Introduction

The Neotropic Cormorant, known until recently as the Olivaceous Cormorant, has also been called the Mexican Cormorant and Brazilian Cormorant (Banks 1988). These former names are misleading and unduly restrictive since this bird is essentially dark purplish as an adult and ranges over much of the western hemisphere. Although this is one of the most widely distributed of cormorants, and is remarkably versatile in its use of habitat, many aspects of its life history remain poorly known and in need of study.

The breeding range of this species extends from the sub-antarctic coast of Cape Horn, South America, northward to n.-central Texas, se. Oklahoma, sw. Arkansas, sw. Louisiana, s. New Mexico, and s. Arizona. The species’ spring through winter wandering in North America is widely scattered: westward into California, northward into Saskatchewan, and eastward into Pennsylvania. Within this vast range, it occurs from sea level to lakes (in the Andes) at elevations up to 5,000 meters.

This cormorant is tolerant of all but close and disruptive human activities. Unlike other North American cormorants, it is highly adaptive, occupying diverse habitats and climates. It perches on slender twigs and even wires and prefers to nest in trees.  Although the Neotropic Cormorant feeds primarily by pursuit-diving from the water’s surface, as is typical of other cormorants, it is the only cormorant also known to feed by plunge-diving; this may enable it to exploit fishes not available to its congeners.

This species’ distinctive piglike voice is responsible for some descriptive but uncomplimentary Spanish names, among them Pato cerdo, “pig duck”; Pato puerco, “filthy duck” (i.e., dirty like a pig); Pato chancho, “unclean duck”; and Pato gruñón, “grunting duck” (Birkenstein and Tomlinson 1981).

In the 1960s, the U.S. population of the Neotropic Cormorant declined severely for reasons not well understood but perhaps related to coastal development or persistent pesticides. Since 1970, this population has fluctuated but increased steadily. 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 of inland colonies quite distant from the coast.

Compared to the Double-crested Cormorant, for which there is extensive information, the Neotropic Cormorant is little known. Prior to the work of Morrison and colleagues in the late 1970s, most aspects of the biology and ecology of this species remained unstudied; Morrison et al.’s work on the upper Texas coast provided baseline information about diet, feeding behavior, reproduction, population trends, and effects of pollutants (Morrison and Slack 1977a, 1977b; Morrison et al. 1977, 1978a, 1978c, 1979, 1983). Subsequent work of King and Krynitsky (1986) and King (1989) provided more data about food habits and organochlorine contaminants. Beginning in the early 1980s, a long-term study of an inland population by Telfair and colleagues has yielded additional information about diet (Telfair et al. 1982) and population dynamics (Telfair 1995).