Buffleheads, the smallest of the diving ducks, were thought to be nesting on the Horicon Marsh State Wildlife Area in 2009, though confirmation could not be obtained.
In 2011 the first confirmed successful breeding of this cavity nesting species did occur on the marsh. Buffleheads normally nest in the spruce forests of Canada and the northwestern U.S. Previously, only one record existed of this species nesting in Wisconsin. This occurred more than 150 years ago.
This year at Horicon Marsh a bufflehead nested again and had a very interesting nest. It contained seven eggs of the bufflehead and four eggs of a wood duck. This type of nest is called a compound nest and is fairly common, occurring when the same or different species of bird deposits eggs in the nest of another bird.
The chance of this occurring between wood ducks and buffleheads is, however, very unlikely. Under normal circumstances, these two species have breeding ranges that barely overlap. As a result, they rarely encounter each other in their nesting grounds. Where they do coexist, the bufflehead generally uses a hole that is too small for a wood duck to enter and lay its eggs. A bufflehead would not likely be sitting on wood duck eggs under these circumstances.
The buffleheads seen on Horicon Marsh were far from their normal breeding range. The exact dynamics/circumstances of the “Horicon” bufflehead taking over a nest box site with a large hole suitable for wood ducks and hooded mergansers was not observed and is unknown. The resulting compound nest was unique.
On June 18, a hen bufflehead was observed calling from the water below the nestbox in a rapid and insistent manner to bring the young out. She then lead a mixed brood of seven buffys and four woodys from the “wood duck nest box” in the Horicon Marsh State Wildlife Area to seek food and safety for her brood in the 30,000-acre marsh.