Factors affecting the length of incubation in upland nesting dabbling ducks

In general, waterfowl incubation periods have been reported as single values. However, there are a growing number of studies which suggest that variable incubation periods within a species may be the norm for waterfowl, and that this variation may be correlated with other life-history factors. In this study, I examined variation in length of incubation, clutch size, and nest initiation date for 297 nests of Blue-winged Teal (Anas discors), Mallards (A. platyrhynchos), Gadwalls (A. strepera), Northern Pintails (A. acuta), and Northern Shovelers (A. clypeata) in northeastern North Dakota during 1995 and 1996. In addition, I conducted a manipulative experiment to directly assess the relationship between clutch size and incubation period in Blue-winged Teal. Incubation periods were highly variable and were negatively correlated with nest initiation date for all species and in all years. After controlling for nest initiation date, incubation periods were unaffected by clutch size except during one of two years in Mallards. When Blue-winged Teal clutch sizes were manipulated, enlarged clutches had significantly longer incubation periods than control or reduced clutches, but reduced and control clutches had similar incubation lengths. Clutch size manipulation data supported the hypothesis that the cost of incubation increases with clutch size, and that incubation may work with other factors to help limit clutch size. These results suggest that variation in incubation periods may be an important life-history component of dabbling ducks.