Ecology and behavior of black-bellied whistling-duck broods in South Texas
Heins, Marianna Newton
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Black-bellied whistling-duck (Dendrocygna autumnalis) broods were observed in south Texas during 1980 and 1981 to quantify diurnal activity budgets and visibility, progressive mortality, and habitat use and preference. Observations of brood activities were evaluated by age-class and time of day. Broods were visible an average of 31% of each day, with no differences between age-classes. Peak visibility occurred in early morning and late evening, with a slightly lower mid-day peak. Parents were least visible in mid-morning. Few between-class differences were found in the amount of time spent in any activity. Feeding was the most common activity, occupying 62% of a brood's day. Increasing periodicity of activity with increasing age of a brood was evident. This likely was due to developmental differences that require younger ducklings to feed more frequently, resulting in an evenness of daily activity. Parental time budgets generally coincided with that of their broods. Parent black-bellied whistling-ducks spent 60% of their day in attentive behavior. Both parents were absent from their brood 29% of the day. Average brood size was nine ducklings, ranging from eleven in Class Ia to seven in Class III. Six juveniles/adult pair fledged. Broods suffered a 45% mortality rate, but this estimate is conservative as losses of entire broods were not included. Major causes of death appeared to be (1) abandonment at the nest, (2) overland travel, (3) predation, and (4) inclement weather. Broods fed primarily in water lacking emergent vegetation. Feeding methods were dabbling on the surface and diving. Ducklings of all age-classes readily submerged completely to obtain food. Brood use of rearing ponds was restricted to water depths <100 cm and to zones of emergents interspersed with open water. Dense stands of lotus, bulrush, and cattail were avoided, as were large expanses of deep, open water. Two broods hatched adjacent to water, but their parents led them 0.6 km and 3.9 km to different ponds. Analysis of habitat parameters revealed that the two ponds with high use by broods were smaller, shallower, and had more shoreline/ha of surface water than the two ponds with low brood use. Amount and types of vegatation did not differ significantly between high-use and low-use ponds. Management recommendations for black-bellied whistling-duck broods in Texas include (1) censusing broods in early morning or late evening to achieve greatest accuracy and time-efficiency, (2) erecting nest boxes near suitable brood ponds to minimize mortality resulting from overland travels, and (3) creating desirable brood habitat in the vicinity of nest sites by building or modifying stock ponds to (a) be at least 1 ha with side slopes of 5:1 or greater and with irregular shorelines; (b) restrict livestock access to one segment of shore; (c) encourage aquatic plant development; and (d) provide loafing sites.