting the attacking birds were in a heightened state of tension and aggression. A previously unpublished behaviour possibly linked to interspecific aggression was also documented: the distinctly powerful swimming style with a strong bow wave pushed ahead of the approaching diver, bearing striking similarity to the busking display of aggressive mute swans, though its status as a consistent behavioural pattern rather than an incidental event remains open.

The two adult great crested grebes attacked in the fatal encounter were in all probability a territorial pair that had bred successfully in the cove and was still feeding juveniles as late as November. Preceding breeding activity and an associated hormonal state might explain the pair’s readiness to defend their territory against the attacking winter guest rather than retreating as grebes did in other cases. After the pair’s separation due to the fatal attack, no pairs of great crested grebes were observed in the cove, and only in February 2025 did a new, possibly newly formed pair begin territorial and courtship display. Beyond the ethological puzzle, the study contributes to very limited knowledge of the species’ occurrence in inland Georgia, showing that the black-throated diver is a regular passage migrant and winter guest at Tbilisi reservoir and emphasising the reservoir’s importance as a migration stop-over for the species.

For now, the reasons for the divers’ risky attacks in their far-away overwintering areas remain mysterious. Given the anecdotal nature of current knowledge of this puzzling issue, the author refrains from providing any explanation for interspecific aggression per se, limiting the contribution to adding further pieces to the mosaic and drawing attention to a possibly common pattern of behaviour. One earlier assessment considered comparable interspecific aggression of great northern divers during the breeding season as examples of aberrant or nonadaptive behaviour, a dictum that lends itself well to the corresponding, even more puzzling behaviour shown by wintering divers in Georgia and elsewhere. What is clear from the Tbilisi observations is that the aggression is not random: it is directed, ritualised, executed with the full territorial display repertoire of the species, and accompanied by vocalisations normally heard only on northern breeding lakes, all deployed against a slightly smaller but formidably well-adapted aquatic opponent in a contest whose ultimate biological purpose remains, for the moment, unresolved.

Subject of Research: Interspecific aggression by overwintering black-throated divers toward great crested grebes at Tbilisi reservoir, Georgia

Article Title: Yodelling death – on interspecific aggression in overwintering Black-throated Divers ( Gavia arctica ) ( Aves : Gaviidae )

Article References: Schröter, A. (2025). Yodelling death – on interspecific aggression in overwintering Black-throated Divers (Gavia arctica) (Aves: Gaviidae). Caucasiana, 4, 81-94. https://doi.org/10.3897/caucasiana.4.e151878

Image Credits: AI Generated

DOI: 10.3897/caucasiana.4.e151878

Keywords: Black-throated Diver, Gavia arctica, great crested grebe, interspecific aggression, overwintering behaviour, Tbilisi reservoir, Georgia, yodel call, territorial display, avian ethology, Podiceps cristatus, wintering ecology