The Neotropic Cormorant, formerly Olivac- eous Cormorant, is a neotropical species that is one of the most widely distributed cormorants – ranging northward from Tierra del Fuego to the southern United States. It is the only cormorant inhabiting the entire tropical American region. Breeding is restricted to coasts, lakes, and reservoirs. Until 1972, all known breeding colonies in the U.S. were along coasts of Texas and Louisiana, >75% of these between Galveston Bay and Sabine Lake on the extreme upper coast of Texas (Morrison 1977, Portnoy 1977, Clapp 1982). Elsewhere the first noted breeding occured in south-central New Mexico in 1972 (Hundertmark 1974), southwest Arkansas in 1996 (Purrington 1996, Coldren et al. 1998), southeast Oklahoma in 2001, perhaps 2000 (W. David Arbour and Matt White pers. comm.), and south Arizona in 2004 (Stevenson and Rosenberg 2004). Since the mid-1970s, colonies have been established in reservoirs, some far into the interior of Texas (Zinn 1977, Telfair 1980, 1995) and, since the 1990s, colonies have developed inland in Louisiana (Cooley 2002, Green et al. 2006). These cormorants are sedentary breeders throughout most of their breeding range, with wide-spread spring through winter wandering: westward into California, northward into Saskatchewan, and eastward into Pennsylvania. Sightings have been recorded for California, Nevada, Colorado, Saskatchewan, South Dakota, Nebraska, Kansas, Minnesota, Iowa, Wisconsin, Illinois, Tennessee, Mississippi, Pennsylvania, and Maryland (DeSante and Pyle 1986, Am. Ornithol. Union 1998, Seasonal reports, Am. Birds 1980-1994, Field Notes 1994-1998,, North America Birds 1999-2006, Telfair and Morrison 1995, 2005). In the non-breeding season, most of the population of the U. S. is found along the Gulf Coast from southwest Louisiana south throughout the Texas coast.
Texas records from 1883-1973 indicate that Neotropic Cormorant nesting colonies were located mostly on the upper coast (Oberholser 1974). During the 1960s, its population declined severely. Reasons for this decline are not documented; but, may involve coastal development since World War II (Oberholser 1974) and persistent pesticides (King 1989). Since 1970, its population has fluctuated (Morrison et al. 1983); however, the overall trend has been a steady increase. Two major population changes have occurred: 1) increases in total population, number of breeding colonies, and size of breeding colonies; and, 2) establishment of new nesting colonies along the coast and some inland colonies quite distant from the coast (Morrison and Slack 1977, Zinn 1977, Telfair 1980, Seasonal reports, Am. Birds 1980- 1994, Morrison et al. 1983, Telfair 1995, Telfair and Morrison 1995, 2005).
DISTRIBUTION: Texas Colonial Waterbird Census Summaries (Tex. Parks and Wildl. Dept./Tex. Colonial Waterbird Soc. 1981-2004) and the TBBA reveal that most Neotropic Cormorants breed on the upper coast, but breeding colonies occur along the entire coast and are scattered inland in reservoirs within 4 ecoregions: East Texas Pine/Hardwood Timberlands, Post Oak Savannah, Rolling Plains, and Rio Grande Plains. Most inland colonies are associated with heronries, especially those containing Cattle Egrets (Bubulcus ibis). Number and distribution of breeding colonies by geographic region since 1973 (Mullins et al. 1982, Texas Parks and Wildl. Dept./Texas Colonial Waterbird Soc.1981-2004, TBBA 1987-1992) are: 78 coastal (64 upper, 13 central, 1 lower) and 39 inland (11 northeast, 16 southeast, 8 south, 2 north-central, and 2 west). Of the total breeding population in Texas in 1990, 87% were coastal, 13% were inland; by 2004, the ratio had reversed (27% coastal, 73% inland).
Since 1981, 1,832 banded/color-marked Neotropic Cormorant chicks have fledged at Cedar Creek Islands Wildlife Management Area (Cedar Creek Reservoir, Henderson County, Texas, latilong 32096-quad C2). Although band recoveries (9, 0.5%) and post-breeding sightings of color-marked juvenile/immature birds (17, 0.9%) are few, dispersal of most young birds is apparently not far from the natal colony (up to 50 km [31 mi]). However, one 4 month old bird was seen at Millwood Lake in southwest Arkansas (259 km, 161 mi). Three other sightings and one band recovery of immature birds indicate a coastward fall/winter movement from inland areas may occur during which birds may move south 233-721km (145-448 mi), thus supporting the observation of Oberholser (1974) that few birds remain above 27oN latitude during winter (1 sighting of an immature bird at Marlin, Falls County, January.-February., 1995). This distribution is reflected in National Audubon Society Christmas Bird Counts; between 1957-1958 to 1969-1970, 99% of the population was coastal, 1% inland; however, between 1970-1971 to 2005-2006, 83% was coastal (46-99% annually), 17% inland (1-54% annually), but few inland birds were above 30ºN.
SEASONAL OCCURRENCE: The breeding season for this species is very long (Oberholser 1974, Arnold 1987, Lockwood et al. 2002) extending from early January (1st eggs) to late December (last fledglings); peak breeding occurs during May-July. Inland, at Cedar Creek Islands Wildlife Management Area, Henderson County (latilong 32096-quad C2), some years there may be a second brood in the fall (Telfair and Morrison 2005).
BREEDING HABITAT: In Texas, the Neotropic Cormorant breeds in single or multi-species colonies. Single pairs may nest alone. Colony size may vary from 2 to 2500 pairs. Nesting is predominantly coastal in living or dead trees and shrubs or on duck blinds. Inland nesting occurs in swamps and reservoirs in living or dead trees and shrubs. Nests are coarse platforms of sticks 1-6 m (3-20 ft) above ground or water. Nest lining consists of twigs, green leaves, grass or seaweed. Nest material may be cemented together with guano.
STATUS: Until 1972, all known breeding colonies within the United States occurred along the coasts of Texas and Louisiana, >75% of these between Galveston Bay and Sabine Lake on the extreme upper Texas coast (Clapp et al. 1982). After a severe population decline in the 1960s to a low of about 14 pairs in 1967, the species has increased and established new coastal and inland colonies since 1973. Although the population fluctuates annually or biennially, the overall trend is an increase with a population of about 3700 pairs (1000 coastal, 2700 inland) in 2004 (Telfair and Morrison 2005). Until 1973, with the exception of two records (1936 in Colorado County and 1941 in Travis county), breeding colonies were coastal, especially on the upper coast (Oberholser 1974). The winter population has increased about 11% per year (1961-2005) and the breeding population has increased about 15% (1967-2004). However, the breeding population has developed two phases, from 1967-1987 a somewhat cyclic though rapidly increasing acceleration (28.85% annually); and, from 1987-2004), a more pronounced cyclic, though slight deceleration (-1.12% annually), (Telfair and Morrison 2005).
Of a total of 117 colonies, only 8 (6.8 %) have been consistently reestablished annually for more than 6 years: 2 inland colonies (Telfair Island Cedar Creek Islands Wildlife Management Area, Cedar Creek Reservoir, Henderson County (latilong 32
096-quad C2), since 1976) and Bird Island, Lake Fork Reservoir, Wood County (32095-G5), 1996-2004; and 6 upper coast colonies (Vingt-et-un Islands, Chambers County (29094-E7), 1967-1990; Sydney Island, Orange County (29093-H7), 1971-82; North Deer Island (29094-C8 ), 1975-1985 and 1994-2003; and Rollover Pass (29094-E5), Galveston County, 1981-1990; Willow Slough, Jefferson County (29094-G2), 1983-1992; and Trinity River Mouth, Chambers County (29094-G7), 1998-2004. Among coastal breeding colonies, about 15-43% are active each year; 53% are used for no more than one consecutive year, 38% are used for 2-6 years, and only 9% are used annually for a period of 7-24 years (Telfair and Morrison 2005). Furthermore, the location of colonies does not appear to have a discernible pattern, either spacially or temporally. However some of their locations, especially inland may reflect large heronries that may serve as “beacons” to successful breeding colonies. The large noisy heronries, especially those containing Cattle Egrets, may also serve as deterrents to predators (Telfair 1980, Forbes 1989). Neotropic Cormorants are able to adapt to diverse climates and environments (del Hoyo et al. 1992).
Inland colonies are not subject to most of the adverse factors that affect coastal colonies, e. g. erosion, starvation, predation, human disturbance, contaminants and marine storms (Morrison et al. 1978 and 1979, King 1989, Cain 1993). Coastal colonies may be subject to population cycles driven by the climatic effects of El Niño years which affect food fish availability (Walton and Green 1883).
Text by Raymond C. Telfair II (2006)
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