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Cormorant Ecology - FAQ


Shortcut to sections:
General Profile & TaxonomyProtection & Population Status
AppearancePopulation Dynamics
Biology, Ecology & BehaviourDiet & Feeding
Reproduction & Cormorant Life CycleEffects On Fish Stocks
Distribution & Migration 

General Profile & Taxonomy

Q: What are the main characteristics of cormorants?
A: The complete scientific name of ‘our’ cormorant is Phalacrocorax carbo (Linnaeus 1758). It appears black at a distance, but closer inspection reveals that the individual feathers have a green-blue sheen bordered with black, which produces a scale-like effect. The cormorant has broad, webbed feet, a rather long neck and a large, strong bill with a sharp hook at its end. In summer, adults show white patches on the face and a neat circular patch on the flanks. Immature birds are more brownish-black and most have a whitish belly in particular in their first year of life.

Cormorants are generally gregarious, nesting in colonies, gathering in flocks and often also hunting together in groups, which sometimes number up to 4,000 birds. They catch fish by diving from the surface, chasing their prey under water and seizing it with the hooked bill. In Europe, it can hardly be confused with any other bird. Only the shag Phalacrocorax aristotelis looks similar, but this species is much smaller.

An adult female in full breeding plumage.

An adult female in full breeding
plumage.
An adult female in full breeding plumage.

An immature cormorant.

Q: To which family do the cormorants in Europe belong and what other species of cormorants exist?
A: ‘Our’ cormorant, Phalacrocorax carbo, belongs to a larger family, the Phalacrocoracidae, which is closely related to pelicans. Overall, there are 36 species worldwide, and all of them are fish eaters, living at sea or on large inland waters. They all nest in colonies, which in some species can be extremely large. Other cormorant species include, for examples, the Double-Crested Cormorant P. auritus (in North America), the Neotropic Cormorant P. brasilianus (in South America), the Socotra Cormorant P. nigrogularis (in the Middle East), the Javanese Cormorant P. niger (in Southern Asia) and the Cape Cormorant P. capensis (in South Africa). Members of the cormorant family can be found all over the world, on all continents, including one species in Antarctica.

Q: Is the ‘Great Cormorant’ a different species which is bigger than the cormorants common in Europe?
A: No, the cormorant in Europe Phalacrocorax carbo is commonly called the ‘Great Cormorant’. It is appropriate as it is the largest of all living cormorant species. In Europe there are two other cormorant species - the Shag Phalacrocorax aristotelis which is smaller, and the Pygmy Cormorant Phalacrocorax pygmaeus, which is much smaller.

Q: In Europe, ornithologists distinguish two cormorant sub-species in Europe: Phalacrocorax carbo carbo and P. carbo sinensis. What is the difference between them?
A: One main difference lies in their geographical distribution. The carbo sub-species lives on the Atlantic coast (the ‘Atlantic race’), whereas the sub-species sinensis (the ‘continental race’) lives on the continent from western Europe across the whole Asian continent to China and India. There is some overlap in the distribution of the two sub-species in western Europe. The differences between the birds are slight, the main one being the greater size and weight of the average carbo cormorant. Experts are able to distinguish the two sub-species based on close examination of certain minor morphological differences, but this is rarely practical in the field. One of the genetic studies of cormorants in Europe suggests that a third subspecies P. c. norvegicus is recognized.



The bird in the top belongs to the subspecies P. c. carbo and the other bird at the bottom to the subspecies P. c. sinensis.

Q: I have read an article which claims that the sinensis sub-species is not a native bird in Europe but an 'alien' that was imported from China in the 17th century. Is that true?
A: There is no evidence for this. It is true that ‘sinensis’ means ‘Chinese’, but this name was given to the sinensis sub-species only as late as 1930, when the experts decided to categorise all cormorants on the Eurasian continent, from Europe to China, as one sub-species. Before that the sinensis cormorants in Europe had been classified as separate subspecies e.g. Phalacrocorax carbo subcormoranus or Phalacroc orax carbo medius. There are a large number of pre-historic records of cormorants in the Baltic region suggesting that the species did breed here in pre-historic times. However, at present most indications are that these cormorants belonged to the subspecies ‘carbo’.



The subspecies ‘sinensis’ also breeds in China.

Q: What is the geographical range of Phalacrocorax carbo?
A: The geographical range of ‘our’ cormorant is very large, from the east coast of North America, over the whole of Europe and Asia to Australia, and from Greenland down to South Africa. The only continents where Phalacrocorax carbo is absent are South America and Antarctica.

The wide range demonstrates the exceptional flexibility and adaptability of this species. It can not only adapt to an enormous variety of habitat types, but also cope with extremely different temperature zones, from arctic Greenland to tropical Asia and Africa.

Within this range, however, ornithologists usually distinguish 6-7 subspecies of Phalacrocorax carbo.

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Appearance

Q: To what size do cormorants grow?
A: The average length of Phalacrocorax carbo from tip of the beak to the end of the tail is 80–95cm and the average weight about 2.5kg, but there is wide variation - from 1.5 kg to 3.5 kg or more. The carbo sub-species is on average 500g heavier than the sinensis sub-species. Males are usually somewhat bigger and heavier than females.

Q: How can I distinguish juvenile and adult birds?
A: Young cormorants are more brownish than black and most juveniles have a whitish breast. Over the first years of life the plumage becomes successively darker as the birds become older. Normally, a bird reaches the final adult appearance and plumage in its 3rd or 4th year, after which the plumage remains unchanged.



Juvenile (left) and adult cormorant.

Q: Do female and male cormorants look different?
A: The plumage and colouring of male and female cormorants is identical. This also applies during courtship, when both develop white patches on their flanks and white feathers on their heads. However, on average females tend to be smaller and have somewhat slimmer beaks and slimmer wings, but these differences are hard to see in the field.

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Biology, Ecology & Behaviour

Q: What is the lifespan and mortality rate of cormorants?
A: The observed maximum age is 24 years, but due to a high mortality – death – rate among juvenile birds the average life expectancy is far less. Usually at least 40% of those birds leaving the nest die in their first year of life. According to a detailed Danish study, first year mortality ranged from 25 - 68% depending on year. Mean (‘average’) mortality in the second year of life was 13%, and mean natural annual mortality among older birds was 12%. This study demonstrated that mortality among cormorants in the Danish breeding population was markedly higher in cold winters, especially after the expansion of the European population of cormorants. Other studies indicate that only 30% of the fledged young (those that leave the nest in which they were born) reach the age of three, but that survival chances becomes much better after the first and second winter, with estimates for annual mortality among adult birds varying between 10 - 20%.



Resightings of colour-ringed individuals can be used to estimate survival.

Q: How big is the largest cormorant colony in Europe?
A: Until 2006 the colony Katy Rybackie at the Baltic coast in Poland was the largest colony with up to 11,600 pairs nesting in trees. However, in 2006 an even larger colony with 14,200 nests was recorded (also in trees) at an islet near the Crimean peninsula in Ukraine.

Q: What are cormorants’ natural enemies?
A: There are no predators that regularly hunt cormorants for food. White-tailed Eagles will sometimes kill young and more rarely adult cormorants. They can lower breeding success in a cormorant colony through disturbance and predation. Regular presence of White-tailed eagles can also provoke adults to completely abandon a colony. Consequently, eagles can locally affect population development. Another impact that White-tailed Eagles have is the theft of fish from cormorants. Cormorants can also be affected by nest robbers like gulls and crows and in ground colonies by foxes. Lately, there have been records that in some German colonies non-native raccoons have severely reduced the breeding success of cormorants.



Q: What factors influence the cormorants’ choice of their feeding sites?
A: As with any predator, cormorants attempt to catch the necessary food with minimum effort (energy expenditure) at maximum perceived security. In energetic terms, flying ‘costs' are at least eight times – and diving six times – than the birds’ resting metabolism. The choice of feeding sites is therefore particularly governed by a) the distance they are from the roost or nesting area, b) fish densities, and c) the experience with certain prey and foraging sites. Other factors such as ‘how safe the birds feel’ at the site can also influence foraging site selection.

Q: How fast can cormorants fly and for how long can they fly without resting?
A: Cormorants have a flapping flight style which enables them to achieve medium speeds for birds – 54km/h has been measured over long distances. The length of individual, unbroken flights has not been studied in detail, but distances of several hundred kilometers are possible (e.g. there are indications that the birds can fly without rest from the Austrian Danube to the Italian Adriatic, a distance of about 600km in perhaps 12 hours of continuous flying). Long flights require a lot of energy which the birds must draw from their stored fat reserves.



Q: To what depth can cormorants dive?
A: There is a record from Lake Constance where a drowned cormorant had been found in a net which had been set 75 metres deep. However, such diving behaviour is exceptional since diving to this depth uses up a lot of energy. More typically, cormorants dive to depths of up to 10 metres.

Q: Cormorants can feed in flocks – do the birds communicate with each other?
A: It is debatable whether cormorants actually communicate or whether they merely imitate and copy one another. The communal feeding strategies adopted by cormorants at some sites is an adaptation to ‘herd’ shoals of fish and improve prey capture rates; this technique can have particular benefits in more turbid (‘cloudy’, unclear) waters but is used at a variety of sites.

Q: I have seen cormorants with their wings outstretched. Why do they do this?
A: A number of theories have been advanced for this common behaviour, but it has been concluded that the birds are merely drying their plumage. After diving for food, a cormorant’s feathers have absorbed water, and the bird will adopt an outstretched wings posture – usually at a convenient perch (e.g a tree, pole, rock or the ground) close to the feeding areas – until its wings have dried.



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Reproduction & Cormorant Life Cycle

Q: What are the typical life stages of a cormorant?
A: A clutch of cormorant eggs will have hatched about 30 days after the first egg was laid. The chicks usually remain in the nest for almost two months. Great Cormorants normally begin breeding at the age of 2-4 years. One study estimated that 44% of the 2-year-old and 70% of the 3-year-old females were breeding. For males the proportions were 25% for the 2-year-olds and 53% for the 3-year- olds. At the age of 8 years almost all individuals were breeding. Generally, when food availability near a colony is optimal, cormorants tend to start breeding at earlier ages, but it seems that in ‘old’ saturated colonies the average age of first breeding often increases. This may be linked to a localised decline in the availability of suitable prey fish.



Q: Where do cormorants build their nests?
A: Cormorants are very flexible and adaptive in this respect. Carbo cormorants in Iceland, Norway, UK and on the French coasts normally breed on rocky cliffs or islets. Sinensis cormorants are commonly described as tree-breeders, but where small, treeless islets are available they also breed on the ground – e.g. in Denmark, Sweden and Finland. Sinensis may also occasionally breed on steep cliffs (e.g. on the Crimean peninsula) and man-made structures such as abandoned platforms, piers and even electricity masts.

Q: How do cormorants feed their young?
A: The young are fed fish which are regurgitated by the adult birds (so this food is usually partially digested) and the chicks retrieve this from the throats of their parents.



Q: How much fish per day does a cormorant chick require?
A: According to a study in the Netherlands a chick needs an average of 386g of fish per day in its first 30 days, with a peak food requirement of 632g per day in the period of fastest growth. This means that a pair of cormorants raising three chicks will need to catch over 1.1kg to 1.9kg fish per day for their young, plus about 2 x 500g to cover their own energy needs.

Q: On average, how many chicks are raised each year by a pair of cormorants?
A: On average, a female cormorant lays 3-5 eggs and, under extremely good conditions, survival to the fledgling stage may be as high as 90%. However, it is usually far less (2.0-2.5 birds/nest is common), and under unfavourable conditions - food shortage, long distance to foraging sites, bad weather, etc. - chick survival may decrease to a colony average of 0.5 birds per nest. Strong, experienced parents appear are able to rear more young than less experienced younger birds, even in the same nesting colony.

Q: About 40% of juveniles and about 12% of adult cormorants die each year. What are the main causes of these mortalities?
A: There are a number of causes, albeit that they have rarely been studied in detail. It is likely that exhaustion and starvation are the most frequent reasons for premature deaths, and this will likely be linked to prevailing weather conditions. Shooting has become a more important factor in recent years, with some estimates of shot birds in Europe exceeding 100,000 birds per year. Drowning in fishing gear is a common cause of death in some areas, especially in the first autumn of life. Predation by White-tailed Eagles occurs, but seems to be of marginal importance. Very little is known about the occurrence and effects of disease and the impacts of parasites.

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Distribution & Migration

Q: Are cormorants migratory birds and, if so, where do they migrate?
A: Cormorants do migrate, sometimes over long distances, but unlike many other migratory birds, they do not all migrate at the same time or to the same areas. For example, migration habits in Europe depend to some extent on the geographical position of the breeding colony. Many birds from northern countries and central Europe migrate south in winter. However, distances vary extensively among individuals from the same colony and with the severity of the winter – thus, some birds just move 100km southwards whereas others fly in steps across the Mediterranean to the coast of North Africa.



The map above shows the distribution of cormorants born and ringed in Denmark and found dead in winter. Note the huge variation in the distance that individual cormorants from the same breeding area migrate to reach their wintering area. (Source: Aarhus University, Denmark).

Q: Surely, cormorants are considered seabirds – why are they found inland?
A: It is true that the Atlantic race (carbo sub-species) has commonly been regarded as a seabird. This sub-species breeds almost exclusively on the rocky coasts of Norway, Great Britain and western France, and some birds moved inland recently to over winter and feed on freshwater habitats. In addition, some birds of this sub-species now choose to breed at inland sites where they can mix with birds of the sinensis sub-species.

The continental race (sinensis sub-species), in contrast, has historically always utilised inland as well as coastal sites. Even when numbers were relatively low in the past, there were some breeding colonies on inland lakes and large, lowland river systems. In winter many individuals from coastal sinensis colonies would cross over inland regions on their flyway south, resting and feeding on inland waters. Since the recent large increase in cormorant numbers across Europe, many more birds of this sub-species now occur at inland sites during as well as outside the breeding season. ‘Surplus birds’ from breeding colonies have emigrated and founded new colonies at inland waters.

The reason that more birds of both sub-species now occur at inland sites reflects, in part at least, the increase in their numbers and the availability of food and suitable habitat. Whereas most major European rivers have experienced a decline in fish abundance, a number of sites like open reservoirs, man-made gravel pits and a large number of fish farms provide good foraging opportunities for cormorants.

Overall the vast majority of cormorants present in Europe are found near to large waterbodies and wetlands both during and outside the breeding season. Moreover, the current legal protection afforded to cormorants make it possible for the birds to breed, roost and feed much closer to densely populated areas than they would have been allowed to in the past. At present, both coastal areas and inland waters provide favourable conditions for cormorants, and the birds are able to adapt their behaviour to exploit the feeding and breeding opportunities available to them.

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Protection & Population Status

Q: Are cormorants protected by law and, if so, do they carry special protection status?
A: Yes, in EU Member States all wild bird species are protected by the EU Birds Directive. But the cormorant has no special protection status; it is not more and not less protected than most other bird species. However, the Directive provides member state governments with the opportunity to ‘derogate’ (be excused under certain circumstances) from the provisions of Articles 5 and 8. Specifically, in Article 9(1), it is possible to apply for permission for various measures, including licenses to kill cormorants when the danger of ‘serious damage’ can be demonstrated or when it is necessary for the protection of flora and fauna. The EU Birds Directive but see also the general description of the Birds Directive under DG Environment.

Q: What is the current population status of the Great Cormorant?
A: The Great Cormorant is probably now more frequent and widespread in Europe than at any time in the last 150 years at least. The current population trend in Europe is considered as 'Increasing' (Wetlands International 2006). In addition almost all national trends of breeding numbers are stable or increasing (BirdLife International 2004).

According to the IUCN Red List, the Great Cormorant is considered to be of ‘least concern’ due to its extremely large distribution and apparent increasing population trend.

Previously the sinensis sub-species was listed in Annex I of the EU Birds Directive (2009/147 EC) because its numbers were low, but as a consequence of the phenomenal population growth it was released from Annex I in 1997. Cormorant conservation is also covered in the Bern Convention and in AEWA (African Eurasian Waterbird Agreement) under the Bonn Convention.

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Population Dynamics

Q: Regarding the size of the cormorant population in ‘core’ Europe, there are highly different numbers quoted, ranging from 500,000 to 2.4 million. Why are the estimates so different?
A: It is recognised that the reporting of different population figures has created confusion. Some figures appear to have been inaccurate due to incomplete coverage. In addition, some confusion has been caused because figures were presented without clearly indicating to which sub-species, to which geographical region or to which year they related.

When talking about numbers it is important to clearly define:
  • the sub-species (carbo or sinensis or both)
  • the geographical range of the countries included (e.g. only western Europe or ‘wider’ Europe including Ukraine & Russia)
  • whether referring to breeding pairs or total population (individuals including non-breeders) the year of reference
  • if dealing with the total population, the time of year (because due to high winter mortality the numbers are always much lower at the start of the breeding season than at the end of the summer, when the new young have fledged)
  • the margins of confidence (e.g. many counts are based on a proportion of available sites in an area and to provide an estimate of the whole population in that area it would be necessary to estimate numbers in areas that were not counted)
In the past, there was often no generally agreed 'authoritative' set of figures. As a contribution, the CorMan project will deliver results of counts made in the 2012 breeding season and in the winter 2012/13. However, a specific project would be needed to best estimate how many cormorants we have in Europe outside the breeding season. Such estimates would require that data on demography and distribution are collated and that a number of assumptions are made and checked before mathematical population dynamics models could be developed.

Q: How are cormorant numbers assessed, and why are there separate figures for breeding pairs and for the total population (number of individuals)?
A: The total number of cormorants is the sum of breeding birds plus young birds and other non-breeders. The number of breeding pairs is assessed by directly counting the AONs (‘apparently occupied nests’) in the colonies during the breeding season. If all colonies are known and counted, these data are generally very accurate.

Like many other long-lived birds, the age at which cormorants begin to breed is 2–5 years. The proportion of these younger birds as well as of the older birds that attempt to breed can vary among breeding seasons and among areas depending on the body condition of the birds as well as the availability of food near the colonies. Hence all 1-year-old cormorants and a variable proportion of the 2-5-year olds and older birds will be non-breeders. The total number of individuals will be the sum of breeders (i.e. numbers of breeding pairs x 2) plus the non-breeders.

An estimate of the total population is thus typically made on the basis of (a) the number of breeding pairs, (b) estimates of the average number of fledged young that each pair produces, (c) the use of mortality rates for different age classes of birds, and (d) assumptions about the proportion of birds breeding of the different age-classes. Counts are also undertaken when birds are more widely dispersed outside the breeding season. The counts made in winter are much more difficult to co-ordinate and undertake than the breeding counts and aim largely at providing information on the distribution of birds at this time of year (see next question).

Note: The number of individuals varies greatly by time of year: It is highest at the end of the summer, when all the young-of-the-year have fledged, but due to high winter mortality it is about 30% lower in late spring (pre-breeding season). In ornithological publications the total population is usually given as an estimate of the mid-winter population.

Q: I have read that cormorants are also regularly counted in January of each year. How are these counts done and do the numbers cover the total cormorant population?
A: There are two types of winter counts:
  • The general mid-winter water bird counts:
    These are standard counts (also named “International Waterbirds Census” – IWC) that have been conducted for many decades, usually on the second weekend in January, where observers count all the species of waterbirds they can see on relevant water bodies, rivers and lakes. These counts in daytime delivers good information on trends within a certain observed region, but it is not able to cover the total numbers of cormorants as these birds are usually dispersed also on sites or habitat not covered by the IWC). However, some attempts have been made to correct for uncovered areas with the aim of scaling up to provide an estimate of the overall numbers in a particular area.
  • Special cormorant counts at sleeping roosts (night roosts):
    In their wintering regions the cormorants spend the night in communal sleeping roosts. When counts are conducted simultaneously at all roosts in a given area, it is possible to derive reliable estimates of all the birds present in the region. In some regions, it is, however, difficult to ensure that all sleeping roosts are covered. In January 2003, such simultaneous counts were organised by IUCN/Wetlands International Cormorant Research Group in (almost) all countries of Europe.
Both counting methods deliver very valuable insights into the regional distribution and numbers of wintering birds in the surveyed areas. Some areas like the coast of North Africa are not easily surveyed and the percentage of the European population spending the winter here is at present unknown but thought to be less than it used to be in the recent past. Although the winter counts do not give an absolute accurate measure of the total size of the European cormorant population they provide a good general estimate of the numbers involved and highly valuable information about how cormorants are distributed throughout Europe in winter. In the well-covered countries, especially where counts are made once or twice a month roost counts are delivering very reliable estimates of trends and numbers.

Q: The cormorant breeding population has shown a substantial increase since the early 1970s. How have the numbers changed?
A: The population of the carbo sub-species has increased only moderately. The numbers of breeding pairs in Norway, UK and western France increased from about 33,000 pairs in 1970 to around 52,000 pairs in 2006. However, the sinensis sub-species in ‘core Europe’ (Europe excluding Russia, Belarus, Moldova and Ukraine) has increased dramatically. In around 1970, the breeding population of these birds was probably only about 10,000 pairs, but this rose to roughly 80,000 pairs in 1990 and some 170,000 in 2000. In 2006 over 233,000 breeding pairs were counted.

Q: How often are breeding counts conducted and how reliable are the results?
A: Information on breeding colonies and the number of breeding pairs is generally good to very good. The coverage is very good in countries in western and central Europe. In many countries breeding counts of sinensis have been conducted virtually every year, providing an almost gapless time series of the population development. In eastern Europe monitoring was less frequent in the past. However, the Cormorant Research Group Pan-European census in 2006 and other recent national counts have also provided a reasonably reliable picture of present breeding numbers.

Q: Have cormorants also expanded their geographical range?
A: There has been little change in the distribution of the carbo sub-species. In contrast, there has been a marked expansion in the distribution of sinensis cormorants: in 1965 there were only a few breeding colonies in a handful of countries; now they breed in virtually all European countries.

Q: Why were cormorant numbers so much lower in the middle of the last century and what were the causes of this?
A: Historically, persecution is believed to have been a key factor when population sizes of cormorants were very low across Europe. The carbo birds, which were almost exclusively breeding and feeding in the sea, generally did not suffer systematic persecution, were never endangered and always kept a fairly stable, if modest, population size. The really dramatic decline relates to the sinensis cormorants in western continental Europe. In the 19th century intensified persecution was officially demanded by law in some countries, and sometimes even military troops were ordered to shoot into the colonies. In combination with habitat loss, this led to almost complete extinction in many countries of western continental Europe. Around 1900 only a few colonies of the sinensis cormorants were left in the Netherlands, the eastern parts of Germany and in Poland. (In eastern Europe, especially along the Danube from lower Austria to Romania, sizeable colonies survived well into the 20th century).

In the following decades cormorants received basic legal protection in some countries and breeding cormorants were protected in a few reserves, but numbers were kept at modest levels. However, in the mid 20th century (1950-1965) there was a further decline in cormorant numbers of sinensis, even in the totally protected Dutch colonies. This time the reason in some colonies was not persecution but a consequence of the use of DDT and other pesticides.

Q: Why are there so many cormorants now?
A: The reasons are complex and not fully understood. They also are apparently somewhat different in separate countries. However, in general terms the key influences have included:

  • A significant decline in persecution of the birds and far greater protection afforded during as well as outside the breeding season
  • Prohibition of the pesticides most dangerous to the birds
  • Better availability of supplies of suitable fish in some waters (although not necessarily more fish per se) as a consequence of anti-pollution legislation, habitat protection and quality, new gravel pits and other man-made water bodies, inland fish farming, fish introductions and other fisheries management policies, eutrophication (nutrient enrichment) of water bodies, etc.

Q: Have cormorant numbers risen in just a few countries?
A: No, in the last 30 years the increase has occurred in virtually all European countries as well as countries such as Japan. Similar increases have also occurred with other cormorant species, such as the Double-Crested Cormorant in North America.

Q: Is the cormorant population still growing?
A: The breeding population of cormorants is still growing in some of the areas of Europe and the Black Sea that have been colonized within the last 10-20 years. Breeding numbers have been fairly stable or even declining in many of the areas that have hosted the vast majority of the breeding sinensis population.

Between 2000 and 2006, the total cormorant population in Europe increased further, and possibly total numbers have continued to increase but only at a very low rate. In the last years there have been signs that numbers are in decline in several of the core breeding areas. These declines are partly explained by extra cold winters affecting survival and individual body condition prior to breeding.

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Diet & Feeding

Q: Many fish-eating birds and mammals eat other food (mice, amphibians, etc.). Is this the case with cormorants?
Cormorants almost exclusively eat fish. Very occasionally they eat other food (e.g. crabs, shrimps, crayfish), but this is rare except at some localities.



Q: Do cormorants actually prefer certain fish for food?
A: In common with most predators, cormorants are opportunistic (so-called ‘generalist’) feeders, and they prey on the fish that they can catch most easily. There is no real evidence that the birds will actively select certain species in preference to others, although size and shape probably play a role (larger and more powerful fish are certainly harder to catch and consume, and slim fish are easier to swallow than species with deep bodies). The diet can also vary markedly over the year, but this is thought to reflect seasonal variation in where cormorants forage and in the abundance and distribution of different fish species. Cormorants can consume dead fish, but field observations suggest that this rarely takes place – regurgitated fish are often present beneath roosts but are not eaten again by cormorants (but may be consumed by scavengers).

Q: How much fish does a cormorant eat per day?
A: The figure which is usually cited for the daily food intake (DFI) is about 500g of fish. However, DFI varies, depending on the size and body mass of the bird, its activity level (time spent for flying and diving, which needs much more energy than the basic metabolism) and the fat content of the prey fish. Moreover, on some days a cormorant may eat more than it actually needs, and sometimes it will consume less. Birds also take substantially more fish during the breeding season when they are feeding their young. For more detail about how to estimate cormorants’ food consumption in a certain situation see “How much do cormorants eat?”

Q: How does the digestive system of cormorants cope with fish bones and scales?
A: Fish are digested entirely, including the bones, but larger and thicker bones are not always fully digested. These remains are regurgitated by the birds in the form of discrete oral pellets; this usually occurs on a daily basis and typically at first light before the birds leave the roost to feed.

Q: How long can a cormorant live without food?
There are no direct research data, but indications are that this time is rather limited. Cormorants cannot store large fat reserves, and flying and diving costs them a lot of energy. They will obviously try to consume enough food each day to sustain themselves because, without food (for example during periods of cold weather), their energy reserves will probably decline within a week or so to levels at which they become too weak to recover.

Q: Some fish can be wounded by but escape from a cormorant’s beak. How often does this occur?
A: It largely depends on the size of the fish – small fish can be easily taken and swallowed, while bigger and stronger fish (>30 cm) are more likely to struggle and escape. However, the age and experience of the birds likely also influences the observed levels of damage. The occurrence of damaged fish at sites frequented by cormorants is quite common, and the level of damage is commonly related to the number of birds.

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Effects On Fish Stocks

Q: There are other fish-eating birds. Why is the discussion about the cormorant so intense?
A: One obvious reason is that because of their numbers, size and their means of hunting, cormorants probably cause greater damage to some fisheries in a shorter time than can any other fish-eating bird in European waters. Like seabirds cormorants can gather in rather large concentrations when they have located areas with easily available fish of the right size. Another reason for discussing cormorants is related to the fact that after heavy persecution and almost extinction in many countries, cormorants have shown a fast numerical increase and a quick expansion, especially into areas where they had been virtually unknown for centuries.

Q: Can cormorants affect the habitat in which they live?
A: The main effect relates to the fact that the cormorant faeces cause white discolouration of the trees in which large numbers of birds roost or nest, often causing the trees to eventually die. This is normally of only marginal importance to such things as the forestry industry but can be a problem where the trees have high landscape value e.g. on the numerous islands in Finland where people have built summer holiday homes. There also examples of cormorants that damage rare trees. Combined with the often unpleasant smell around cormorant colonies, landowners may experience a reduction of the recreational and financial value of their property.

Q: Are cormorants capable of threatening the survival of fish species?
A: No, not in the sense that a fish species would become extinct in an entire region or a whole river system. Even when cormorants reduce local fish stocks substantially, surviving individuals and those which immigrate from adjacent stretches are often able to reproduce successfully and cause repopulation. However, this is different in smaller enclosed water bodies. Here, cormorants can reduce the number of mature fish below the numbers required to maintain a viable, self-sustaining population.

Q: Can cormorants cause damage to fish stocks and fisheries?
A: It is recognised that cormorants can impact on fish stocks with potential consequences for human interests. The level of impact varies considerably between different sites from very substantial at some sites to negligible or non-existent at others. While cormorants are normally protected by law, the European Bird Directive allows countries to take action against cormorants (derogations), including killing the birds, where the birds are causing serious damage to a site, or there is a reasonable expectation that this will happen, and where there are no other satisfactory solutions. The European Commission is developing guidance on Article 9 of the Birds Directive which relates to the use of derogations and interpretation of serious damage. The European Commission is presently developing guidance on when and how derogations can be applied in particular in relation to cormorants.