IS THE CONTINENTAL GREAT CORMORANT – PHALACROCORAX CARBO SINENSIS – INDIGENOUS TO EUROPE? (PART 2 OF 3)

Subfossil cormorant remains indicate that prehistoric populations in Europe and Asia were separated by a massive natural dispersal barrier in the Central Asian highlands, dividing the birds into two distinct populations. All evidence suggests that the westward spread of the continental great cormorant from Asia to Europe could only have occurred with human assistance, providing strong evidence that the European subspecies sinensis ssp. is not native to the Danish and European fauna.

 

BY JENS BURSELL, BIOLOGIST. PHOTOS: LARS LAURSEN & ADOBE STOCK

 

As established in part 1, there is neither subfossil nor genetic evidence demonstrating that the sinensis subspecies of the great cormorant was originally part of the Danish fauna. And since the relatively few bone finds in Denmark cannot reliably distinguish between the carbo and sinensis subspecies, the same naturally applies to the rest of Europe, where there is currently neither subfossil nor genetic evidence showing that sinensis was originally part of the native fauna.

Most previous studies of the history of European cormorants have taken a relatively narrow approach, without considering their history from a broader temporal and geographical perspective. To clarify the origin and arrival of sinensis in Europe, it therefore makes sense to look more closely at subfossil remains, zoogeographical zones and natural dispersal barriers across prehistoric Eurasia throughout the Holocene and Pleistocene. Combined with an interdisciplinary evolutionary perspective, this may provide a new, evidence-based interpretation of the dispersal history of the cormorant — Phalacrocorax carbo — and its two subspecies, the Great Cormorant (carbo ssp.) and the continental cormorant (sinensis ssp.).

 

The gradual human-assisted and “natural” westward spread of sinensis from Asia to Europe

My interpretation of what best explains the present-day global distribution of the two cormorant subspecies is that P. c. carbo is the original European subspecies, having been geographically separated from P. c. sinensis in East Asia by the vast Central Asian dispersal barrier. From there, a gradual and diffuse westward spread of sinensis, assisted by humans, may have taken place from East Asia across the Central Asian dispersal barrier and into Eastern and Southern Europe over a period perhaps spanning 500–1.000 years. This could have occurred along the extensive network of terrestrial and maritime trade routes associated with the Silk Road, potentially combined with more direct transportation or deliberate importation (Olburs 2008).

From there, it is plausible that both (a) further “natural” and (b) human-assisted dispersal of P. c. sinensis took place westward, northwestward and northeastward through Europe. In this scenario, (a) P. c. sinensis gradually outcompeted P. c. carbo, while (b) possible direct introductions from Asia and/or re-domestication of “wild-immigrating” cormorants of Asian origin may have further accelerated the northward spread of P. c. sinensis. The latter could readily have occurred after interest in cormorant fishing intensified in Venice from the beginning of the 16th century (Olburs 2008; Beike 2013, 2014), and was later reinforced by an interest in cormorant fishing in Renaissance France, the Netherlands and England (Olburs 2008; Beike 2013, 2014).

This new theory of a gradual introduction of Asian sinensis into Europe across a broad geographical and temporal front — combined with the subsequent “natural spread” of its Asian genes throughout Europe — does not conflict with the factual information and specific findings of previous studies. Rather, it helps support and explain a number of the questions left unanswered by earlier research and reviews of the literature.

Here is the background to my interpretation:

7,500 km between European and Asian great cormorants in the Pleistocene

The period preceding 11,700 years ago is known as the Pleistocene. During this period, great cormorant remains have been recorded in most European countries, including England, Ireland, Germany, the Czech Republic, Italy, Switzerland and Spain, as well as in Israel, Egypt and, furthest east, Azerbaijan. The two closest Pleistocene finds to Denmark were in Bavaria and Norfolk — both approximately 800 kilometres away (Tyrberg 1998).

Across Central and East Asia, there is only one location where great cormorant remains have been found from this period: Japan (Tyrberg 1998). What is striking here is the abundance of finds in Europe, the complete absence of finds across Western and Central Asia, and then the two finds in Japan.

Already at this point, it seems reasonable to consider the possibility that the western European/Middle Eastern subspecies may have been ancestral to the atlantic great cormorant (Phalacrocorax carbo carbo), while the eastern subspecies may have been ancestral to the continental great cormorant (Phalacrocorax carbo sinensis). The two may originally have been separated allopatrically by a natural dispersal barrier approximately 5,000–6,000 kilometres wide, stretching across the deserts and mountain ranges of Iran, the mountains of northern Pakistan, and the Hindu Kush, Karakoram, Pamir, Alai and Tien Shan ranges, with the Himalayas, the Tibetan Plateau, the Taklamakan and Gobi Deserts, and the Mongolian highlands further to the east.

In addition, large parts of the region extending from the eastern shores of the Caspian Sea to the western edge of the Central Asian highlands consisted of dry steppes and deserts, most of which would have provided unsuitable cormorant habitat. This would have further impeded any east–west dispersal of the two subspecies (see map below).

The absence of Pleistocene great cormorant remains from this belt stretching across Asia from Japan to Azerbaijan cannot be attributed to a lack of relevant archaeological excavations. In fact, 37 excavations involving analyses of subfossil bird bones have been carried out in Iran, Iraq, Kazakhstan, Kyrgyzstan, Uzbekistan, Mongolia, China and the Asian part of Russia (Tyrberg 1998) — without a single great cormorant being identified.

 

European and Asian Cormorants in the Holocene — 11,700 Years Ago to the Present

The Holocene also provides numerous cormorant finds from Europe (Bregnballe and Gregersen 1995; Ericson and Carrasquilla 1997; Bullied and Grey 1911–17; Stewart 2002).

When searching for Asian subfossil cormorant remains from the Holocene, there are finds from southeastern Turkey, in the Upper Tigris region, dating to 11,700–10,500 BP (Zeder & Spitzer 2016), as well as finds from Saudi Arabia dating to approximately 5,000 BP (Monchot 2026), which could plausibly belong to the western great cormorant. Northeast of this area lie the Persian Gulf and the deserts of Iran and Afghanistan, forming a massive dispersal barrier extending northeastward toward the Central Asian plateau.

There are also great cormorant subfossils dating to 9,020–8,650 BP from Lake Baikal in Siberia (Nomokonova et al. 2015). However, because this site lies north of the species’ present-day northernmost distribution — probably as a result of the Holocene Thermal Maximum, which occurred approximately 10,300–7,900 BP — this find is less relevant to the question of possible human-assisted westward dispersal of the continental cormorant. It predates by a considerable margin the period when cormorants were domesticated, as discussed below.

The coldest period on the northern Central Asian plateau occurred between approximately 4.000 and 200 BP (Bliedtner 2022). This would presumably have made natural westward dispersal by Chinese continental great cormorants or fishing cormorants particularly difficult during the several thousand years in which cormorant domestication may have emerged and subsequently developed — coinciding almost exactly with the period when trade along the Silk Road reached its peak.

There are also subfossil great cormorant remains from Asia dating to approximately 4.000 BP that have been verified through ancient DNA (aDNA) analysis from the Tibetan Plateau, 300 kilometres west of Lhasa (Wang et al. 2025). These could plausibly represent the eastern continental cormorant population. The Lhasa River eventually flows into the Brahmaputra, which joins the Ganges before emptying into the Bay of Bengal, suggesting that continental cormorants may also have been present in northern India at the time. In addition, cormorant remains dating to 7,000–4,500 BP have been found in the lowlands of eastern China (Liang et al. 2023).

Taken together, this provides further support for the possibility that, well into the Holocene, there were still two geographically distinct cormorant populations or subspecies separated by the Central Asian highlands. By this point, the dispersal barrier between the northeastern and western cormorant populations was approximately 4,500 kilometres wide. The narrowest point in this barrier may have been the approximately five-kilometre-high Karakoram Pass near the headwaters of the Ganges, adjacent to the Karakoram range. This would have had to be crossed in order to reach the wetlands of Kyrgyzstan.

A massive natural dispersal barrier between Europe and Asia for the great cormorant

The mountains, steppes, dry steppes and deserts of the western and Central Asian highlands together form a contiguous area of approximately 9 million square kilometres spanning several zoogeographical zones, most of which do not constitute attractive cormorant habitat. This geographical barrier is of considerable importance when considering the evolutionary history and dispersal patterns of the great cormorant.

Allopatric speciation typically occurs when a physical barrier to dispersal arises within the range of a species, separating populations and restricting gene flow. For example, there may once have been an uninterrupted distribution of the ancestors of the great cormorant across the Eurasian continent, from Europe to Japan, before the Indian subcontinent collided with Eurasia, creating the Himalayas and raising the Central Asian highlands to approximately their present elevation 13–15 million years ago during the Miocene. In this process, the two populations may have become effectively physically separated, eventually giving rise to the ancestors of the atlantic great cormorant in the west and the continental great cormorant in the east.

It is difficult to imagine either the eastern or western cormorant crossing this region without human assistance. One hypothetical explanation for natural dispersal would be that the continental cormorant subsequently spread westward around the northern edge of the Central Asian mountain ranges, through southern Siberia, during the Holocene Thermal Maximum (10,300–7,900 BP). But if this were the case, one would also expect to find subfossil remains of great cormorants throughout the 4,200-kilometre-wide and approximately 3,000-kilometre-long belt between Lake Baikal and Europe, as well as a more continuous present-day distribution across this region. Neither is the case, despite approximately 37 archaeological excavations of Pleistocene layers in the area (Tyrberg 1998), as well as 22 archaeological excavations of Holocene bird remains dating from the 5th to the 17th centuries along the Volga River (Galimova et al. 2014), which forms the boundary between Europe and Asia north of the Caspian Sea.

The fact that great continental cormorants appear unlikely to have spread naturally westward through the northern part of the East Asian section of the great Palearctic zoogeographical bird region (Holt 2023) is readily explained by the fact that the area is, on average, simply too cold. It is therefore unlikely to be a coincidence that the present-day northern pan-Eurasian distribution of the continental cormorant almost exactly follows the southern edge of this zoogeographical bird region, which is based on both distributional and phylogenetic data from a wide range of Eurasian bird species.

There is, however, a present-day occurrence of continental great cormorants in the southern part of the Palearctic bird region between Tyumen in Russia and Shalkar in Kazakhstan. This area lies so far west of the Central Asian dispersal barrier that it does not support the idea of natural eastward dispersal by continental cormorants along this route. And had they arrived during the warmer period between 10.300 and 7.900 BP, one would also expect to find cormorant remains in the numerous excavations mentioned above. This part of the current breeding range may more plausibly have resulted from a westward expansion of the species’ range mediated by human activity.

The interactive map of ongoing cormorant observations across the region, including migration periods, provided by Bird Count India, offers a good visual representation of the absence of continental great cormorants across the Central Asian dispersal barrier. It also illustrates very clearly that the northern Central Asian breeding populations of continental great cormorants likewise fly around the barrier on their way to their southern wintering grounds.

Data from ringed European atlantic great cormorants, as presented on Migration Atlas, likewise provide little support for the idea that the two original Eurasian cormorant populations could have mixed simply through accidental dispersal during maigrations. Furthermore, the prevailing winds across the Central Asian plateau blow from west to east, which further reduces the likelihood that an east-to-west crossing of the Central Asian dispersal barrier could have occurred naturally — for example, through a vagrant individual from the eastern population.

It therefore appears unlikely that the two great cormorant populations, which were separated at the time, mixed naturally during either the Pleistocene or the Holocene. This leaves human-mediated dispersal as the most plausible explanation for the present-day distribution pattern. See also the excellent overview of great cormorant distribution on eBird.

 

 

Did the two great cormorant subspecies spread through trade from east to west — or vice versa?

The present-day distribution of the continental great cormorant, as shown by BirdLife’s data zone, extends more or less continuously from Ireland in the west to Japan in the east, although there are considerably larger gaps in its distribution, particularly across West and Central Asia.

But we have just seen that the two subspecies were separated by a natural dispersal barrier more than 4,000 kilometres wide as recently as 4,000 years ago. The question, therefore, is whether the eastern population spread westward — or vice versa.

Evidence for cormorant fishing with domesticated continental cormorants dating back to 2,200 BP (He and Gremillet 2009), combined with extensive trade in these birds between all provinces of China (Gudger 1926), makes it highly plausible that they could have spread through local trade networks all the way to the great Central Asian dispersal barrier in the west. With human assistance, they may also have crossed the barrier and reached the western Asian steppes — even without any European demand for them.

Knowledge of cormorant fishing — and thus the potential beginnings of European demand for domesticated Asian continental cormorants — had nevertheless reached Europe by the middle of the 13th century at the latest, through the Dutch explorer William van Ruysbroeck (Olbur 2008). This, combined with extensive trade with chinese continental great cormorants for fishing, driven by regional and perhaps also emerging international demand, makes it plausible that Asian continental great cormorants were gradually, and perhaps in many separate waves over several centuries, transported westward by local traders and/or larger trading caravans along the Silk Road. They may have progressed in stages until eventually reaching the oases of fx Kyrgyzstan and, later, the Caspian Sea, from where they could have spread further westward “naturally” — without further human assistance.

The opposite scenario — the eastward dispersal of atlantic great cormorants from the west — appears unlikely. For one thing, the atlantic great cormorant does not currently have a distribution extending into either western or eastern Asia. More importantly, the present-day pattern suggests that the continental cormorant has gradually been moving its range farther north, northwest and northeast. There is no indication that the atlantic great cormorant was ever a traded commodity that could have been transported eastward.

There is therefore little evidence to suggest that the presence of continental great cormorants west of the Central Asian dispersal barrier is natural. Conversely, there is a strong basis for arguing that Asian continental great cormorants used for fishing could have been introduced into Europe either through direct importation (Olbur 2008) or through the gradual, diffuse, human-assisted dispersal outlined above.

Indeed, it seems almost implausible that such an obvious shortcut to several highly prestigious status symbols — involving a species so conspicuous and exotic in its use to Europeans — would not have been imported into Europe in one way or another over such a long period, just as other Asian bird species, such as pheasants and peafowl, were transported westward.

 

Marcus Beike does not disprove Christer Olburs’ theory of the introduction of Asian continental great cormorants

Olburs’ theory (2008) proposed that European seafarers in the 15th and 16th centuries introduced Chinese fishing cormorants to the Netherlands, after which cormorant fishing — essentially the fishermen’s equivalent of “underwater falconry” — spread as a gentleman’s sport among European elites and royal courts throughout the 16th–19th centuries. Escaped birds could readily have become the source of the European population of continental cormorants.

The chronology and scenario fit well with the possibility that this may have occurred in the wake of Vasco da Gama’s discovery of the sea route to India in 1498, followed by Jorge Álvares establishing direct maritime trade routes to China in 1513. However, although the scenario is plausible, Christer Olburs’ theory never really gained widespread acceptance — probably largely because the German researcher Marcus Beike (2024) was considered to have refuted it.

Beike neither proves nor establishes that the continental great cormorant is native to Europe

Marcus Beike presented, among others, three main criticisms of Olburs’ theory:

1) There was no concrete evidence of a specific, documented shipment taking place at the appropriate time to support Olburs’ theory. This is correct, insofar as the earliest currently known written account of a specific and successful shipment of Chinese fishing cormorants from Asia to Europe dates to 1880 (Gudger 1926).

2) Beike argued that the Dutch fishing cormorants actually originated from birds captured and subsequently trained in England (Beike 2014), which could, in principle, quite plausibly have been atlantic great cormorants (Phalacrocorax carbo carbo).

3) Medieval accounts from 840 and 990 — the latter representing the earliest description of a cormorant in the European natural environment — together with references from the 12th–16th centuries, including the first European description of cormorants breeding in 1377, indicated to Beike that “continental great cormorants” were already present in Europe before the period in which Olburs proposed that they could have been introduced: “The analyses show that there is no evidence for the introduction of birds from China. Instead, a medieval breeding record as well as regular appearance of the cormorant in religious and secular literature from the middle ages show that the bird was at least temporarily well-known in deeper inland regions of the German-speaking area.” This led him to conclude that the evidence raised strong doubts about the hypothesis that Phalacrocorax carbo sinensiswas an introduced species.

This conclusion has subsequently been (incorrectly) interpreted by some as proof that the continental cormorant was originally native to Europe. The fact is, however, that Beike (2014) does not prove this. What his analysis actually establishes is simply that, so far, no documentation has been found for a specific shipment of Chinese continental cormorants to Europe. That does not establish whether the cormorants present in Europe at the time were sinensis or carbo.

Nor do the written sources referring to European cormorants in medieval Central Europe document or make it particularly likely that these birds were sinensis. There are several reasons for this.

First, sinensis was not formally defined as a subspecies by Blumenbach until 1798 — many centuries after the medieval accounts in question.

Second, reports of cormorants nesting in trees do not demonstrate that the birds involved were sinensis. Tree nesting is not a sufficiently conservative trait from an evolutionary perspective to serve as reliable evidence of subspecies identity. Both subspecies have been documented nesting on the ground, on cliffs and in trees (Brown 1935; Berrow et al. 2024, among others).

Third, the theory of gradual westward expansion through Europe, driven by competitive displacement, provides a plausible alternative explanation. What we today regard as the atlantic great cormorant’s principal breeding habitat — particularly cliffs — could simply be the result of sinensis gradually displacing carbo northwestward into the Atlantic region. In practical terms, coastal cliffs would have been among the most obvious, secure and readily available nesting habitats remaining there.

If Beike’s personal interpretations are set aside, none of the factual information presented in his articles is actually incompatible with a theory in which continental great cormorants of Asian origin gradually spread through Europe and progressively displaced Great Cormorants over a period perhaps spanning fx 500–1.500 years.

 

 

Direct and diffuse dispersal theories for the spread of continental cormorants from Asia to Europe are not mutually exclusive

The two proposed models for the human-mediated spread of Asian continental cormorants into Europe — diffuse dispersal and direct introduction — are not inherently contradictory. Nor does one exclude the other.

Even if domesticated Asian continental great cormorants were very likely transported to Europe through trade over many centuries, across a broad network of trans-Asian trading routes, this does not rule out the possibility that specific direct shipments were also commissioned or undertaken by Europeans. We simply have not yet found documentary evidence of such shipments.

The earliest recorded example illustrating European interest in this possibility is a Dutch envoy who, as early as 1655–57, attempted to purchase a fishing cormorant in China (Gudger 1926). The owner refused to sell the bird, but the episode clearly demonstrates that there was already European interest in obtaining “fishing cormorants.” It is therefore entirely possible that historical documents documenting direct shipments of fishing cormorants from China to Europe do exist, but have either been lost or have simply not yet been discovered.

As the digitisation of historical documents accelerates, combined with rapidly improving AI-assisted translation and searches of original sources from across Eurasia and East Africa — regions to which many of the maritime Silk Road routes also extended — it is quite conceivable that substantially more information will emerge in the future. Such discoveries could shed new light on the possible trade and human-mediated dispersal of fishing cormorants from east to west.

The Silk Road — diffuse human-mediated dispersal of continental great cormorants from Asia to Europe

As mentioned, cormorant fishing was widespread among poor communities throughout China, while Asian continental cormorants were traded extensively between all of China’s provinces and presumably in many other parts of Asia as well. There must therefore have been enormous numbers of domesticated continental cormorants in the Far East — perhaps as early as the 5th–10th centuries, or even considerably earlier. With such large numbers of domesticated birds, and the resulting potential for escapes, wild populations of continental great cormorants in Asia may also have carried a significant proportion of domestication-related genes associated with highly efficient, selectively bred fishing abilities.

In my view, diffuse dispersal through both time and space — whereby trade between neighbouring villages gradually transported domesticated birds, or genes carried by escaped and subsequently wild birds, farther and farther westward — is therefore highly plausible. This may have occurred in stages of just a few kilometres at a time, either overland or with the cormorants transported in small boats.

Along the way, and over the course of many years, escaped domesticated birds may have interbred with wild birds and established small local populations. Birds from these populations may subsequently have been captured and trained again, then transported farther west, and so on. Such a process could have continued continuously for many hundreds, or perhaps more than a thousand, years.

It may have occurred along countless parallel routes, connected to some of the north–south-oriented trade routes that eventually converged with the main Silk Road routes across the Central Asian highlands. From there, the birds could readily have been transported with human assistance along the many branches of the Silk Road — for example to oases and wetlands in Tajikistan, Kyrgyzstan and the border region between Xinjiang and Kazakhstan, where the birds would have had access to abundant water and fish.

This could have happened via the northern routes, as well as via the southern Silk Road through the Ganges Valley and across the Karakoram Pass to the same region.

A gradual, human-mediated dispersal over shorter distances may, in practical terms, have been easier than long, uninterrupted journeys. Obviously, the more stops there were along the way, the easier it would have been to keep the cormorants alive and in good condition by providing them with food and water.

He (2009), however, notes that the diet of young cormorants was often supplemented with tofu, a food that has been consumed in China for approximately 2,000 years. Tofu could therefore readily have been used to keep cormorants alive during longer journeys, where low weight, limited space requirements and shelf life would have been crucial: Tofu can be dried and stored for up to a year before being rehydrated in water prior to consumption. These properties could conceivably have made it easier to transport food for the cormorants over long distances through arid regions. Even at the smallest spring, stream or oasis where no fish were available, traders could have supplemented the birds’ diet — or fed them rehydrated, air-dried tofu — allowing them to survive the journey.

From the Central Asian oases and wetlands, continental cormorants could either have been transported and traded farther west, and/or established wild populations from escaped birds. These populations could gradually have expanded westward through wetland corridors, combined with shorter flights across dry areas to reach new suitable habitats farther west across the steppes of western Asia.

In this way, Asian continental cormorants may eventually have reached the Caspian Sea and potentially what was once the eastern edge of the original range of the atlantic great cormorant, represented by subfossil great cormorant remains from Azerbaijan (Tyrberg 1998).

Another obvious route westward would have involved overland transport combined with the many shorter stages of the maritime Silk Road. Through the ports of India and the Middle East, Asian continental cormorants could gradually have been transported as far as the Red Sea and Egypt. There, they would have reached what may originally have been the southeastern range of the western great cormorant — a possibility supported, as mentioned earlier, by Pleistocene finds from Egypt and Holocene finds from Saudi Arabia.

 

The continental great cormorant arrives from the east and conquers Europe

If this theory is correct, and the Asian continental great cormorant gradually conquered Europe from southeast to west and north, either as a breeding species and/or as a resident population, then the Asian continental great cormorant must have possessed a particularly significant advantage.

How else could it potentially have outcompeted the somewhat larger Great Cormorant as it expanded westward? In many cases, larger animals of the same type tend to dominate smaller ones. One possible answer may lie in the fact that, for perhaps more than 2,000 years, humans have artificially selected continental great cormorants very strongly for superior fishing efficiency.

Studies have shown that increased foraging efficiency, unsurprisingly, results in greater breeding success (Daunt et al. 2007) — and could therefore also provide a greater capacity to expand its range by subjecting the atlantic great cormorant to intense interspecific competition.

Domesticated continental great cormorants in Asia were forced and trained to catch far more fish than wild birds because they first had to catch fish for human consumption and then catch additional fish for themselves and their offspring. Based on extensive studies of Chinese literature and field observations, He and Grémillet (2009) report that a cormorant often had to catch around eight fish before it was allowed to keep one for itself — typically one of the smaller fish. From a purely fishing perspective, Chinese fishing cormorants therefore had to perform considerably more than wild great cormorants, which “only” had to catch enough fish for themselves and their offspring.

As with other animals domesticated by humans over thousands of years, the best-performing continental cormorants would naturally have been selected for breeding. Over hundreds of generations, this could have resulted in strong directional selection for increasingly efficient fishing abilities, potentially giving Asian continental great cormorants a set of characteristics that subsequently enabled them to outcompete the Great Cormorant.

The Baltic Sea — when the great cormorant disappeared and the continental great cormorant took over

The distribution pattern we see today, in which the continental cormorant has progressively expanded farther westward and northward through Europe, fits well with the theory that the Asian continental great cormorant gradually replaced the original atlantic great cormorant from the east. This is also consistent with earlier studies (Stewart 2002; Ericson & Carrasquilla 1997).

Based on extensive morphometric studies of cormorant bones from the Baltic region, the latter authors were able to establish that up until the 13th–15th centuries, and perhaps considerably later, the Baltic region was inhabited exclusively by Great Cormorants. The last documented record is depicted in a painting from 1695 (Olburs 2008).

Ericson and Carrasquilla link the disappearance of the Great Cormorant to a decline in the salinity of the Littorina Sea:

“Maybe the salinity sank below some threshold value after 1500 AD causing the P. c. carbo to cease breeding in the Baltic. Simultaneously, the environment slowly transformed into that preferred by P. c. sinensis, which seems to have colonized the region soon after.”

This interpretation, however, does not fit well with palaeosalinity models and proxy values for Baltic Sea salinity during the same period (Vestmand et al. 1999). These indicate that salinity in the Baltic Sea has remained essentially unchanged since the 6th century. And even when salinity peaked in the Baltic approximately 5.000–6.000 BP, it was apparently not too high for a continental cormorant, judging from where the subspecies normally breeds and forages today — both in the Atlantic and in the oceanic salinity found along the Chinese coast.

The potential competitive advantage of the continental great cormorant over the atlantic great cormorant is therefore unlikely to have been related to lower salinity. If it were, one would expect to see an increasing proportion of continental great cormorants in the bone assemblages throughout the Subboreal (5.000–2.500 BP) and Subatlantic (2.500 BP to present) periods, leading up to the point when the continental cormorant eventually became the dominant breeding subspecies in the Baltic region. This is not the case.

If salinity really was a decisive factor for the two cormorant subspecies, it also seems peculiar that the atlantic great cormorant apparently has no problem today overwintering and foraging in precisely the same salinity conditions that it supposedly abandoned in the 13th–15th centuries — or perhaps not until as late as the 17th–18th centuries.

This could indicate that the transition from an original atlantic great cormorant population in the Baltic region to an increasing dominance of continental great cormorants at breeding colonies from around the 17th–18th centuries was actually caused by other factors. One possibility is that this period simply happens to coincide with the point at which the continental cormorant, during its gradual conquest of Europe toward the north, reached the Baltic Sea.

The continental cormorant was first recorded in Denmark, Sweden, England, Finland, the Baltic States and Norway at various points between the 19th century and the end of the 20th century. The fact that the first specimens identified to subspecies in these regions date from this period does not necessarily tell us much about the precise year of arrival. It may instead be an artefact of the fact that the continental great cormorant was not formally defined as a subspecies until 1798, followed by increasing awareness of whether an observed great cormorant belonged to one subspecies or the other. Even with the best binoculars available today, and a trained eye, the two subspecies can be extremely difficult to distinguish in the field.

 

Allopatric subspecies differentiation in cormorants is more plausible than sympatric differentiation

Today, the two subspecies, atlantic great cormorant and continental great cormorant, occur sympatrically in Europe — that is, they occupy the same geographical areas without an impassable natural dispersal barrier separating them.

If we assume that the continental cormorant was indeed native to Europe and had been present here for, say, hundreds of thousands of years, we would also need to explain how the separation between the two subspecies could have been maintained over such a long period. There would have to be some mechanism maintaining the distinction — for example, preferences for different habitats.

If a preference for saline coastal habitats versus freshwater inland habitats were the basis for the differentiation between the Great Cormorant and continental cormorant, we would expect two things:

1. The formation of two similarly stable and distinct subspecies in China.
2. If atlantic great cormorants and continental great cormorants had genuinely coexisted naturally in Europe for, say, hundreds of thousands of years, with their status as separate subspecies being stably maintained over time through niche differentiation based on preferences for saltwater and freshwater habitats, we would expect them not to hybridize, but instead to diverge genetically.

Neither of these predictions appears to be supported.

First, there is only one clearly established subspecies in China — the continental great cormorant — and it occurs both inland in freshwater habitats and along the East Asian coast, where it forages in waters with salinities of up to 30–40‰ (Huang 2025). The japanese great cormorant — Phalacrocorax carbo hanadae — which is almost identical to the continental great cormorant, is normally grouped together with the continental cormorant as sinensis/hanadae, since, as indicated above, there is uncertainty as to whether it actually constitutes a distinct subspecies.

Second, recent genetic studies have documented introgressive hybridization between Great Cormorants and continental cormorants in Norway and Britain (Winney et al. 2000; Grøndahl & Johnsen 2024). Rather than helping to maintain the distinction between the subspecies, such gene flow would instead tend to erode that distinction over time. Grøndahl and Johnsen (2024) conclude: “Our results suggests that gene flow is more common from sinensis into carbo than vice-versa…”

The snapshot we see in 2026 — with the majority of continental great cormorants occurring inland and around freshwater and brackish-water habitats, while the majority of atlantic great cormorants are found along the Atlantic coast — could therefore simply be an artefact of the eastern continental great cormorant having expanded westward, and toward the west and northwest through Europe, with human assistance and through interspecific competition.

The continental great cormorant — non-native and invasive in Europe?

Based on the current state of knowledge about the continental great cormorant, everything, in my view, currently points towards there being considerably more scientifically based evidence that the European continental great cormorant is non-native and potentially invasive than there is evidence that it was originally native to Europe.

A definitive clarification of the continental great cormorant’s dispersal history and its status as native or non-native in Europe will require an extensive genetic study — ideally involving aDNA from subfossil great cormorant remains from across Eurasia, combined with DNA analyses of contemporary great cormorants from throughout their present range.

However, this is unlikely to be straightforward. While a deliberate introduction of Asian continental cormorants, as proposed by Olburs (2008), would presumably leave a genetic bottleneck, a more diffuse, human-mediated dispersal across a broad front in both time and space would probably produce a far more diluted genetic signal.

Until the continental great cormorant’s dispersal history has been conclusively clarified through DNA analyses, there is, from a management perspective, an argument for applying the precautionary principle. For the time being, this should entail strong regulation of continental cormorant populations in Europe, so that they:

1. are reduced to a population level that neither places the probably original atlantic great cormorant at risk of irreversible genetic contamination or dilution; nor
2. negatively impacts existing native fishspecies, particularly those that are vulnerable.

This, together with the question of whether the EU Birds Directive should be revised, and whether the continental great cormorant should, from a management perspective, be formally categorized as an invasive species, will be the focus of the third and final article in the series, coming soon to fiskogfri.dk.

GREAT CORMORANT LITTERATURE:

Beike, M: “The history of Cormorant fishing in Europe”, Die Vogelvelt 133: 1-21 (digital engelsk version), 2012.

Beike, M: “Phalacrocorax carbo sinensis in Europe – indigenous or introduced?” Ornis Fennica 91: 48-56. 2014.

Berrow et al: ”Cormorants Phalacrocorax carbo breeding in Special Protection Areas in the Shannon Estuary and adjacent waters”, Irish Birds 47: 1–8, 2025.

Bliedtner, M. et al: “Holocene Temperature Variations in Semi-Arid Central Mongolia—A Chronological and Sedimentological Perspective From a 7400-year Lake Sediment Record From the Khangai Mountains”, Sec. Quaternary Science, Geomorphology and Paleoenvironment, Vol. 10, 2022.

Boev, Z.N: “The Holocene avifauna of Bulgaria (A review of the orni-archeological studies)”, Historia naturalis bulgarica, 6, 59-81, 1996.

Boev, Z.N: “Fossil birds in the National Museum of Natural History, Sofia: composition, development and scientific value. Zool. Med. Leiden 79-3 (4), 35-44, 30-ix. 2005.

Bregnballe, T og Gregersen, J: ” Udviklingen i ynglebestanden af Skarv Phalacrocorax carbo sinensis i Danmark 1938-1994#, Dansk Om. Foren. Tidsskr. 89: 119-134, 1995.

Browne, T: “An account of birds found in Norfolk. In Sir Thomas Browne´s works, Vol. 4 (ed. Wilkin, S) 311-323, Pickering, London.

Bulleid, A., & Gray, S. G: “The Glastonbury Lake Village”. Glastonbury Antiq. Soc. 2: 632-637, 1911-17

Daunt et al: “Older and wiser: improvements in breeding success are linked to better foraging performance in European shags”, Functional Ecology, vol. 21,3. 2007.

Ericson, G, P og Carrasquilla, H: ”Subspecific identity of prehistoric Baltic cormorants Phalacrocorax carbo”, Ardea 85: 1-7. 1997

Galimova, D, N et al: “Bird Remains from the 5th – 17th Century AD Archaeological Sites in the Middle Volga Region of Russis”, International Journal of Osteoarchaeology, Int. J. Osteoarchaeol., 2024.

Grøndahl, F, A og Johnsen, A: ”Combining biometrics and genetics to distinguish two subspecies of the Great Cormorant Phalacrocorax carbo carbo and P. c. sinensis at an inland lake in southeast Norway. Ornis Norvegica 47: 25-40, 2024.

He, L og Grémillet, D: ”A review of the current knowledge and status of tame Great Cormorants Phalacrocorax carbo in China”, BirdingASIA, 12, 103-106, 2009.

Holt et al: ”An Update of Wallace’s Zoogeographic Regions of the World”, Science 339, p. 76. 2013.

Huang, S et al: “Long-term monitoring data inform conservation strategies for waterbirds in the abandoned tropical salt pans of Budai, southwestern Taiwan”, Global Ecology and Conservation, 62:e03736, 2025.

Liang, Q. et al: ”Subsistence strategies in the Late Neolithic and Bronze Age in Nenjiang River Basin: A zooarchaeological and stable isotope analysis of faunal remains at Honghe site, Northeast China. International Journal of Osteoarchaeology, 1–14. 2023.

Monchot, H:” Arabian Animal Remains from the Bronze Age to the Historical Period: a State of Research”, In Athirat: Journal of Ancient Atabia 2 (2026) 165–196, 2026

Nomokonova, T:” A 9,000 Year History of Seal Hunting on Lake Baikal, Siberia: The Zooarchaeologyof Sagan-Zaba II, 2015.

Olburs, C: “The chinese cormorant Phalacrocorax carbo sinensis Blumenbach 1798 – An attempt to understand a complex biological question”, 2008.

Stevart, J, R: “Sea-birds from coastal and non-coastal, archaeological and“natural” Pleistocene deposits or not all unexpected depositionis of human origin”, Acta zoologica cracoviensia, 45(special issue): 167-178, Kraków, 2002.

Tyrberg, T. ”Pleistocene birds of Palearctic – a catalouge”, 1998.

Wang et al, “Ancient DNA insights into Neolithic bone-tool use on the Tibetan Plateau, Journal of Archeologival Science , article 106183, Vol. 177, 2025.

Westman et al: “Salinity change in the Baltic Sea during the last 8,500 years: evidence, causes and models”, Oceanus Havsundersökningar, Göteborg, 1999.

Winney, B, J er al: “The subspecific origin of the inland breeding colonies of the cormorant Phalacrocorax carbo in Britain”, Heredity 86, 45-52, 2001.

Zeder, M & Spitzer, M: ”New insights into broad spectrum communities of the Early Holocene Near East: The birds of Hallan Çemi“,Quaternary Science Reviews 151(2):140-159, 2016.

NOTES:

– Ericson and Carrasquilla (1997) mention three “sinensis” bones from Bulgaria (3,000 BP), which were used to investigate whether the continental cormorant may have changed in size over time. However, these do not document the presence of an original continental cormorant population in Europe, because reliable identification can only be achieved through larger-scale morphometric studies, as explained in Part 1. This is probably also why no Bulgarian subfossil collections contain Phalacrocorax carbo bones identified to subspecies level (Boev 1996 and 2005).

– Beike (2014) argues that a cormorant specimen found in inland Europe (Brehm, 1824), later identified by Claud B. Ticehurst (1923), was identical to Blumenbach’s type specimen of sinensis*. In my view, the discussion of why the continental cormorant was given the subspecies name* sinensis*, meaning “the Chinese cormorant,” is based on anecdotal ambiguity from which it is difficult to draw any firm conclusions. The origin of the name* sinensis is ultimately irrelevant, since there is substantial evidence that the subspecies could have spread to Europe with human assistance in several different ways — regardless of what its Latin name happens to be.