Nesting Interference in a Dense Population of Wood Ducks

Abstract

A breeding colony of wood ducks (Aix sponsa) was built up over a 9-year period by erecting nesting boxes along a small slough in the Sacramento Valley. The number of breeding pairs increased faster than boxes could be added, which led to nesting interference, compound nesting, nest desertion, and generally inefficient reproduction. Effective production of young per pair was inversely proportional to density, and in the latter years the population became essentially self-limiting. Lack of territorial defense of the nest site by an established pair was primarily responsible for the inefficiency of nesting in a high density population. This study was conducted on the Cattail Duck Club occupying a 28-acre marsh on the Coburn Haskell ranch 10 miles south of Marysville, Yuba County, California. The marsh is a sump, receiving drainage water from surrounding irrigated croplands and pastures. Continuous flooding during the summer growing season encourages the growth of dense stands of cat-tail (Typha latifolia) and locally of round-stem bulrush (Scirpus acutus) which occupy much of the basin. Five shooting ponds have been opened and maintained by hand-cutting the cat-tails and bulrushes with scythes (see Fig. 1). The sump drains through a galvanized culvert into a flood bypass adjoining the duck marsh on the northwest. Channels in the bypass, which carry water throughout the year, are bordered by willow and poplar trees averaging 40 ft in height. In 1957 three wood duck nest boxes were ounted in trees along the bypass. Although no wood ducks had been seen previously, two pairs appeared and shared occupancy of one box. Encouraged by this sign of interest, members of the club added three more boxes in 1958, and five of the six boxes were used by ducks. In subsequent years new boxes were added both along the bypass and at the upper end of the sump near the clubhouse, until 16 boxes in all had been erected. Box locations designated by numbers, are shown in Fig. 1. From 1957 to 1961, Leopold and W. W. Middlekauff, partners in the hunting enterprise, recorded the breeding activities of wood ducks, mostly during weekends in spring and early summer, and tallied the box contents after breeding terminated in late summer. In the years 1962-64, Jones undertook a more intensive study of the