Biogeography of Double-crested Cormorant

by Samuel J. Anderson, student in Geography 316

Thank you for visiting our site. This web pages was written by a student in Geography 316: Biogeography and edited by the instructor, Barbara Holzman, PhD. All photos and maps are posted with specific copyright permission for the express use of education on these web pages. The students have tried to be as accurate as possible with the information provided and sources and references are cited at the end of each page.

Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Pelecaniformes
Family: Phalacrocoracidae
Genus: Phalacrocorax
Species: Phalacrocorax auritus

Description of Species:

Physical Characteristics--
Cormorants are "somber-looking birds with long bodies, long necks, long beaks with a hooked upper mandible, completely webbed feet, short legs and wedge-shaped tails. Plumage is predominantly black with a greenish or bronze sheen in both sexes. The wings are short and do not permit long flights out to sea" (unknown, 1967). P. auritus can be 19-40 inches long, but they are generally 33 inches in length (Landsborough, 1964). In the summer, adults are glossy greenish-black, coppery-gray with narrow distinct black edges and black shafted feathers on back and wings. There are two curly black crests on the head. P. auritus shows orange on throat sac and lores, dusky bill, black feet, green iris, and blue eyelids. In the winter adults lack crests, show no blue on eyelids, have a yellow bill, with red on their gular sac, and yellow behind the ocher. The young appear plain dark brown with grayish or whitish coloring underneath (Pearson, 1936).

Natural History:

Behavior--
Phalacrocorax auritus can be found in small groups. They feed during the day by swimming and diving for fish (Kern, 1995). After diving, a cormorant seeks out an elevated spot to perch with its wings outspread. It is supposed that this is done in order to dry out their feather. This behavior is not directly in response to water, however, as captive cormorants that do not dive for their food still perch to dry their feathers after eating. I think that it is just instinct. Nests are built of twigs or seaweed. The nests are sometimes found in trees, but are more commonly found along the ground in crowded colonial sites (Kern, 1995).

Food habits--
Frequenting coasts and large rivers, Phalacrocorax auritus swim low in the water and do not spend a lot of time in the water, except for feeding. For P. auritus the majority of their diet consists of fish, although crustaceans and amphibians from fresh water are also in their menu (Landsborough, 1964). Cormorants dive and catch their prey underwater (unknown, 1967). "Sometimes the bird submerges gently, but more often it takes a forward leap almost clear out of the water. It swims underwater without aid from the wings except in braking. It may reach a considerable depth, but it commonly brings its catch to the surface before swallowing it, although small fish are apparently sometimes swallowed underwater" (Landsborough Thomson, 1964). P. auritus displays an 88% rate of efficiency in converting consumed food energy into energy (Brooke, 1991).

Reproduction--
Cormorants breed in colonies of up to three thousand pairs. According to Pearson (1936), the males have elaborate courtship dances, including dances in the water where he presents the female with material to build a nest. The male will also dance to mark out a nesting site for the couple. Phalocrocorax auritus will lose its double crest on the crown after pairing. The eggs of P. auritus are about two and one-half inches long and pale bluish-white in color with a chalky coating (Pearson, 1936). A clutch is usually 2-4. Both parents take part in incubation as well as care for the young (Landsborough, 1964). The young are covered in black down and have insatiable appetites, eating food regurgitated by the parents (unknown, 1967). The young are fed about six times a day by their parents. Once they are a month old, the young are fed about six times a day by their parents. Once they are a month old, the young begin to explore outside of their nesting site in groups. The birds begin to fly after five to six weeks (Perrins, 1990).

Habitat--
Phalcrocorax auritus inhabit marine and inland waters, including the temperate coastal, freshwater lake, and freshwater river biomes (Perrins, 1990). Cormorants are easily seen "on the Seal Rocks near the Cliff House at San Francisco, on Black Horse Island off the coast of Maine, and on almost every buoy and Channel-stake about the harbors of Florida" (Pearson, 1936). They even inhabit Lake Merced of San Francisco (Johnson, 2000).

I know for a fact that they inhabit tall, steep, ocean-side cliffs. They build their nests on any rocky hummock or crevice available in the cliff face. I know that there are one or two nesting sites at Pillar Point Naval Base Area in Half Moon Bay, California. On Monday, October 14, 2002, I went surfing at Ross' Cove, which is adjacent to the fabled big-wave surf spot "Mavericks". This is right at Pillar Point. To get to the surf spot one must hike, and paddle, over a reef trail right under the cliffs. I was able to observe about 25 cormorants, each guarding its nest, about halfway up the cliff face. The immediate area below it was covered with a whitewash from the bird's droppings. A little further into the cove I was able to observe about 30 more cormorants sitting on top of a large sea stack, which is a geological formation made out of weather-resistant rock. I do not believe that there were nests here. Rather, it is more likely that the P. auritus used this perch as a jumping-off place to go fishing.

Evolution:

Siegel-Causey (1998) took a phylogenetic analysis of the recent taxa of the family Phalacrocoracidae (of which the cormorant belongs). He did this by using "qualitative osteological characters." Siegel-Causey feels that there has been little controversy over which taxa comprised the Phalacrocoracidae. More pertinent is determining the relationships of the family with in Pelicaniformes, and that of the order to the rest of the birds (1988).

Siegel-Causey (1998) found that the family is comprised of two subfamilies. the Phalacrocorax (cormorants) comprise four genera of all-dark, littorine species. The Leucocarboninae (shags) comprise five genera of variably plumaged, littorine, and pelagic species. The relationship of the darters (Anhingidae) to the Phalacrocoracidae remains problematical and unresolved. As far as homoplasy, or parallel evolutionary traits, in the family is concerned, cormorants are characterized by convergences, shags by reversals in the phylogenetic tree. Plumage patterns in the birds apparently have functional correlates, but phylogenetic history will be the overriding factor Siegel-Causey(1998). Which means that phenotypes are not as important as genotypes.

Distribution:

The Double-crested cormorant (Phalocrocorax auritus) is a magnificent and graceful bird. It is widely distributed across the United States. It can also be found in Canada, NW Mexico, the Yucatan peninsula, Belize, Cuba and the Bahamas (Molina, 1999).

According to Thomas (2000), the Double-crested cormorant breeds throughout the St. Lawrence Seaway, the Great Lakes, and central North America. It also maintains year-round populations in Alaska, and on the southwest and southeast coasts of the United States. The wintering range includes the western and southern United States coastlines, and various islands in the Gulf of Mexico. The Double-crested cormorant maintains many colonies throughout the Great Lakes, with the highest numbers of birds being present on Lakes Huron and Ontario. This species is a common to abundant migrant, but is only rarely seen in winter (Thomas, 2000).

Status and range in California: Remsen (1978) notes that the Double-crested cormorant formerly bred on coastal cliffs and offshore islands along the California coast from Marin County south to La Jolla, San Diego County, and in the interior in northeastern California, the Sacramento Valley, the San Joaquin Valley, and the Salton Sea. Recent surveys have revealed breeding populations from Marin County north to the Oregon border (Remsen, 1978). This species also nests along the Colorado River, but nesting sites are on the Arizona side of the river (Remsen, 1978). It has disappeared as a breeding bird from the Sacramento and San Joaquin valleys and the Salton Sea (Remsen, 1978). Tulare Lake in the San Joaquin Valley, where hundreds of nests once were, is now unsuitable as nesting habitat. Similarly, Buena Vista Lake, which is in Southern California, where this species once bred in "immense numbers", is no longer suitable. No nests have been noted at the Salton Sea in many years, although 100 nests were present in one rookery at the Northeastern end alone as recently as 1956. Coastal breeding populations have also declined in southern and central California. This species no longer nests in San Diego Co. and populations on the Farallon Islands once numbered in the thousands in the 19th century but were reduced to 20-40 pairs by the early 20th century (Remsen, 1978).

According to Molina (1999) the P. auritus still occurs quite frequently in California. Although, in 2002 the California Department of Fish and Game lists the Double-crested cormorant as a species of special concern. Molina (1999) also states that it occurs widely in freshwater and marine habitats along coastlines and throughout the interior of the continent. Breeding cormorants are known from 41 of 50 states in the U.S. and among all 10 provinces in Canada. It is strictly a North American species.

In California double-crested cormorants are found over much of the state. They occur along most of the state’s coast, although in Southern California they breed only on a few of the Channel Islands and on the Coronado’s Island in adjacent Baja California Norte (Molina, 1999).

This species is widespread, but a “rare to uncommon transient” throughout the deserts of Southern California (Garrett, 1981). The Double-crested cormorant is considered a Species of Special Concern. One might wonder, what is a Species of Special Concern? According to the Department of Fish and Game’s Habitat Conservation Planning Branch, "Species of Special Concern" (SSC) status applies to animals not listed under the federal Endangered Species Act or the California Endangered Species Act, but which nonetheless 1) are declining at a rate that could result in listing, or 2) historically occurred in low numbers and known threats to their persistence currently exist (CDF&G, 2002). Let us hope that we can all work towards preserving this species for future generations to enjoy.

Abundance Map:
Highest abundance rating in each state for the Double- crested Cormorant in the United States.

Map and abundance legend:

No records in i-bird database.
Recorded as erratic, transient, casual, vagrant, or rare.
Present, but uncommon. Not likely to be seen except by extensive searching over several days.
Common. Usually seen daily in proper habitat and season.
Seen daily in good numbers in several habitats and seasons.

Other interesting issues:

There is more information on the Double-crested cormorant at San Francisco University Geography Graduate Student Doug Johnson's website about Lake Merced, http://bss.sfsu.edu/envstudies/lakemerced/birds.htm. Lake Merced is located adjacent to the University. Cormorants here face some hardships that are described. One issue that has been of concern for decades is potential lead poisoning from submerged lead shot that results from trapshooting over the lake from the Pacific Rod and Gun Club. The significant danger is for diving ducks that eat by straining benthic sediments. If they happen to ingest a lead BB, the shot will dissolve in their gullet and kill the bird. If the bird is ingested by a predator or scavenger, secondary poisoning will occur. Shooters are now required to use non-lead shot, and the lake has been dredged in efforts to remove the old shot (Johnson, 2000).

Johnson (2000) also asserts that shore nesters can be greatly affected by changing lake levels. In recognition of this, current operating plans call for lake levels to be held steady between April 15 and July 15, and for lake levels to be changed at certain maximum rates at other times to allow organisms time to move. Of course, the declining health of aquatic life (also linked to water levels, pollutants, and other factors), a primary food source for many bird species, is significant as well.

Recreation, at Lake Merced in San Francisco, specifically, potentially impacts the integrity of bird habitat. The noise of the Rod and Gun Club and police shooting range may limit nearby bird presence. With all these inhibiting factors, Lake Merced is still one of the most important remaining wild places in San Francisco.

Brooke, Karen and Bruce Birkhead. 1991. The Cambridge Encyclopedia of Ornithology. Cambridge University Press, Cambridge.

Garrett, K. and J. Dunn. 1981. Birds of southern California: Status and Distribution. Los Angeles, CA. Los Angeles Audubon Society.

Kern, Mozzotti, Schaefer and Tanner. 1995. Florida Wildlife Resource Handbook. University of Florida, Florida.

Landsborough, Thomson. 1964. A New Dictionary of Birds. McGraw-Hill Book Company, New York.

Molina, Kathy. (November 12, 1999). Double-crested Cormorant Phalacrocorax auritus Author: PDF. [Online]. Available: http://www.ca.blm.gov/cdd/Dcco1.pdf

Perrins, Luke. 1990. The Illustrated Encyclopedia of Birds. Prentice Hall Press, New York.

Siegel-Causey, Douglas. 1988. "Phylogeny of the Phalacrocoracidae." Condor 90 (4): 885-905.

University of Michigan. (May 2, 2000). Animal Diversity Web. [Online]. Available: http://www.address.edu/

Unknown. 1967. The Larousse Encyclopedia of Animal Life. McGraw-Hill Book Company, New York.

© 2000 International Birding Information Resource Data

Other interesting issues: