Double-crested cormorant

Nannopterum auritum

What does a cormorant look like?

The cormorant is characterized by a long, thick neck, stout body, short, rounded wings, medium length tail with stiff feathers, and legs that are positioned far back on the body. Webs connect all four toes on each foot and provide additional thrust in swimming. The cormorant appears black or grayish black, but it has iridescent greenish and purple highlights on the back. It has a small fleshy yellowish-orange throat pouch, and the eyes have a deep, turquoise-colored iris. The long slender bill has a small hook at the tip. This species gets its name from the presence of tufted feathers on both sides of the head, just above the eyes, that are referred to as “crests.” The crests are present only during the nesting season.

In flight, the cormorant can be identified at great distances by its behavior of alternately flapping and gliding while flying in flocks. In contrast, ducks and geese have a steady wingbeat. While perched or swimming, the bill is usually held at an upturned angle. In contrast, ducks, loons, and grebes usually hold their bills parallel to the surface of the water. Cormorants often dry their feathers after swimming by holding their wings out, creating a distinctive posture that generates interest and curiosity about this species.

How large is a cormorant?

The cormorant ranges from 29 to 36 inches long from bill to tail, and the wingspread may reach 54 inches. Weights range from 4 to 6 pounds.

When do cormorants nest?

Cormorants do not nest until they are 3 years old. Nesting begins shortly after ice-out in April and extends into July. Three to four eggs are usually laid in a nest, and they require 24 to 25 days for incubation. The young fledge 35 to 42 days after hatching.

What is a colonial nester?

A colonial nesting bird is a species that nests near others of its species and sometimes near other waterbirds. These colonies are usually located on the ground, or in cliffs, dirt banks, or trees. Examples of colonial nesting waterbirds in Minnesota include great blue herons, American white pelicans, double-crested cormorants, black-crowned night-herons, great egrets, ring-billed gulls, Franklin’s gulls, common terns, and Forster’s terns. Black-crowned night-herons and great egrets may nest in marshes, either on bare ground or with nests attached to Phragmites stalks. Black terns and Forster’s terns nest in openings in freshwater marshes.

The cormorant is called a colonial nester and may nest either in stick nests that they build on the ground on islands or peninsulas, or in live or dead trees at or near the shoreline of lakes and rivers. In Minnesota, cormorant colonies may range in size from 4 to 2,500 nests.

How common and abundant was the cormorant in historic times?

The cormorant was a common and abundant breeding bird throughout much of the state in the pre-settlement era but was nearly extirpated by the late nineteenth century.

How many cormorants nest in Minnesota?

In 2015, a statewide count of cormorant nesting colonies estimated 15,421 nesting pairs, a total of 30,842 nesting birds. In 2015, the statewide population, including non-breeding cormorants, was estimated at about 40,000 birds. Non-breeding cormorants may be present at colony sites until they reach breeding age at 3 to 4 years of age. Though cormorants were documented nesting at 36 sites in 2015, 75% of the state’s population occurred on just ten lakes.

The statewide breeding survey was planned again in the summer of 2020. In 2020, pandemic considerations resulted in a partial survey. In 2021 a follow up survey was conducted to visit colony sites that had not been surveyed or for which only a partial survey was completed in 2020. Because some sites that were surveyed in 2020 were not resurveyed in 2021, statewide population estimates were not obtained.

In addition to nesting cormorants, there are also large numbers of cormorants that migrate through Minnesota to Canadian nesting grounds in the spring and again through the state as they migrate south in the fall.

Is the cormorant population increasing or decreasing?

The cormorant population in Minnesota experienced a rapid growth phase between 1981 and 1997. During this time, cormorants experienced rapid growth across much of the continent as populations recovered from DDT and benefited from legislation and protection. Though statewide surveys were not conducted during this period, monitoring in Minnesota suggested an increase from about 8,000 nesting pairs in 1988 to an estimated 12,000-14,000 nesting pairs in 1997.

In 2004, the first statewide breeding survey for cormorants and pelicans in Minnesota was conducted. This survey established important baseline data for these species, describing their distribution and estimating the sizes of their breeding populations. The statewide breeding survey was repeated in 2010, 2015 and 2020/2021. Total cormorant nest estimates were very similar across the first three survey years (2004, 2010, 2015). Breeding distribution also remained approximately the same, with colonies located across much of the state, except the northwest, southwest and southeast corners. Due to the pandemic in 2020, the statewide survey was not entirely completed in one year so statewide population estimates were not obtained in the most recent survey. The statewide breeding survey was intended to estimate nest/pair counts and does not estimate actual species abundance, because unpaired birds and those that do not nest are not counted in survey efforts. Data obtained between 2004 and 2015 suggest the rapid growth of the cormorant population has ended. The state’s breeding population of cormorants appears to be stable.

How much does the cormorant eat and what does it eat?

In Minnesota, cormorants have been found to consume from 1.0 to 1.5 pounds per bird per day. Most of their diet is made up of fish (99.9% in Leech Lake), but diets can also include snails, crayfish, and amphibians to a lesser extent. Ninety percent of the fish they consume are less than 6.0 inches long and have an average weight of 4 ounces. Cormorants are opportunistic feeders and will prey on a variety of native and introduced species that varies seasonally. In northern Minnesota lakes this often includes yellow perch, minnows, cisco, and walleye. In southern Minnesota lakes, diets can be expected to reflect the fish species composition of lakes and reservoirs at similar latitudes and include a higher proportion of bullheads, freshwater drum, and where present, shads. The diet typically reflects the relative abundance and sizes of prey species present at the time of foraging. For example, during spring diets are comprised of slightly larger fishes age-1 and older because the fish hatching that same spring have yet to grow large enough to be eaten. In Minnesota, age-0 fishes start showing up in cormorant diets around July 1, and by August diets are primarily comprised of various smaller, age-0 fishes. For these reasons, species composition fish sizes within diets varies considerably within and among years at the same location.

How much of a cormorant’s diet is comprised of walleyes?

Diet studies have been conducted on several systems throughout the Great Lakes region and reflect the relative abundance of small fishes present during that particular year sampled in that lake. By weight, composition of walleye in diets in a single year is typically less than 5% but has ranged as high as 10%. In most cases, cormorant consumption of walleye and other sportfishes is negligible. However, when high enough numbers of foraging cormorants are sustained throughout spring to sufficiently reduce the abundance of age-1 and older fish, these are the rare circumstances when cormorant management may be considered. Demonstrating this occurrence requires a considerable amount of fisheries and cormorant data to be collected over a long enough duration to separate the potential effect of cormorants, which are occasional, from the natural variation that already occurs in survival of young fish, which is common.

A study of cormorant food habits on Lake Winnipegosis in Canada revealed that of 10,911 fish eaten by cormorants, only 30 fish were walleyes (0.20%). Similarly, cormorant consumption of walleye and yellow perch Lake Winnebago, WI was less than 5% by weight each year. A study in Lake Oneida in New York showed that walleyes comprised 3-5% of cormorant diets in a year, mostly eaten during the fall bird migration. Walleye contribution to Leech Lake cormorant diets was similar to that on Lake Oneida with walleye comprising only 4.6% of the average diet. Walleye composition ranged from 1.4% to 11.1% across the 4 years that diets were examined. By weight this consisted primarily of age-1 and age-2 walleyes. The variability in Leech Lake walleye consumption was associated with the number of small walleyes available and the presence of other alternative prey sources, particularly cisco. During years that cisco may be more vulnerable, such as during a summer kill (as occurred in 2006), predation on other species such as walleye and yellow perch was lower, as was total consumption.

Do cormorants negatively impact fish at a population level?

Cormorant foraging can, under the right combination of circumstances, potentially have a negative impact on recreational fishing at a localized level. Such impacts are very difficult to detect and measure, as many factors operating within the ecosystem can affect fish populations. Such factors include fish stocking, fishing regulation changes, invasive species introductions and population expansion, normal variability in year-class strength, predation, competition, loss of critical habitat caused by shoreline development, and excessive commercial and/or sport fishing pressure by people.

Cormorant population management on Leech Lake is often cited as a situation where control was necessary to improve the local walleye fishery in Minnesota. In this situation there was circumstantial evidence at the time control was initiated that cormorants were reducing survival of small walleyes, which in turn translated to fewer catchable-sized fish for anglers. Some walleye stocking occurred to address the walleye decline.

Fisheries biologists examined cormorant diets and re-constructed walleye year classes produced from 2000 to 2004 to see what the walleye population structure might have looked like in the absence of cormorant predation. This analysis indicated that the predation pressure by the expanding cormorant population could have been sufficient to impact walleye survival to older age classes.

The use of a lake by cormorants does not mean the fishery is at risk. For example, Lake of the Woods supports one of the most abundant cormorant colonies in North America and it also supports one of the most recognizable walleye and sauger fisheries in the country. Similarly, management activities at Leech Lake have reduced the cormorant population to a level where fish consumption by cormorants is expected to have little effect on the fishery. This removal is considerably different than eradication and, based on increases in fish population surveys, appears to be appropriate.

What does it mean that most cormorant predation on fish is usually “compensatory” and not “additive?”

Cormorants commonly eat small fish under 6 inches in length. Sport fish of this size are not typically caught by anglers. Small-sized fish are more often eaten by predatory fish as well as waterbirds and mammals. A 4-to 6-inch-long fish has a relatively low chance of growing to become a legal or desirable size for an angler. Why? Because if it does not die of one cause, it will die from another. This is called “compensatory mortality. In other words, most of the fish eaten by cormorants would have probably been eaten by larger fish or other predators. This is common in all animal populations and this type of mortality does not decrease fish populations.

Additive mortality occurs if there are unusually high predation or harvest rates on a particular group of fish that reduces the survival of those year classes in an unusual manner. High predation was documented at Oneida Lake in New York from 1995 to 2000 for yellow perch and walleyes, on Leech Lake for walleye and yellow perch during 2000-2004, and on eastern Lake Ontario, New York for smallmouth bass during the 1990’s prior to the implementation of cormorant management in 1999.

Larger fish that are at least 12 to 14 inches long (for example, walleyes in the 3-year or greater age classes) may occasionally be taken by cormorants, but that is the exception rather than the rule. Excessive mortality from any cause in these older age classes becomes “additive” because such mortality can decrease the fish population if it occurs at excessive levels.

Can cormorants cause problems for baitfish production or fish farmers?

Yes, cormorants, as well as other fish-eating birds, can be a problem in small natural ponds where bait dealers release minnows for later seining. Minnows in these small ponds are vulnerable to predators because the ponds are typically untended natural waters easily discovered by fish-eating birds. Ponds are also shallow, making the minnows easy for cormorants to catch. Similar issues can exist at aquaculture facilities. Cormorant damage at aquaculture facilities is managed under a depredation permit system, whereby the U.S. Fish and Wildlife Service permits the lethal take of double-crested cormorants at commercial freshwater aquaculture facilities when non-lethal methods are insufficient in preventing depredation.

Do cormorants damage vegetation in rookery areas where large numbers of cormorants nest?

The natural vegetation at all waterbird colonies typically dies from the long-term accumulation of guano at the site. This tree decline is a natural, dynamic process in which tree-nesting birds may subsequently move to nearby groves of trees. Some species like cormorants may move from tree-nesting sites to the ground. With the open ground cover created by the loss of trees and shrubs, ground-nesting waterbirds can move in, including American white pelicans, common terns, and ring-billed gulls.

Is the loss of vegetation at a cormorant colony a cause for concern?

Typically, this loss is not an environmental concern because the plants affected are common trees, shrubs, and forbs like cottonwoods, willows, and stinging nettles. Where vegetation changes occur on public lands, agency resource professionals assess whether rare plants are at risk. If not, cormorant control based on vegetation changes is not warranted. Some vegetation changes may create suitable habitat for Species in Greatest Conservation Need like the American white pelican or endangered species like the common tern. In other states, cormorant control has been used in selected cases to preserve trees and shrubs for nesting waterbirds.

What about the loss of vegetation on private land caused by cormorants?

Where there is significant vegetation damage by cormorants to private property, provisions exist to address the damage. Cormorant control is not allowed if it negatively impacts other colonial waterbirds that are present. In these instances, control is usually carried out by professional control agents through the Animal and Plant Health Inspection Service (APHIS) part of the U.S. Dept. of Agriculture, for a fee paid by the landowner.

Is the cormorant a threat to aircraft from collisions?

Cormorants could potentially be struck by aircraft, but the likelihood is extremely low. A greater threat is posed by Canada geese, eagles, hawks, cranes, ducks, and gulls. The FAA Wildlife Strike Database for Minnesota showed that from January of 1990 through June 2023, 2,900cases of aircraft striking birds or other wildlife were recorded. Only four of these records were collisions involving cormorants.

Is the cormorant a human health threat as a vector of disease like West Nile Virus?

There is no scientific evidence indicating that cormorants pose a greater risk of spreading West Nile Virus than other affected bird species. A total of 225 wild bird species have been found to carry West Nile Virus, and the species serving as a primary source for transmitting this disease have not been identified.

Is the cormorant a threat to commercial poultry operations because it carries Newcastle Disease?

While it is theoretically possible for Newcastle Disease to be transmitted to poultry, past spread of the disease in poultry operations was primarily caused by humans who transported the disease in live birds, equipment, poultry products, and poultry feed. Minnesota has never had a case of Newcastle Disease in poultry. Effective farm biosecurity measures are important to protecting poultry from disease.

Does the guano deposited at a cormorant nesting colony pose a public health risk?

There are no documented cases of lakes in Minnesota becoming contaminated by cormorant guano or posing any public health risk from diseases like salmonella or aspergillosis.

Are cormorants a protected bird species?

The cormorant is federally protected by the Migratory Bird Treaty Act. This act prohibits killing cormorants, their eggs or young and also prohibits the disturbance of the nests or young, except as provided for through federal permits. The cormorant is not technically protected by Minnesota statute, but federal regulations cover its protection. Any authority for taking or control of this species is under the jurisdiction of the federal government.

Why is there no hunting season on cormorants? Are they good to eat?

Cormorants are not considered a game species or traditional sporting species by hunters in the United States. Generally, they are not thought of as good to eat due to their primarily fish diet.

What agencies have the authority to control numbers of cormorants if they are deemed to be causing damage?

In Minnesota, if cormorants are documented to be causing damage to private property or public resources, or to be a human health and safety concern, one or more of the following agencies has authority to alleviate these problems, as explained below:

• U. S. Fish and Wildlife Service (USFWS)
• U. S. Department of Agriculture/Wildlife Services (WS)
• Tribal Natural Resource Management Agencies
• Minnesota Department of Natural Resources

What types of regulatory tools can be used to address cormorant damage or conflicts with human interests?

Depredation Permits. Under the Migratory Bird Treaty Act no take of cormorants is allowed without a permit from the USFWS. Depredation permits may be issued to alleviate damage to private property (such as vegetation or stocked fish on private property) and risks to human health and safety. Before a permit application can be considered, the permit applicant must consult with the USDA Wildlife Services (WS) office. The WS will assess the situation and on the WS Form 37, they will document the damages and non-lethal options for addressing them and, if non-lethal approaches are insufficient, recommend an appropriate level of take of birds, eggs, or active nests. The USFWS then considers the permit application and WS Form 37 before deciding whether to issue a depredation permit. In some cases, the USFWS may require that WS does the control work (at the landowner’s expense) to minimize the likelihood that it will affect other birds that co-occur with the cormorants.

As part of ongoing efforts to address conflicts between cormorants and wild and stocked fisheries, the USFWS published a final rule in December 2020 establishing a new special cormorant permit. State and tribal fish and wildlife agencies may work with the USFWS to apply for a special permit. Under this permit state or tribal fish and wildlife agencies may be authorized to take cormorants for a variety of reasons including depredation of wild and publicly stocked fish managed by the agency (DOI 2020).

For more information see the USFWS page on cormorants.

Harassment. Federal law does not prohibit the harassment of cormorants unless it causes the death of adults or the death of eggs or young at their nests. Additionally, harassment may not cause the death of adults, eggs, or young of any other bird species that might nest or roost near cormorants. Minnesota state law prohibits chasing or killing cormorants and other wild animals by persons using motor vehicles, including boats and personal watercraft. (Minnesota Statutes 97B.091 and 97A.015 Subd. 32).

Are all bird species that nest or roost near cormorants protected by the Migratory Bird Treaty Act?

No actions may be taken toward cormorants (killing or harassment) that would cause the death of adults, eggs or young of other bird species. Incidental take of non-target bird species is not allowed under the Act.

Could high populations of cormorants damage the attraction of an area as a tourism destination?

There is no evidence that the presence of cormorants can reduce the attraction of an area as an important fishing and tourism destination. Anglers on Lake of the Woods frequently enjoy exceptional walleye fishing within sight of islands where hundreds of cormorants are nesting.

Are cormorants interesting birds to watch?

Cormorants at their colonies are very interesting birds to observe. They have intricate courtship behaviors and mates often pay close attention to one another. They are excellent parents, frequently bringing fish and sometimes water to their young. The sounds of a cormorant colony are also diverse. Many people enjoy viewing and photographing this species.

For example, the Minnesota DNR Nongame Wildlife Program responded to requests from the Hutchinson Bird Club in 2006 because the Department of Transportation Pigeon Lake Overlook on Trunk Highway 15 north of Hutchinson had become obscured by trees. Travelers could no longer stop to view the colony of American white pelicans, cormorants, great blue herons, and great egrets nesting on the island. This site is one of the best and most accessible spots for viewing nesting pelicans and cormorants in Minnesota. The DNR had Conservation Corps Minnesota workers clear brush and trees at the site so people could see and enjoy this wildlife spectacle.

Birdwatching at waterbird colonies (including those with cormorants) attracts a diverse recreational group that ranges from avid birdwatchers to individuals and families that enjoy watching wildlife. Wildlife-based tourism helps to strengthen and diversify local economies. Further information about wildlife viewing is available at the following websites:

• American Birding Association: Area birding trails – a list of birding trails by state.
• Audubon: Birding in the United States - A state-by-state guide to birding hotspots.
• eBird - a global real-time, online checklist program for bird tracking
• The Great River Birding Trail – Highlights birdwatching sites along the Mississippi River.
• Pine to Prairie International Birding Trail - The trail includes more than 70 bird watching sites in Minnesota and Manitoba.
• Minnesota River Valley Birding Trail – Highlights birdwatching sites along the Minnesota River.
• Minnesota Ornithologists Union: Birding Hotspots - Sixteen Quality Birding Locations Around the State

For further information:

• Wires, L.R., K.V. Haws, and F.J. Cuthbert. 2005. The Double-crested Cormorant and American White Pelican in Minnesota: a statewide status assessment. submitted to the Nongame Wildlife Program. 28 pp. State Wildlife Grants Program final report.
• Wires, L.R., F.J. Cuthbert and D. Hamilton. 2011. The American White Pelican and Double-crested Cormorant in Minnesota in 2010: Distribution, abundance, and population change. Final report submitted to the Nongame Wildlife program. 28 pp. State Wildlife Grants Program final report. Final report.
• Wires, L. R., and Cuthbert, F. J. Minnesota Fish Producers Report on Losses to Birds. 4 pp. 2004. (A20)
• Wires et al. 2001. Status of the Double-crested Cormorant (Phalacrocorax auritus) in North America (updated April 12, 2013)
• Expanding Management of Conflicts Associated with Double-crested Cormorants | U.S. Fish & Wildlife Service (fws.gov)
• USFWS. 2020. Final Environmental Impact Statement: Management of Conflicts Associated with Double-crested Cormorants. U.S. Dept. of the Interior, USFWS, 5275 Leesburg Pike, Falls Church, VA 22041. October 2020.
• Cuthbert, F.J. and L.R. Wires. 2011. The fourth decadal U.S. Great Lakes colonial waterbird survey (2007-2010): Results and recommendations to improve the scientific basis for conservation and management. Final Report (February 2011) to U.S. Fish and Wildlife Service, Ft. Snelling, MN.
• Hamilton, D. and F.J. Cuthbert. 2016. Assessing Distribution, Abundance, and Population Change in the American White Pelican and Double-crested Cormorant in Minnesota: Comparison to Three Census Periods, 2004/05, 2010, and 2015. Final Report to Minnesota Department of Natural Resources. Final report.
• Bracey, A., Kolbe, S., Grinde, A., and Cuthbert, F. 2021. Minnesota Department of Natural Resources 2021 Minnesota Colonial Waterbird Surveys. Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2021/36. 25 p. + 4 appendices. Final report.
• Dorr, B. S., J. J. Hatch, and D. V. Weseloh (2021). Double-crested Cormorant (Nannopterum auritum), version 1.1. In Birds of the World (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.