The Fulvous Whistling-Duck (formerly Fulvous Tree Duck) is one of the most widely distributed species of waterfowl in the world, occurring mostly in tropical and subtropical regions but also in temperate areas of the Americas, Africa, and Asia. This medium-sized duck expanded its distribution northward into the southern United States beginning in the mid- to late nineteenth century, becoming established in California and rice (Oryza sativa)-growing regions of the U.S. Gulf Coastal Plain in the early to mid-twentieth century. Erratic wanderings by individuals after the nesting season, especially since the 1950s, have produced numerous reports of vagrants in the Mississippi River Basin, eastern Great Lakes region, and along the Pacific and Atlantic Coasts to southern Canada. U.S. breeding distribution, however, remains restricted to the Gulf Coastal Plain of Texas and Louisiana and localities in southern California and south- and east-central Florida.
Fulvous Whistling-Ducks are migratory in northern portions of their range, but elsewhere they exhibit only local movements. Their return to Gulf Coast nesting areas in February and March coincides with the onset of planting in rice-growing areas. Individuals feed almost exclusively on seeds, especially seeds of native moist-soil plants, which may be abundant in flooded ricefields. They also feed opportunistically on grain in seeded ricefields. Flooding of ricefields in preparation for planting stimulates ground-nesting by birds on ricefield levees and in pastures, haylands, and small grain fields adjacent to ricefields. More commonly, however, Fulvous Whistling-Ducks nest in flooded ricefields when plants are of sufficient stature to support eggs. The species makes only limited use of undisturbed, nonagricultural habitats for nesting in the United States.
Whereas most ducks nesting in temperate regions pair seasonally for only 1 reproductive cycle, pair bonds of Fulvous Whistling-Ducks are suspected to be long-term, extending over multiple nesting seasons or the lifetime of individuals, as in geese and swans. Males and females share equally in incubation, and males also assist with rearing of young. Thus, males contribute to parental care to a greater extent in this species than in other small- to medium-sized waterfowl. Intraspecific brood parasitism is widespread in ground and over-water nests. Additionally, unlike other North American waterfowl that undergo wing molt immediately after the nesting season on breeding areas or at remote northern sites traditionally used for molt, onset of wing molt is delayed in migratory Fulvous Whistling-Ducks until their return to southern wintering areas.
Limited survey information available for this species suggests that Fulvous Whistling-Duck numbers in the United States have fluctuated dramatically since the early twentieth century, when breeding was first documented. Factors possibly contributing to these fluctuations include irregular movements by the species, pesticide contamination, habitat loss and degradation, disturbances associated with agricultural practices, changing agricultural practices, and hunting. In California in the early to mid-1900s, this species commonly bred in southern coastal areas from the San Joaquin Valley to the southern end of San Francisco Bay, and in marshes in southeastern deserts. By the late 1970s, however, breeding was restricted to the Imperial Valley, and in the 1990s, fewer than 5 pairs were regularly observed at remaining nesting locales in the Imperial Valley. Except for Caliornia, however, U.S. breeding populations of Fulvous Whistling-Ducks currently are thought to be stable or increasing. The species' abundance outside of the United States is poorly known.
This species is perhaps the least studied of common North American waterfowl. To date, studies have been conducted primarily in agricultural regions of North and South America at the fringes of the species' breeding distribution and have focused mostly on the duck's importance as an agricultural pest ( Meanley and Meanley 1959 , Mccartney 1963 , Flickinger and King 1972 , Bruzual and Bruzual 1983 , Dallmeier 1991 , Hohman et al. 1996 , Wyss 1996 , Peris et al. 1998 ). Consequently, many aspects of the species' behavior, movements, life history, and demography are unknown or known only from captive studies or studies conducted outside of core breeding and wintering areas.