Wood ducks and the flooded nest box

The nest box tilts slightly over dark water near Clarksville, its entrance hole facing the cattail marsh. Inside, a wood duck hen sits on eleven cream-colored eggs, her iridescent green head catching fragments of light that filter through the wooden walls. The box was mounted here in early spring when the water was lower, but June rains have raised the pond level until the structure appears to float above the shallows.

Wood ducks are cavity nesters by necessity. Unlike mallards or geese that build ground nests in tall grass, wood ducks seek the protection of tree hollows or nest boxes elevated above predators and flood waters. The hen selected this box in May, inspecting the interior with her bill before laying her first egg. She has been incubating for three weeks now, leaving only briefly each morning to feed on duckweed and aquatic insects in the surrounding marsh. Her drake departed after mating, molting somewhere deeper in the wetland complex where he will remain flightless through midsummer.

The timing of wood duck nesting aligns precisely with the peak emergence of aquatic insects and the longest days of the year. Ducklings that hatch in early summer will have maximum daylight hours for foraging and the richest invertebrate populations to sustain their rapid growth. Mayflies, caddisflies, and midges pulse from the water in waves throughout June and July, providing the protein-rich diet that young wood ducks require. The hen times her nesting so that her brood will hatch just as these insects reach peak abundance. A clutch that hatches too early faces cold nights and sparse insect life; too late, and the ducklings must compete with juvenile waterfowl from earlier broods for diminishing food resources.

The flooded condition of the nest box creates both opportunity and risk. Higher water levels bring the box closer to prime feeding habitat, shortening the distance ducklings must travel on their first day of life. When they hatch, the young will tumble from the entrance hole into the water below, using their natal down as flotation while they learn to dive for insects and small crustaceans. The deeper water also provides escape routes from terrestrial predators and access to submerged vegetation that supports the invertebrates they need. But flooding can also trap adult birds inside boxes with entrance holes too close to the waterline, and sudden water level changes can strand developing eggs if the hen abandons a compromised nest.

Listen for the soft murmuring calls that drift from the box in the early morning hours. The hen talks quietly to her developing brood, a low conversational sound that will help the ducklings recognize her voice when they hatch. In another week, if the water holds steady and the weather remains warm, eleven small heads will appear at the entrance hole, ready to leap into the world below.