While it is common for this species of sea bird to migrate south for the winter to ice-free fishing grounds, in a mild winter, cormorants can also be seen along Estonia's coastline, ornithologist Marko Mägi noted in a report on the bird, named as Estonia's bird of the year.

The Great Cormorant (Phalacrocorax carbo sinensis), one of several species of cormorants, is the bird in question.

Examples have been ringed when young, to track their migration routes and wintering grounds, while "today, modern tracking technology is also used," Mägi added.

The cormorant population has grown significantly across Europe and in Estonia.

For instance, in 1983, approximately 91,000 of the birds were recorded to have wintered in Europe; by 2013, that figure had risen to 690,000.

Today, the population is even larger than it was a decade ago, but shows signs of stabilizing.

As numbers have risen, conflicts with fishermen and fish farmers have emerged too. Cormorants not only deplete fish stocks but may also over-winter in fish farm areas, intensifying the need for solutions.

Tracking cormorant movements

The origin of cormorants wintering in Central Europe was for many years unclear, which served to raise doubts about the effectiveness of checking wintering populations.

The hope was that managing wintering birds could help regulate breeding colonies, but these colonies can be located far and wide, such as in Estonia's coastal waters.

To clarify migration patterns, researchers analyzed 30 years' worth of data from 220,000 cormorants banded as chicks in European breeding areas.

Among them were 21,500 birds ringed in the Baltic States, including in Estonia, and also in northwestern Russia.

From the wintering grounds, researchers recovered 4,500 ringed birds—2,000 of which had been hunted and 2,500 found dead.

This dataset allowed scientists to study the birds' movements, population changes, and the factors influencing them, such as migration behavior or breeding success.

Banding data showed that 41 percent of juvenile cormorants survive their first year. Survival rates double thereafter—if a cormorant survives that hazardous first year, it has an 80 percent chance of surviving the next year.

Over time, the likelihood of recovering ringed cormorants decreased across all populations, however. This can be explained by a general increase in population size, which naturally reduces the proportion of banded birds among the total populace.

In total, 741 recoveries were recorded along the coasts of Baltic Sea countries.

Western cormorants shift their wintering grounds

The study also revealed that the distribution of wintering birds has moved over time.

Birds originating from Denmark and the Netherlands now winter more frequently in western and northern areas along the Baltic Sea, and also on the French coast.

At the same time, they have abandoned traditional wintering areas in Italy and the western Balkans.

Changes in wintering distributions have contributed to a later increase in cormorant numbers in the eastern Baltic, such as Finland and the Baltic states, compared with earlier increases in western Europe.

This has altered the proportion of Baltic cormorants in different wintering regions.

Managing populations in one place won't resolve the problem

Now that researchers understand where different populations winter, could this knowledge help in controlling cormorant numbers?

While some bird populations are more likely to over-winter in specific areas, ring data show that cormorants from a single breeding ground tend to disperse across multiple wintering locations.

As a result, monitoring a single breeding population will not resolve conflicts between fishermen and cormorants.

For example, Baltic cormorants do not winter in one location but are spread across Europe and even North Africa.

Similarly, monitoring a particular breeding population will not necessarily reduce conflicts in wintering areas, as birds from different breeding grounds tend to merge during migration.

This means that culling birds in one region will not have a significant impact as the losses would be distributed across multiple populations.

Survivors can often compensate for small declines in numbers too. A case study in France confirms this: Despite culling from 6 to 43 percent of the cormorant population between 1996 and 2013, wintering numbers did not decline in subsequent years in that area.

Future challenges: Climate change and conservation strategies

According to the cormorant study's authors, the composition of wintering cormorant populations is projected to change further in the coming decades.

This implies that focusing on specific breeding populations is not the best approach to reducing conflicts and economic damage.

Thus far, the most common method of deterrence has been culling.

However, alternative strategies—such as restricting access to fishponds and implementing conservation measures at local and international levels—may be more effective in protecting fish stocks.

It is crucial to adapt management strategies based on scientifically gathered data.

In Estonia and in other northern countries, the most effective natural check on cormorant numbers has traditionally been harsh winters.

However, with a warming climate, severe winters which would naturally limit populations have become rarer.

As both cormorant numbers and negative attitudes toward them rise, the challenge remains.

"It is easy to vilify the black bird—anyone can do that—but understanding requires looking beyond one's immediate perspective," Mägi noted.

The original study on which this article was based was published in the Journal of Applied Ecology.