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 A previously unpublished behaviour possibly linked to interspecific aggression of black-throated divers is described.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.

arctic diver, arctic loon, black-throated loon, concurrence, ethology, food competition, Gavia immer, overwintering, Southern Caucasus ecoregion

The five extant species of divers (or loons as they are called in North America) of the order Gaviiformes are a well-defined group of medium-sized to large, mainly piscivorous waterbirds of freshwater lakes of the subarctic and arctic zones. As the colloquial name implies, all divers are diving birds superbly adapted to hunting for food under water and are currently placed in the genus Gavia (Winkler et al. 2020; Paruk et al. 2021). They are famous for their loud, haunting, melodic, and far-reaching, yodelling calls, for which in the public’s perception they have become icons, redolent of the solitude and grandeur of the far North and its traditions (see Kauppinen 2010; Mager and Walcott 2014).

On their breeding grounds, divers are generally known to be highly territorial and often aggressive in defence of their nesting sites and chicks. Territorial fights between males may get violent and even result in death (Carboneras 1992; Parrett et al. 2023). Additional to their readiness to fiercely defend their territory against conspecifics, during the breeding season divers can regularly be seen chasing off other co-occurring water birds, such as Anatidae and Podicipedidae, from their nesting sites (Kirkham and Johnson 1988; Bildström 2006; Own unpublished data). Due to their dagger-shaped bill such interspecific attacks can lead to fatal injuries. Spectacular cases of ferocious interspecific attacks have been observed, especially by the larger species, most notably the great northern diver Gavia immer (Brünnich, 1764). D’Auria (2020) reported the discovery of a dead bald eagle Haliaeetus leucocephalus (Linnaeus, 1766) floating on the water near a nesting pair of divers, along with a dead diver chick, from Highland Lake in Bridgton, Maine, United States of America. Radiographic examination of the eagle revealed a puncture wound the size of a diver’s bill which extended straight into the heart, and which presumably had killed the eagle instantaneously; the dead young diver, on the other hand, showed marks consistent with shape and size of bald eagle talons. All these pieces of evidence clearly pointed to a preceding dramatic fight during which the bald eagle, an otherwise unrivalled avian apex predator, was stabbed by one of the divers defending its chicks. Cases like this demonstrate dramatically the aggression potential and determined behaviour of divers; the incident attracted attention amongst biologists and even made its way into the mainstream international press (Der Spiegel 2020).

The medium sized black-throated diver Gavia arctica (Linnaeus, 1758), the Palaearctic counterpart of the East Siberian-North American Pacific diver G. pacifica, ranges over much of northern Eurasia from the British Isles in the west to the Bering Sea in the east, extending into the Nearctic realm at Seward Peninsula in the westernmost part of Alaska (Russell 2020).

Like its two larger congeners, the yellow-billed diver G. adamsii (Gray, 1859) and the great northern diver, the black-throated diver is also known for territorial fights which might become violent and result in severe injuries and even death of one of the combatants (Sjölander 1968; Uher-Koch et al. 2019; Parrett et al. 2023). Besides aggression against conspecifics, violent and occasionally fatal attacks against co-occurring water birds during the breeding season have been reported (Bildström 2006). The biological significance of such interspecific aggression in black-throated diver and its congeners remains unclear and contentious (Pynnönen 1971; Morton and Pereyra 2011). Established concepts, such as the defence of youngs against predators or defence of food resources, don’t seem to offer adequate explanations as predominantly non-piscivorous Anatidae or Anserini were attacked, which don’t pose any threat to offspring of divers. Interspecific aggression was regularly observed in single divers, making also sexual selection hypothesis unlikely in view of the absence of a mate during attacks (Kirkham and Johnson 1988).

In the following, I report two cases of interspecific aggression of wintering black-throated divers at Tbilisi reservoir, an artificial lake within the Georgian capital area. The course of events in both cases, one of which resulted in the death of a great crested grebe Podiceps cristatus (Linnaeus, 1758) following attack, are described, and my observations are discussed in the context of common hypotheses and published literature.

Compared to other interspecific interactions, such as predation and parasitism, knowledge about the evolutionary effects of interspecific competition seems to be still limited, prompting Grether et al. (2013) to call the subject “curiously understudied”. Besides the lack of a well-established theoretical framework, Grether et al. (2013) emphasised that unclear and confusing terminology remained a major shortcoming, the literature on interspecific aggression being “a morass of overlapping terms with unclear meanings”.

Despite the fact of recent progress and the extension of the theoretical framework of direct competition to interspecific aggression (see for instance, Mikami and Kawata 2004; Peiman and Robinson 2010; Grether et al. 2020), diver attacks as mentioned above cannot readily be explained by any of the existing concepts. The main reason for the general lack of applicability of according theories seems to be their narrow scope covering only very specific ecological contexts. This applies even more to violent interspecific attacks taking place outside the breeding season in the overwintering grounds. In the following I will show that any potential adaptive benefit of this behaviour remains a mystery (see Miller 2017).

The coastlines of the Black Sea are the most important wintering areas for black-throated divers from European Russia and Western Siberia (Bauer and Glutz von Blotzheim 1966a; Cramp et al. 1977a; Flint 1982; Russell 2020) and the species is known as a regular passage migrant and winter visitor along the Georgian Black Sea coast. Beyond the species’ coastal habitats, however, virtually nothing seems to be known about migration and overwintering behaviour in inland Georgia (Abuladze 2018). Therefore, with this paper, I aim not only to contribute to specific puzzling aspects of the ethology of wintering black-throated divers, but also to add to the very limited general knowledge of the species’ occurrence in inland Georgia.

Black-throated divers were studied at Tbilisi reservoir situated on the north-eastern outskirts of the Georgian capital. The reservoir lake was built in 1953 during the Soviet era in a natural upland depression mainly for irrigation and drinking water. It covers an area of approximately 11.5 km² and is fed by water extracted from Iori river, a tributary of the Kura river. The basin has an elongated shape stretching northwest to southeast, with a shoreline of about 24 km. The reservoir’s initial maximum depth of about 45 metres had decreased to about 28 metres by 1984 due to sedimentation, the storage capacity of 308 million cubic metres halved within that period (cf. World Bank 2001; Tsivtsivadze et al. 2019; Kuljanishvili et al. 2021).

Observations and photographs were obtained at the south-eastern third of the reservoir, including the small basin connected with the main part of the lake by a narrow natural channel. This particular area of about 6 km of shoreline along the southern banks is my “personal patch” (stretching from 41°42'54N, 44°53'38E to 41°43'57N, 44°51'57E) where I have counted birds on a regular basis since 2021, with more than 1000 hours of observation spread over 165 days, and with an average time expenditure of about 5-7 hours per round in 2024 (Schröter 2025). This area, like the greater part of the reservoir, offers excellent opportunities to study wintering divers. The riparian zone of this patch is mainly treeless and except for sparse scrub and a small Salix-Populus wood in a wind-sheltered bay, the rocky banks are easily accessible, allowing me to keep track of divers which often cover considerable distances under water (Lehtonen 1970).

All photographs were taken using an Olympus camera OM-D E-M1X and 300 mm F4.0 telephoto lenses (Micro Four Thirds System, 600mm 35mm equivalent) plus 1.4x teleconverter; a carbon monopod by Leofoto provided additional stability. Furthermore, Leica and Swarovski binoculars and spotting scopes were used.

Piscivorous birds can be seen preying on a wide range of fish in Tbilisi reservoir, most notably on Alburnus derjugini, Rutilus lacustris, Syngnathus abaster, Neogobius melanostomus, Abramis brama, Perca fluviatilis, Sander lucioperca, Esox lucius (Own unpublished data; Kuljanishvili et al. 2020). Other water bird species present during reported observation days relevant for this paper were the great crested grebe, the black-necked grebe P. nigricollis Brehm, CL, 1831, and the little grebe Tachybaptus ruficollis (Pallas, 1764) as well as mallard Anas platyrhynchos Linnaeus, 1758, gadwall Mareca strepera (Linnaeus, 1758), and common goldeneye Bucephala clangula (Linnaeus, 1758).

Whenever possible, black-throated divers were photographically documented to establish individual identity and numbers based on individual plumage characters. I have noticed interspecific aggression in divers annually since 2021 but only started to study and to document this behaviour more thoroughly since the winter season of 2024. Observation data can be accessed on eBird online data base (Schröter 2025).

Terminology of specific body postures linked to territoriality and aggression follows Lehtonen (1970) and Sjölander and Ågren (1972, 1976), except for the crouch posture (Rummel and Goetzinger 1975).

Between November 2021 and February 2025, I recorded a total of 141 individual sightings of black-throated divers Gavia arctica on 73 observation days. Based on photographic analysis, I estimated the actual number of individuals present on 73 consecutive days to be at least 65. During that period, I observed 12 cases of interspecific aggression by black-throated divers. In the majority of cases only partial sequences and snapshots of events could be observed and documented, often missing significant details of the start or final outcome of an encounter. In the following I describe and illustrate two selected events of interspecific aggression which I had observed from start to finish and from a close distance.

Observation 2

During a routine bird count at the reservoir on 25 December 2024, I noticed five black-necked and three great crested grebes in a cove with riparian willow scrub at the north-western border of my patch. The great crested grebes, two adults and one juvenile, were foraging, their smaller congeners mainly resting and preening about 15-20 m from the shore. Suddenly, I became aware of a black-throated diver in the middle of the lake which was slowly swimming towards the cove.

At about 20 m distance from the group of grebes, the diver accelerated and switched into a powerful swimming style, pushing a strong swelling bow wave before itself. The diver switched to alert posture with raised neck and began to swim in circles around the grebes, still keeping a distance of about 8-10 m. The behaviour of the diver gradually became more and more agitated, gazing around, switching aim from one grebe after another. Suddenly the diver adopted the crouch posture (Fig. 1; see Fig. 2) and slowly swam towards two of the great crested grebes, forcing the group of black-necked grebes and the juvenile great crested grebe to begin retreating.

The two adult great crested grebes, a territorial pair known to breed regularly in the cove, started to respond aggressively, uttering harsh throaty calls and grunts. Moments later the diver lifted its wings and adopted the upright fencing posture with its bill turned towards the breast and the neck arched upwards. A few seconds later the diver bow-jumped (i.e., the bird bowed down and briefly submerged its head only to quickly change into upright fencing posture again); both great crested grebes dived in response. After some bow-jumps, and just a fraction of a second after a very loud and piercing call, the diver plunged down for under water attack. Fig. 4 shows the calling diver at the very moment when the upright fencing posture ceased, and the underwater attack was launched.

What followed was a series of quick splash dives, under water chases, and fights just below the surface. Due to splashes and very quick movements mainly under water it was very difficult to obtain an accurate picture of the sequence of events. However, in the midst of the fight one of the two great crested grebes emerged and frantically flew off, followed by the diver emerging partially directly afterwards, stretching its neck and head out of the water, the bill wide open. Still half submerged, the diver gave a second call, harsher, shriller and shorter than the first one, only to dive down again a moment later. After another round of fierce underwater chases and fights which produced two more massive splashes, the remaining adult grebe surfaced and dragged itself into flooded riparian willow scrub, clearly injured with its left wing hanging limp, which hindered flight and wing-paddling. The splash was an estimated 90-100 cm in diameter and gives an impression of the intensity of the fight occurring below.

Having chased all the grebes away – with the final opponent hiding injured in nearby scrub, apparently unnoticed – the emerged diver settled down quickly: after three rounds of wing-flapping alternating with repeated bill-dipping (see Fig. 3), the diver changed to a normal posture again and headed back to the centre of the reservoir at a leisurely pace. The entire event, from first signs of aggression by the diver approaching the cove to settling down after the last underwater fight, lasted only about four minutes, from 13:39 and 13:43.

I was unable to locate the injured great crested grebe despite immediately examining the flooded willow scrub after the diver had left. However, the following day I found a dead adult great crested grebe in exposed roots of bankside trees about 50 m away, presumably the defeated duellist. The dead grebe showed large haematomas under the upper rib cage of the left side, as well as a deep ventral wound where the diver’s bill had slipped on the sternum, incising the skin for about two centimetres. Moreover, one of the toe lobes of the right leg was almost detached at its base, hanging on by a thin shred of tissue.

Figure 1. 

Black-throated diver Gavia arctica in aggressive crouching posture. 25 December, 2024, Tbilisi reservoir, Georgia. Photo: AS.

Figure 2. 

Black-throated diver Gavia arctica at the moment of attack. 20 November, 2024, Tbilisi reservoir, Georgia. Photo: AS.

Figure 3. 

Black-throated diver Gavia arctica in bill-dipping posture. 25 December, 2024, Tbilisi reservoir, Georgia. Photo: AS.

Figure 4. 

Black-throated diver Gavia arctica in aggressive fencing posture about to launch an underwater attack. 25 December, 2024, Tbilisi reservoir, Georgia. Photo: AS.

Why did the two black-throated divers at their overwintering habitat at Tbilisi reservoir show such aggressive behaviour? Why did they leap with such vehemence into physical conflict with grebes? This question about the adaptive benefit seems all the more puzzling when considering the presumed risk the diver took to take up fight even with a pair of great crested grebes, obviously dangerous opponents only very slightly smaller and hardly any less well adapted to the aquatic environment (Bauer and Glutz von Blotzheim 1966b).

In their review on recent efforts to incorporate interspecific aggression into existing theoretical frameworks to explain ecological and evolutionary consequences, Grether at al. (2013) analysed opportunities and limits of the two most promising approaches: Character Displacement Theory and Optimal Aggression Theory. The former is based on the evolutionary process of character displacement where closely related sympatric species occupying similar niches, develop different morphological, behavioural, or ecological traits. This process is driven by natural selection and minimises competition by favouring individuals within each species showing traits which allow them to reduce interspecific competition (see Brown and Wilson 1956). Optimality models, on which the second approach is based, could predict the exact nature or extent of a specific behaviour under defined circumstances. However, results generated by these theories, of which several have never been tested empirically (see Grether et al. 2013), depend highly on to what extent model organisms are assumed to have perfect information and control over their territories. Therefore, studies using optimality models, frequently come to opposed positions. Peiman and Robinson (2010) for instance, considered interspecific aggression as usually adaptive, whereas Ord et al. (2011) doubt its adaptive value.

However, to my current knowledge, interspecific aggression of wintering black-throated divers as described above, cannot be explained by any of these theories or hypotheses. The reasons, very simply, are prerequisites and basic assumptions of the theoretical framework. Territoriality and competition over resources, for instance, are obligatorily related to sexuality, mating and reproduction of closely related species which are sympatrically distributed. Therefore, all these theories inevitably fail to account for an immature, sexually non-active representative of a species which shows aggressive behaviour outside its breeding range and breeding saison against other species to which it is not closely related to!

The question of the biological significance of such interspecific aggression shown by overwintering divers has repeatedly been discussed in literature, most recently by Miller (2017) and Byrkjedal (2017) with reference to great northern divers. My observations provided in this paper are anecdotal, as is our entire knowledge on this phenomenon. Therefore, I refrain from any attempts at explanation and shall limit myself to add further pieces to the mosaic, to draw attention to a possibly common pattern of behaviour.

Several papers dealing with this topic show obvious and interesting parallels to what I observed. In his study in Norway, Byrkjedal (2017) concluded that “… interspecific aggression by great northern divers (G. immer) during winter observed in this study most likely is related to fish capture.” To explain aggression towards non-piscivorous birds, the author suggested they had the potential to disturb the divers’ fish prey. Against this background it seems interesting to note that the main target in all cases of interspecific aggression of black-throated divers I observed at Tbilisi reservoir so far, was the great crested grebe. In terms of body dimension, preferred prey size and abundance surely the strongest potential competitior for food present. The escape and retreat of ducks and the two smaller grebe species from the fighting arena after the divers launched their attacks gave me the impression of mere ‘collateral damage’: the target of the attacks I observed were always great crested grebes. The only case I observed of a diver attack directed against a duck involved a single common goldeneye Bucephala clangula, swimming next to several great crested grebes, hastily flying off after being briefly harassed by the diver; the diver’s trigger for aggression was evidently still the grebes.

So, with due caution given the anecdotal level of our knowledge, I generally share the conclusion of Byrkjedal (2017) that interspecific competition for food, especially fish, and concentrations of prey, might play a role in triggering this behaviour.

It should be mentioned in this context, that 10 of the 12 observations of interspecific attacks I noticed took place in late autumn and in the first half of the winter half-year, as did the two observations described above. A further aspect these observations in late autumn and early winter have in common, is their spatial distribution within my observation patch: close to the shore and in accordingly rather shallow areas of the reservoir. This suggests that the critical factor behind clashes between divers and grebes may be found underwater. The seasonal behaviour and spatial distribution of preferred fish prey, such as swarming, foraging gatherings, etc. in shallower areas within the reservoir, might determine the spatial use behaviour of piscivorous diving birds and thus influence likeliness of encounters of both competitors for fish. If so, these connections of course would not explain aggression per se but nevertheless suggest a possible reason for confrontation.

The two adult great crested grebes attacked by the diver as described in observation 2 were in all probability a territorial pair regularly breeding in the cove and well known to me. This pair had a successful and long breeding season in 2024 and was still seen feeding juveniles as late as November (see photograph in Schröter 2024). Preceding breeding activities and an according hormonal state might explain the pair’s readiness to defend their territory against the attacking winter guest instead of retreating as grebes did in other cases in which apparently overwintering guests from elsewhere were involved (see observation 1). The third great crested grebe, a first-year bird which accompanied the pair, and which fled after the diver became aggressive, might well have been one of the pair’s offspring still being fed only shortly before. What fits in with this overall picture is the fact that right after the grebe pair’s separation due to the fatal attack, no pairs of great crested grebes were observed in the cove and only in February 2025 a new, possibly newly formed pair began territorial and courtship display (Own unpublished data).

The great northern diver is especially well studied and is generally known to exhibit high levels of interspecific aggression during the breeding season (Morton and Pereyra 2011). The black-throated diver shares many ecological similarities with its larger congener, notably “…territorial behaviour, courtship, copulation, nesting behaviour, incubation, and parental behaviour are very much the same.” (Sjölander and Ågren 1972). Based on my observations I can confirm these parallels between the two species also for interspecific aggression in winter, with the sequence of postures and actions, the overall process of threat and subsequent attack showing a strong similarity with the descriptions and drawings provided by Byrkjedal (2017).

The behaviour of black-throated divers studied at Tbilisi reservoir gave the impression of a double-function of several postures with regard to the two moods, ‘agitated’ versus ‘calm’, especially bill-dipping and wing-flapping. Bill-dipping observed in the context of interspecific aggression seemed almost identical to peering about underwater when routinely hunting for food. The same applied for wing-flapping which under ordinary conditions was exhibited after preening, as well as after a series of plunge dives, with quick twisting of the neck to shake off water. Outside the context of plumage care and in the context of aggressive behaviour, flaps were often executed several times in a row as obvious display behaviour, the neck twisting seemingly less pronounced.

The distinctly powerful swimming style as described and illustrated in observation 2 represents a specific detail of obvious aggressive behaviour which to my current knowledge is not covered by literature so far. Although observed and photographed only once, which is why I cannot rule this out as an incidental event, this striking behaviour clearly gives the impression of purposeful behaviour. In fact, the distinct bow wave pushed at the front of the approaching diver, surely signalled both speed and aggression, and in my opinion showed striking similarities to similar bow waves pushed by aggressive swans, e.g., the mute swan Cygnus olor while displaying the spectacular ‘Busking’ pose (Wood et al. 2024). However, as this behaviour in observation 2 cannot readily be recognised in aggressive displays described so far for any of the five Gavia species, the question of its nature as a possibly overlooked behavioural aspect in (non-breeding) black-throated divers must remain open for the moment.

Several further ideas and hypotheses in the literature concerning interspecific attacks of wintering divers I find unconvincing. Based on the fact that all diver species spend their winter primarily in coastal waters (Carboneras 1992; Cramp et al. 1977a), Miller (2017), who reported on an aggressive great northern diver wintering on a freshwater lake in Massachusetts, assumed that “…this bird responded to a specific set of circumstances that were similar to conditions that trigger aggression on the breeding grounds. In particular, Fresh Pond, as a freshwater lake reminiscent of breeding habitat, may provide a stimulus that is lacking at saltwater wintering sites.” Prima facie, this idea appears to be somehow appealing in view of my observations from Tbilisi reservoir. However, with regard to no fewer than 26 cases of interspecific aggression of great northern divers reported from the Norwegian North Sea coast by Byrkjedal (2017) alone, this idea seems unsupported by evidence. With regard to black-throated divers at Tbilisi reservoir, another interesting fact seems to speak against a freshwater stimulus as suggested by Miller (2017). According to Cramp et al. (1977b), Gavia species reach sexual maturity within 2-3 years. In at least eight of twelve observations of interspecific aggression by black-throated divers from Tbilisi reservoir that allowed aging with reasonable certainty, juveniles or immatures were involved, which tends to dispel the idea of a direct link to aggression associated with mating and sexuality.

The two loud calls given by the diver during a sequence of fencing and bow-jumps shortly before underwater attack (see observation 2) were probably consistent with the short call with “sharply rising pitch” mentioned by Cramp et al. (1977a) and the yelp as described by Russell (2020). The second call which was uttered half submerged already in the heat of the fight, was even shorter and harsher than the first one, not unlike the high-pitched cry of a water rail (Rallus aquaticus). However, the first call especially retained the general character and timbre of the typical yodel (Carboneras 1992; Lehtonen and Lappalainen 2022). Therefore, it could as well be described as condensed short form of yodel and equivalent of the choked yodelling, a term coined by Sjölander and Ågren (1976) for yellow-billed divers calling in fencing posture.

All diver species tend to be silent outside the breeding season (Bauer and Glutz von Blotzheim 1966a; Cramp et al. 1977b; Carboneras 1992). Except for unobtrusive goose-like flight calls, in decades of field experience with migrating and wintering black-throated divers along European coasts and inland lakes, I have never heard calls of swimming black-throated divers before – mere anecdotal evidence, which nevertheless may indicate that the diver was in a heightened state of tension and aggression.

However obvious the readiness for defence by the territorial pair of great crested grebes appears to be, as described in observation 2, reasons for the divers risky attacks in their far-away overwintering area remain mysterious. Kirkham and Johnson (1988) considered interspecific aggression of great northern divers during breeding season as “… examples of aberrant or nonadaptive behaviour.” For the time being, this dictum lends well to the corresponding, even more puzzling behaviour shown by wintering divers in Georgia and elsewhere.

I am indebted to Darius Stiels, Kathrin Schidelko, and Vasil Ananian for providing literature, to Reinhard Jödicke for extensive exchange and photographic advice, to Levan Mumladze for ichthyological expert assessment, to Albert G. Orr for correcting the language and for providing valuable comments which greatly improved the manuscript.