Case Studies: Ducks
Nest box placement in wood ducks
Wood ducks (Aix sponsa) were threatened with extinction early in the 20th century but recovered after market hunting was outlawed. Because logging had also destroyed much of the cavity-nesting wood duck's nesting habitat, nest boxes became a popular management tool that persists to this day. Nest boxes were typically placed on poles over open water where they were easy to check and inaccessible to many predators. High egg counts were considered evidence of the nest boxes' success, however, managers noted that clutches often contained eggs from several females.
In 1985, wildlife biologist Brad Semel and animal behaviorist Paul Sherman teamed up to study wood ducks as an example of dump-nesting, or intraspecific brood parasitism. Semel and Sherman (1986) documented a parasitism rate as high as 95% in nest boxes on a reservoir in Missouri. Parasitism resulted in a single female attempting to incubate as many as 37 eggs. Hatching success was reduced in parasitized nests due to abandonment, broken eggs and eggs laid after incubation. Clearly, large numbers of eggs did not equate with large numbers of ducklings.
Because there were many empty boxes that had been used successfully in previous years, Semel and Sherman concluded that dump-nesting was not a response to a lack of suitable nest sites. Rather, the high visibility of nesting activity and the high density of nest boxes in the marsh attracted parasitic females. In the absence of nest boxes, wood ducks nest in natural cavities in trees in the forest away from the water, where reduced sight distance and increased environmental complexity make active nests less conspicuous. Nest boxes erected in more natural circumstances (at a low density throughout the forest) experienced decreased parasitism and increased hatching success relative to marsh nest boxes (Semel et al. 1988).
As a result, wildlife managers began to move nest boxes from poles in the marsh onto tree trunks in nearby forests with great success (Hope 1999). While wood ducks are no longer an endangered species, this research holds important lessons for the management of other cavity nesting birds and birds subject to inter- or intraspecific brood parasitism (e.g., Eadie et al. 1997). It also suggests that high rates of brood-parasitism in behavioral studies of hole-nesting passerines should be interpreted with caution when nest boxes are involved (e.g., bluebirds, tree swallows). But, by the same token, research involving nest boxes plays an important role in elucidating the effects of nest densities and placement on aspects of a species' social dynamics (e.g., intraspecific brood parasitism, territoriality).
Eadie, J. M. 1991. Constraint and opportunity in the evolution of brood parasitism in waterfowl. Acta Congr. Int. Ornithol. 20: 1031-1040.
Eadie, J., Sherman, P. and Semel, B. 1998. Conspecific brood parasitism, population dynamics, and the conservation of cavity-nesting birds. In: Caro, T. (ed.). Behavioral Ecology and Conservation Biology. Oxford University Press, Oxford. pp. 306-340.
Hope, J. 1999. Breaking out of the box. Audubon 101(3): 86-91.
Semel, B. and Sherman, P. W. 1986. Dynamics of nest parasitism in Wood Ducks. Auk 103: 813-816.
Semel, B., Sherman, P. W. and Byers, S. M. 1988. Effects of brood parasitism and nest-box placement on Wood Duck breeding ecology. Condor 90: 920-930.