The Food Habits of Ducks Wintering on Laguna Madre, Texas
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The food habits and distribution of rehead (Aythya americana), pintail (Anas acuta), and lesser scaup (Aythya affinis) ducks were studied in Laguna Madre, Texas, a hypersaline lagoon well known for wintering redhead populations. The availability of major duck foods in the lagoon was also studied. Shoalgrass (Diplanthera wrightii) made up 84 and 88 percent by volume of the representative diets of redheads and pintails. Lesser scaup, on the other hand, showed a predilection for animals foods, chiefly molluscs and crabs. The singularity of shoalgrass in the diet of redhead and pintail ducks results from its particular abundance in Laguna Madre, nearly to the exclusion of any other rooted aquatic plants. Pintails and redheads were distributed in the shallower waters. Pintails fed mostly in water six to 10 inches deep along the shorelines and spoil banks dug from the Intracoastal Waterway which bisects the lagoon. Redheads fed primarily in open shallow water sites, six to 24 inches deep, and in the flats in the lee of Padre Island, the east boundary of the lagoon. Redheads shifted from feeding in very shallow, easily accessible shoalgrass stands early in the season to unused shoalgrass stands in deeper water late in the season. Although redheads and pintails have similar food habits, they were not considered competitors for food in that they usually occupied different sites. Ecologic separation was clearly seen between these two species and lesser scaup, wchi fed chiefly on animals in broad expanses of open water three to seven feet deep. Alterations detrimental to the limited shoalgrass habitat in Upper Laguna Madre, such as its removal by digging or dredging operations, may reduce carrying capacity there and tend to concentrate ducks in areas not so accessible to hunters. Also, permanent lessening of salinity norms in Laguna Madre, through the construction of passes from the Gulf of Mexico, may reduce shoalgrass biomass through the increasing dominance of hypersaline intolerant plant species, some of which are unpalatable to ducks. This occurrence would also disperse ducks from the affected areas.