Illinois Natural History Survey - University of Illinois

Tree Cavity Abundance and Use by Nesting Wood Ducks 

At the turn of the century, the wood duck (Aix sponsa) was considered by many to be on the brink of extinction, and as a result, wood ducks were granted full protection during the 1916-1940 waterfowl hunting seasons. However, the comeback of the wood duck to current abundant population levels rivals that of the white-tailed deer, giant Canada goose, and the wild turkey.

In recent years, wood ducks have been the most common nesting duck in Illinois. Wood ducks also currently rank second in the number of ducks harvested in Illinois and have been second in the harvest of ducks in the Mississippi Flyway for most of the last 33 years. Although wood duck populations in Illinois, the Mississippi Flyway, and the Atlantic Flyway appear to be increasing, little is known of wood duck production and use of natural tree cavities in Illinois. No recent studies have determined the importance of natural tree cavities for nesting wood ducks in Illinois.

A major problem in wood duck management has been the inability to estimate the size of populations because of the species inhabitation of densely forested areas and its secretive nature. Estimation of wood duck population size and trends has been an important management goal in Illinois and the Mississippi Flyway because aerial surveys of this species have proven inadequate. In 1993, states in the Atlantic and Mississippi flyways, along with the U.S. Fish and Wildlife Service, developed a wood duck management strategy to outline databases needed to effectively manage wood duck populations. One specific objective of this strategy was to develop techniques that would provide estimates of regional breeding population sizes. Therefore, information on the size of the regional wood duck breeding population was necessary to enhance management of this species endemic to North America.

Our study on the abundance of natural cavities suitable for nesting by wood ducks, their nesting use by wood ducks, and success of the nests was an important step in estimating wood duck population size and growth in Illinois.

Wood Duck (Aix sponsa)

The study area encompassed portions of southwest Mason, northwest Cass, and eastern Schuyler counties and included 3,835 ha (ha = hectare, which = 2.5 acres) of the state-owned Sanganois Conservation Area. Sanganois is a floodplain forest at the confluence of the Illinois and Sangamon rivers and is considered to be one of the least disturbed bottomland areas along the Illinois River.

Cavity densities suitable for wood duck nesting at Sanganois were 2.12 cavities per ha, or an estimated total of 4,577 natural cavities in the 2,159 ha of forested wetland habitat on the study area. Eighty cavities were inspected for wood duck use during July 1994. Ten (12.5%) cavities were used for nesting by wood ducks yielding a wood duck nest density at Sanganois of 0.206 nests/ha. Of the 10 wood duck nests located in natural cavities, five were destroyed by raccoons, three hatched successfully, one was abandoned for unknown reasons, and the fate of one nest could not be determined. Thus, a simple estimate of nest success in natural cavities was 33.3%.

The density of cavities suitable for nesting by wood ducks at Sanganois was relatively high when compared with other studies. Only three previously reported surveys indicated higher densities of natural cavities. Studies in New York, Minnesota, and New Brunswick reported 3.95 - 5.50 suitable cavities/ha of forested habitat. As expected, suitable cavity densities at Sanganois (2.12 cavities/ha) were more comparable with densities found in Indiana (1.23 cavities/ha) and Kentucky (1.26 cavities/ha) because cavity densities tend to be greater in the northern states than in the southern states. Also, tree species prone to cavity formation in the north are less common in the south, and tree injuries heal more quickly in the southern states, thereby reducing cavity formation rates. Cavity densities in other geographic regions ranged from 0.075 to 0.67 cavities/ha.

Tree cavity used by wood duck for nest. Photo by Steve Havera.

A wood duck natural cavity survey of bottomland forest near Sanganois in 1944 revealed 0.15 suitable cavities/ha. Upland black oak woodlots in central Illinois contained 0.51 suitable wood duck nesting cavities/ha during 1938-1961. These previous surveys in central Illinois revealed much lower densities of cavities suitable for nesting by wood ducks than our survey indicated. The increased density of natural cavities along the Illinois River probably represented the increased age and maturity of the forest stand along with a change in the bottomland forest composition to better cavity producing tree species, such as silver maple and black willow.

Continued monitoring of natural cavities at Sanganois during summer 1995 will provide more information on the importance of natural cavities for nesting wood ducks in Illinois River bottomlands.

Aaron P. Yetter, Christopher S. Hine, and Stephen P. Havera, Center for Wildlife Ecology 

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Subject: INHSPUB-2179
Last Modified 3/19/96

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