Deep Dive - Cormorant FAQs
For additional, detailed information about cormorants, visit the “Deeper Dive” section.
1: Are cormorants a naturally occurring, native species in the Great Lakes Basin?
Yes. Double-crested Cormorants have been present in the Great Lakes for a very long time. In fact:
Double-crested cormorants were recognized as nesting in the region by the American Ornithologist’s Union in the 19th century;
There is specimen-backed documentation for the species nesting over a vast region that includes the Great Lakes;
Indigenous peoples living in the Great Lakes region provided named references to areas that identified cormorants, indicating familiarity with the species;
There are several written observations of cormorants in and around the Great Lakes area that were published prior to 1900. For example, Thomas McIlwraith, in Birds of Hamilton, Canada West (1868) published in the Proceedings of the Essex Institute, mentions specimens of the Double-crested Cormorant collected in Hamilton Bay (where they again are a nesting species) in the spring of 1854 and 1865.
It is certainly counter-intuitive and extremely unlikely that an aquatic, fish-eating and highly mobile species of bird that undisputedly nested from the Pacific coast to Labrador and from the northern prairies to the southern U.S. would somehow skip over the world’s largest source of freshwater and fish in the world, right in the midst of that vast area.
Based on this collective evidence and reasoning, it is likely that prior to European colonization Double-crested Cormorants were in the Great Lakes Basin, but they were probably not as common as they are now, given the enormous changes in the species and sizes of fish, with several non-native species cormorants prefer, now present.
2: Do cormorants modify vegetation?
Yes. Modification of vegetation in and around cormorant colonies is a natural process. Their excrement, called guano, is rich in nutriments of value to plants when diluted by rain and snowmelt, but as is true of any fertilizer, too much can damage or destroy plant growth, with plant species varying in their degree of vulnerability. Guano can coat leaves, thus interfering with photosynthesis. Also, cormorants will remove branches to be used as nesting material, which, while not necessarily fatal to trees, may compromise their health, again some species being more vulnerable than others.
3: Are cormorants ecosystem engineers?
Yes. Cormorants are part of the ecosystem they inhabit and are considered to be “ecosystem engineers”. While all species of wild animal and plant, from microorganisms to Blue Whales, inevitably affect their respective ecosystems, “ecosystem engineer” is a handy way of designating those species that do so to quite noticeable degree. Ecosystem engineers modify habitat through their actions. For example, when a beaver builds a dam, flooding otherwise dry forest floor, or adjoining fields, it is a very obvious modification of habitat that can benefit a vast array of other animal and plant species. An example of habitat destruction, on the other hand, would be turning a forest into a parking lot (and even then, it remains an ecosystem, but one impoverished of species larger than insects or weeds that may break through the pavement in time). Naturally occurring ecosystems are dynamic, always changing in response to whatever characteristics, elements, species and events occur within them.
4: How many fish can one cormorant eat?
It depends. The overly simplistic “rule of thumb” answer usually given to the question is about half a kilogram (about 1 lb) of fish per adult cormorant per day. While cormorants are opportunistic and on rare occasions eat other aquatic animals, most of their diet consists of small (less than 15 cm., or about six inches long) fish, meaning fish that weigh just over 100 grams (3 ½ ounces) each. Fish are virtually the sole source of the energy the birds require to survive and energy needs of the birds will vary through the year, being highest when there are families to be cared for. So, fish must be small enough to catch and swallow but large enough to provide sufficient nourishment and be available in sufficient numbers to replenish the energy it takes the cormorant to catch them. The actual number of small fish caught may vary from day to day.
5: If cormorants eat fish, why do they not eliminate fish populations?
Because fish are virtually the sole source of nutriment for cormorants, they are essential for cormorant survival, but capturing fish in order to fuel the bird’s metabolism requires energy. Cormorants are “opportunistic” feeders who eat what is easiest (uses least energy) to chase, catch and swallow. As a generality, the more fish there are, the less energy is required per capture. Long before fish populations are eliminated, they will decline to a point of diminishing returns whereby the amount of energy expanded by each cormorant to capture each fish is more than obtained from the net amount of fish thus eaten, and the bird has no choice but to move on or starve, but there are still enough fish to quickly replenish the loss. Food webs are structured so that prey are more abundant than predators; fish lay a great many eggs. One Alewife, (an invasive, herring-like fish often eaten by cormorants in the Great Lakes) can lay up to 300,00 eggs, compared to cormorants (usually 3 or 4). While most young fish do not survive, when numbers are down, competition within the species is thus reduced, and survival potential increases for each fish. It is literally impossible for any predator to eliminate its essential prey without itself becoming extinct, and the presence of predators indicates the presence of essential prey, whatever the species. Typically when there are lots of fish-eating waterbirds in an area, it’s a good sign that fish populations are healthy and robust.
6: Are cormorants edible?
Bald Eagles and Great Horned Owls think so. Both are known to prey upon cormorants, but humans generally consider cormorants to be inedible because of their rank, fishy smell and taste. In the Great Lakes region cormorant bones have shown up only in a couple of kitchen middens – waste heaps – left behind centuries ago by indigenous peoples, indicating that the birds may have been eaten, but if so, very rarely.
7: If cormorants are not generally considered to be edible, why are they a game species in Ontario?
It appears to be a political decision made by Ontario Premier Doug Ford, against scientifically-based advice, in response to intense lobbying by Anglers and Hunters, presumably in hope of winning votes in rural areas, where cormorants are often seen to be competitors for “game” and “commercial” fish species, and destructive of trees (ironically at most a tiny fraction of one percent of the trees lost due to changes in laws and regulations enacted by the same government).
8: Why do you oppose them being called a game species anyway?
It is not just us, but government biologists, academics, naturalists and even many sport hunters who oppose the Ontario government’s bizarre decision to turn back more than a century’s worth of progress in wildlife management by ignoring the basic principles of game management, including utilization (game cannot be wasted), fair chase (killing game species often requires some skill whereas cormorants are easily killed) and controls to conserve the species (unlike the legal take of all other game species, the Ontario government has no way to determining the number of cormorants killed, or the size of the population, data essential to effective game management). The government is appealing to slob hunters, who kill for the sake of killing, and to the ignorant, who lack knowledge of basic, ecological principles.
9: Why is Parks Canada culling cormorants on Middle Island?
Because Middle Island has a history of settlement that has kept colonial waterbirds away, there is a plant cover that includes several species which are at the northern end of their respective natural ranges. Middle Island is just a few meters across the (invisible, being in the middle of Lake Erie) border with the U.S., thus these species are listed under the Endangered Species Act of Ontario, being rare in Ontario, and Canada, although they are found in the U.S., some abundantly. The plants also grow on the US mainland, but on the Canadian mainland side there is little place left for them, so Parks Canada wants the very tiny number of them currently eking out a living on Middle Island to stay. In a nonsensical effort to make that happen, Parks Canada has slaughtered thousands of cormorants and driven away other colonial birds that would otherwise nest on the island. It is a very complicated issue best explained, albeit at quite some length, in our Deep Dive section.
10: If you want to protect trees, are there non-lethal ways to do so?
Yes. First, it should be reiterated that modification of vegetation in and around bird colonies is a natural process that occurs throughout the world. Having said that, there are methods to prevent vegetative change if desired. They require chasing the birds from trees before the birds can establish territories (the unwillingness of birds to abandon established nesting locations is called “nest site tenacity” and is stronger in some species, like cormorants, than in others, like herons), knocking down nests in winter, and encouraging ground-nesting elsewhere nearby. It should be noted that cormorants will not start building nests in the first place if simply disturbed. Walking around under the trees when the birds first arrive in the early spring will do the trick.
11: Where have non-lethal controls of cormorants nesting in trees actually worked?
The biggest Double-crested Cormorant colony in eastern North America, perhaps anywhere, is managed by the Toronto Region Conservation Authority (TRCA) in a successful plan that allows co-existence with other tree-nesting colonial birds (Black-crowned Night-Herons and Great Egrets), protects some trees from having any cormorant nests, and allows co-existence with other ground-nesting colonial birds such as terns and gulls. Presqu’ile Provincial Park, after some serious missteps, has also succeeded in encouraging ground nesting by cormorants and protecting trees used by Great Blue Herons, always remembering that bird nesting colonies are inherently dynamic, always changing, even coming and going through time, that being their nature.
12: Isn’t cormorant excrement a natural component of the environment?
Yes. The waste cormorants (and all other aquatic, fish-eating species, including larger fish that eat smaller ones) produce originates from the consumption of the fish themselves – in other words, it is already in the system, recycling the nitrogen, phosphorus and other nutriments from the water back into the water (but for the portion that falls on land and even some of it will ultimately reach the water); nutriment recycling has happened for millions of years. So excrement from colonial nesting birds isn’t typically a water quality problem.
The TRCA colony, about the largest Double-crested Cormorant colony presently established, adjoins water – Lake Ontario – that is continually monitored and found to be clean enough for swimming, except after storm sewers overflow, which has nothing to do with the birds. Excrement can befoul standing water in small, landlocked ponds but is not a problem in open lakes, rivers or oceans.
On the other hand, prior to the cormorants’ recovery, serious befoulment of the Great Lakes occurred when great shoals of introduced Alewives used to wash ashore, and rot, creating a serious health hazard for humans. Cormorants and other predators now reduce those Alewife populations to the point where shorelines covered in rotting fish are far less frequent and less extensive, although they can still be triggered by changes in water temperature, to the detriment of cormorants and other fish-eating species.
13: Do cormorants benefit other, “more desirable species”, such asherons and egrets?
Ecologically all species native to a given ecosystem belong in that ecosystem and collectively determine the nature of the ecosystem in terms of the population size of each species, with no one species “more” or “less” desirable than any other. Ecosystems are defined by their inhabitants. While modifying the health of tall trees may produce conditions favorable for Great Blue Herons, who prefer nesting amid somewhat defoliated branches, cormorants, herons, egrets and night-herons can also compete with each other for nesting materials. The species that make up the mixed-species colony define the levels of “interspecific” (competition for resources between species) that occurs, along with intraspecific competition (competition for resources within the species). While it is certainly true that cormorants’ excrement may eventually kill trees needed by other species, such as egrets, night-herons and Great Blue Herons, that either never or seldom nest on the ground, these species have co-existed through millions of years. They certainly co-existed on Middle Island, Point Pelee National Park, which hosted one of the province’s largest Great Blue Heron colonies at the same time that nesting Double-crested Cormorants were reaching their peak. Sadly, the Great Blue Heron numbers in the colony have dramatically declined since Parks Canada began culling. In balance, Great Egrets appear to be increasing overall in Ontario, possibly as a result of climate change, and with night-herons it is hard to know as their numbers fluctuate and colonies move around.
14: How many cormorants can the Great Lakes Basin support?
It depends. The number is based on the inherent ability of the region to support the species. Left alone there would be as many cormorants as the area they are in can support, and once a population that was depleted restores itself it levels off, not at a fixed population size – nature is dynamic, always changing – but at a size that the conditions support. Most areas in the Great Lakes Basin do not have cormorants; those areas that have them do so because there is enough of what the cormorants require to survive, as is true of all other species present.
15: Are cormorants a popular, well-liked bird?
Most people enjoy looking at wildlife and appreciate wild animals being on the landscape. However, a small number of very vocal people often dislike, and demonize, certain wild animals. If we look at places where cormorants are most vilified and persecuted, we can see that those places more or less may have certain things in common. They are often locations where the cormorants were once nearly wiped out and are recovering, thus numbers are erroneously believed to be increasing “exponentially”. People are not used to them and fear that which is new and strange. Also, there appears to be at least some correlation between their persecution and regions in North America that have lower average levels of education, where basic principles of the science of ecology are poorly comprehended. And finally, they are hated by aquaculturists (fish “farmers”) and some (definitely not all) commercial and sport fishers who see them as “competitors”. Indeed, cormorants can negatively impact profits of fish farmers (much of the pressure to kill cormorants has come from catfish growers in the southern U.S.) although competition with Asian producers has had a far more negative impact on American fish farmers than cormorants ever could, while the role of cormorants in controlling invasive species and removing sick and genetically inferior fish may result to them being beneficial to commercial and sport fishers overall.
16: Are cormorants beautiful?
Beauty is in the eye of the beholder, and to many they are attractive. Cormorants have a sleek streamlining of form and a lovely dark bronze colour to the black-edged feathers of the upper backs and wings, and a subtly iridescent black to the head, neck, belly and lower back. There is a sinuous, at times almost reptilian, suppleness to them, especially when a pair interact affectionately, maintaining their pair bond even after their young are raised and on their own. Their shape, both in flight and on the water, is similar to that of loons, and as is also true of loons, millions of years of evolution has shaped them into streamlined, living submarines. Their eyes are a startling shade of emerald green while they have, particularly when in breeding finery, brilliant blue mouth-linings bordered by the bright orange of face and throat. The double crest, the form of which varies across their range from tightly curled black feathers in the east to long, limp white plumes in wester birds, that gives the species its name is an unique feature borne only during the breeding season.
17: What kinds of fish (or other food) do cormorants eat?
They are opportunistic feeders, meaning they “select for” what is most easily caught and utilized. By far the vast majority of fish consumed by the birds are small, in the 15-cm (six inch) minnow and shiner category, and whatever is most plentiful. Whatever they eat is likely to be abundant. In fact, in the Great Lakes Basin cormorants are particularly inclined to eat invasive, non-native species, Alewives and Round Gobies, both of which are deleterious to the survival of our native “game” and commercially valued fish and other aquatic species.
18: How many Double-crested Cormorants are there?
It is a bit of a trick question because there is no fixed number. The number of birds of any species is always higher, often to a very significant degree, after young birds have hatched, and lowest just before hatching season. Apart from that, in fact no one knows the actual numbers. This is a species that breeds throughout much of North America, from Alaska to Newfoundland and Labrador, and south as far as Mexico and Cuba, although with large gaps where there is unsuitable habitat, although some of those regions in the southern U.S. and elsewhere, extending all the way to the southern tip of South America, hosts nesting populations of the closely related and very similar Neotropic Cormorant. It is probably safe to guess that at any one time there are between one and two million Double-crested Cormorants. To provide a bit of perspective, there are about a dozen cities in the U.S. and Canada with more than a million human inhabitants in each, so the number of Double-crested cormorants living in North America today is miniscule by comparison
19: Do people scapegoat Double-crested Cormorants, even when there’s no evidence they’re actually doing anything wrong?
Yes. In fact, in 2020 when a botanist discovered a non-native invasive week, Japanese Chaff-flower, on Middle Island, Lake Erie, hundreds of kilometers north of where it was otherwise known to grow in North America. Double-crested Cormorants nest on Middle Island and Parks Canada, which manages the park as part of Point Pelee National park, and the media, immediately implicated the cormorants as the likely source of the seeds. The plants, which Parks Canada’s staff had overlooked in their own botanical surveys, were mostly growing under where the cormorants perched, causing the botanist to speculate, but not conclude, that seeds might somehow have been brought there by cormorants. A subsequent investigation found that there were no records of cormorants distributing these or similar seeds whereas there were records of birds related to other kinds that might be found on Middle Island that had done so. However, by far the most frequent source of chaff-flower seed distribution proved to be mammals, including domestic dogs and humans. The whole story is here [LINK] and worth looking at as a prime example of the visceral, if not always rational, nature of antipathy so many people have against cormorants.