PARASITISM IN NATURAL CAVITIES VS. NEST BOXES

Intraspecific nest parasitism (dump nesting) is a normal behavior observed in nesting wood ducks, in addition to more than 40 other species of waterfowl worldwide. Much of the research on dump nesting has focused on artificial cavities (nest boxes) because of the relative ease of observing this behavior in the controlled environment that nest boxes provide.

Research over the last 50 years has shown that large disparities exist between rates of parasitism in nest boxes versus natural cavities (7-26% of nests in natural cavities are parasitized vs. 32-95% of nests in nest boxes), but are those differences real or just a result of the way in which we have previously studied this behavior? Two different methodologies have been used to document nest parasitism. The first method used to identify dump nests is based on total clutch size, where nests that contain eggs above a certain threshold are considered parasitized (often >14 eggs). The other common method used to identify dump nests is documenting the egg laying rate, where if more than one egg is deposited in a nest for each day of the laying period then the nest is considered parasitized. Bias exists for both of these approaches, but they remain the primary way in which waterfowl managers document this interesting behavior.

Recent research by wildlife investigators in Illinois sought to clarify apparent rates of dump nesting behavior in natural cavities by utilizing genetic techniques that promise to be more precise than previous approaches. By using DNA extracted from eggs and hens in natural cavities, researchers were able to accurately identify host and parasite eggs contributing to a nest. Results of this research shed some new light on this interesting nesting strategy in a natural environment. Out of all nests monitored over a three year period, 85% were considered parasitized using this new methodology versus 26% or less using older methods. This rate of nest parasitism was more than three times greater than previously thought to occur in natural cavities.



Another distinction this study made was the difference in the intensity of parasitism between different habitat types. A large difference in the degree of dump nesting was observed between two habitat types examined: floodplain (fragmented forest with large openings) versus upland forests (continuous tracts of forest with widely scattered openings). Documentation showed more hens parasitizing nests in the floodplain habitat, which may have resulted from potential nests being more visible due to the openness of the habitat and higher densities of females using these areas.

So what does this all mean? This research emphasizes the fact that intraspecific nest parasitism is a natural behavior in wood ducks and is not necessarily a remnant of our management of wood ducks through nest box placement, although our management schemes can sometimes exacerbate this behavior. Higher levels of dump nesting observed in floodplain habitats versus upland forests is comparable to the differences we observe in nest boxes placed in these same habitats. These researchers recommend that managers interested in increasing or maintaining wood duck populations should measure the intensity of dump nesting (e.g., clutch sizes) rather than the rate of parasitism (e.g., number of nests parasitized) in addition to dispersing nest boxes to decrease the intensity of dump nesting.

To learn more you can access this research article:

Roy Nielsen C.L., Gates R.J., Parker P.G. (2006) Intraspecific Nest Parasitism of Wood Ducks in Natural Cavities: Comparisons with Nest Boxes. Journal of Wildlife Management: Vol. 70, No. 3 pp. 835–843