Reproductive Success and Population Increase of Black-Bellied Whistling Ducks (Dendrocygna autumnalis) in Newly Placed Artificial Nests in a Tropical Freshwater Marsh

We studied the reproductive success of Dendrocygna autumnalis in artificial nests during 1986–90 and the resulting population increase in Palo Verde National Wildlife Refuge, Costa Rica. We estimated the pre-1986 nesting population in the 500-ha, seasonal freshwater marsh at about 10 pairs before placement of artificial nests. We placed 34, 29, 30, 44, and 55 nesting boxes, respectively, each yr on 2.5-m posts in both exposed and lightly forested areas at the marsh edge. We counted the number of eggs laid, hatched, or depredated once every two wk during the reproductive season (May-November). Nesting activity increased from 8 clutches in 1986, to 14 in 1987, 36 in 1988,49 in 1989, and 42 in 1990. Egg success varied from 13 to 66%, with lower values being more typical. Boas (Boa constrictor) and raccoons (Procyon lotor) were important predators. The number of dump nests increased from 1 in 1986 (12.5% of all nests), to 3 in 1987 (21.4%), to 22 in 1988 (61.1%), and to 37 in 1989 (76%). The nesting population increased over 5 yr to about 200 individuals in 1990. High depredation losses in 1986 were attributable to poor design of anti-predator guards. Improved guards worked well in 1987-88. Increased nest abandonment in 1988 was attributed to increased dump nesting. Because of this, we tested the effect of partially harvesting eggs in dump nests in 1989. Nesting efficiency in dump nests could be increased to 80% in this manner. Thus, we believe abandonment of unharvested dump nests by the highly sociable black-bellied whistling duck may be due to a visual stimulus of“too many eggs”rather than physical interference to the incubating pair, as suggested recently for Aix sponsa. Nest boxes can be effective in increasing local nesting population densities for this species, up to a point where nests should either be partially harvested or, perhaps, placed in denser cover as recently suggested for Aix sponsa to reduce dump nesting.