Recruitment of black-bellied whistling-ducks in South Texas with special reference to the use of nest boxes

In light of an increasing population of black-bellied whistling-ducks and associated nest-box numbers, a study was conducted to measure the contribution of young produced in nest boxes to total production at key breeding areas in south Texas. Nest boxes at five locations throughout south Texas was monitored for two nesting seasons to determine the effect, if any, on production. Although nesting effort was high, rate of nest loss and measurements of the contribution of box-produced young indicated poor performance from nest boxes. A total of 8084 eggs in 266 nests resulted in 79 successful nests yielding 1395 hatchlings. Poor hatching rates, high progressive brood mortality and large numbers of young produced from other kinds of nests (e.g., ground nests) reduced the significance of nest boxes as a management tool. The annual recruitment rate from nesting in boxes was estimated at 1.75-1.99 young per nesting pair. Brood surveys conducted at nest-box study areas did not yield brood densities higher than densities at the headwaters of Lake Corpus Christi, an area without nest boxes.

The data indicated that proper predator deterrents, reduced competition for nest boxes, and proper density and location of boxes can increase the efficiency of a boxmanagement program.

Concentrations of black-bellied whistling-ducks were monitored following the nesting season to examine the social structure of flocks. Feeding ducks were observed at bait sites to determine if whistling-duck family units, unpaired adults, and orphaned fledglings arrive at, and approach, bait differentially. Age ratios of feeding birds changed dramatically as the "time on bait" progressed. The proportion of young in the feeding group was greatest at the onset of feeding activities (x = 75% juveniles). Four rocket-net captures showed similar changes in age ratios (i.e., as "time on bait" increased, the percentage of juveniles in the samples decreased). Family units most often arrived at bail first and defended the feeding area against other arriving birds.

Juvenile black-bellied whistling-ducks captured with rocket nets showed a slight preponderance of females but did not differ (p > 0.05) from a 50:50 ratio.