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 t he coasts. Breeding birds are currently known in 40 of the 50 States o f the United States, all 10 Canadian Provinces, and in Mexico, Cuba an d the Bahamas. There are six more-or-less distinct populations, and fi ve subspecies have been described but their significance is unclear be cause measurements overlap substantially and the distributions of cres t characters are poorly known. Scattered archeological and historical accounts indicate that numbers in the Interior and Atlantic population s 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 1990s into the 1950s when pesticides had major impacts. The Interior populations fell to low po ints about 1970 and the Atlantic population ceased growing. The specie s was recognized as of ''Special Concern'' in several states during th is period. For the past 20 years numbers have been increasing rapidly in most areas, although information on nonbreeding birds is very spars e. Particularly large increases are reported for the subspecies auritu s, both the Interior population centered in the Canadian Prairie Provi nces and Great Lakes (220,000 pairs) and the Atlantic population on th e Northeast Coast (96,000 pairs). These birds migrate to winter in the South, chiefly in coastal areas from Texas to North Carolina, where t here is considerable overlap of birds from diverse breeding areas. The numbers of the resident populations in Florida and the Caribbean comp rising subspecies floridanus and heuretus (14,000 pairs) are poorly kn own and are probably declining. The status of the small population in Alaska (cincinatus) (3,000 pairs) is not known. On the West Coast (alb ociliatus) (31,000 pairs) the numbers in Canada and the United States are increasing rapidly, but the status of Mexican populations is uncle ar. The migrations of the populations in the Pacific are not well char acterized.