Although eggshells of hooded mergansers (Mergus cucullatus) and wood ducks (Aix sponsa) are difficult to distinguish by color, I found that eggshell thickness (0.541 vs. 0.255 mm, respectively) provided reliable identification. J. WILDL. MANAGE. 51(3):534 Distinguishing nesting species of ducks during winter maintenance of nest houses can be difficult. The nest remains of hooded mergansers are most often confused with those of wood ducks, especially when only eggshell fragments are available. Many field technicians simply report "duck use" or frequently misidentify hooded merganser eggshells. The purpose of this paper is to describe a technique to differentiate wood duck and hooded merganser eggshell fragments. I thank S. P. Havera, F. C. Bellrose, B. Bacon, L. E. Nauman, and N. F. Payne for providing comments on the manuscript. While studying wood duck nesting biology and general nest-house use, eggshell fragments were collected from 25 hooded merganser and 8 wood duck nests at Mead Wildlife Area in central Wisconsin and from 22 houses used by wood ducks at Nauvoo Slough in westcentral Illinois. Thickness of 7 eggshell fragments (1-3 mm square in size) from each nest was measured with a tubular micrometer (The Cent. Tool Co., Auburn, R.I.). Fragments from hooded merganser nests averaged 0.541 (SE = 0.0079, range = 0.467-0.630 mm, N = 175) in thickness and those from wood duck nests averaged 0.255 (SE = 0.0039, range = 0.196-0.300 mm, N = 210). No overlap in range of eggshell thickness was observed. Hence, hooded merganser vs. wood duck use can be determined by measuring th thickness of eggshell fragments remaining in nests. Differences in breeding range and site and cavity dimension requirements make use of wood duck houses by other duck species a rare occurrence. However, where common goldeneye (Bucephala clangula), Barrow's goldeneye (B. islandica), or bufflehead (B. albeola) compete for nest sites, eggshell color may aid in species identification. Whereas wood duck eggs are dull white to light brown and hooded merganser eggs are pure white (eggshell fragments often stain to light brown), both goldeneyes have green eggs and those of the bufflehead are ivory yellow to olive buff. Received 18 March 1986. Accepted 7 February 1987. 1 Present address: Illinois Natural History Survey, River Research Laboratory, Box 599, Havana, IL 62644.