cavity nesters are well-known for dropping eggs in nests with other bird eggs. I regularly see both wood ducks and hooded mergansers visiting the same boxes. The wood duck eggs are usually ovid (egg shaped), while the merganser eggs are rounder. They are also slightly different colored, but I am not sure how reliable that is for differentiating them. I try to minimize my handling of the eggs, so I do little more than count the eggs and determine if both are present.
If you realize that a hen lays one egg a day, you see that often multiple hens are using the boxes. It has been hypothesized that egg dumping is a result of a lack of satisfactory cavities, but studies have not supported this simplification. While some boxes are major dump zones, nearby boxes can be ignored. I have been told by one observer, who maintained a string of many wood duck houses for several decades, that if a Wood Duck hen hatches a mixed brood, the merganser chicks will abandon that group and either go off on their own or join with another family of mergansers. Wood duck chicks will continue to follow a merganser hen. I personally have not seen any "orphaned" Wood Duck chicks. I have, at least a couple of times, seen merganser chicks on their own without a hen. Interestingly, these orphans are more successful on our lake than Wood Duck chicks in a traditional family group. Steve Weston on Quiggley Lake in Eagan, MN sweston2 at comcast.net
