Changing populations of Double-crested Cormorants

The Double-crested Cormorant (Phalanocorax auritus) has a wider range in North America than any of the five other breeding cormorants and is the only species to occur extensively in the interior as well as on the coasts. Breeding birds are currently known in 40 of the 50 States of the United States, all 10 Canadian Provinces, and in Mexico, Cuba and the Bahamas. There are six more-or-less distinct populations, and five subspecies have been described but their significance is unclear because measurements overlap substantially and the distributions of crest characters are poorly known. Scattered archeological and historical accounts indicate that numbers in the Interior and Atlantic populations declined during the 19th century, and subsequent increases have been facilitated, but not fully explained, by dam-building and by declines in persecution. Numbers increased front the 1920s into the 1950s when pesticides had major impacts. The Interior populations fell to low points about 1970 and the Atlantic population ceased growing. The species was recognized as of ''Special Concern'' in several states during this period. For the past 20 years numbers have been increasing rapidly in most areas, although information on nonbreeding birds is very sparse. Particularly large increases are reported for the subspecies auritus, both the Interior population centered in the Canadian Prairie Provinces and Great Lakes (220,000 pairs) and the Atlantic population on the Northeast Coast (96,000 pairs). These birds migrate to winter in the South, chiefly in coastal areas from Texas to North Carolina, where there is considerable overlap of birds from diverse breeding areas. The numbers of the resident populations in Florida and the Caribbean comprising subspecies floridanus and heuretus (14,000 pairs) are poorly known and are probably declining. The status of the small population in Alaska (cincinatus) (3,000 pairs) is not known. On the West Coast (albociliatus) (31,000 pairs) the numbers in Canada and the United States are increasing rapidly, but the status of Mexican populations is unclear. The migrations of the populations in the Pacific are not well characterized.