Wood duck hens dump their clutches on another mom

And then there were three: three half-grown wood ducks, survivors from a hatch of 25 eggs. The hen began motherhood with a flotilla of ducklings swimming behind her.

Prior to nesting season, we had five pairs of wood ducks on our little pond. A hen occupied one of our six nest boxes. That left five boxes for the other hens, which initially seemed like a fair division.

But wood ducks are notorious for what is called "dumping." Females mate, then use another hen's nest for laying. They skip nest-building and incubation, letting the designated hen take responsibility.

I think that happened to us — and to one unsuspecting hen.

Survival odds

The other ducks are long gone to places unknown, the parasitic hen(s) leaving her genetic future tucked beneath the chosen mom.

Future for little ducks, however, is an iffy proposition, certainly around here. Twenty-two of those babies disappeared, most likely down a fox or coyote gullet. We saw fox in our yard twice in the weeks after the hatch.

Predators, rough weather, temperature and food supply dictate duckling survival rates. Success varies, but it's commonly estimated that 30 to 40 percent of the brood will survive the first 90 days. We missed that mark widely.

Was dumping a bad survival strategy? If two or three other hens had raised their own ducklings, would the survival rate have been higher? Not necessarily. A study in Massachusetts found similar hatch rates for dump nests and normal nests — 26 percent for the former, 24-27 percent for the latter.

Opportunity knocks

Birds, including ducks, lay one egg per day. Incubation begins when the clutch is complete. That would be 12 to 14 eggs for wood ducks. If the parasitic hen is to be successful she must synchronize her egg production with the resident hen.

Hens would not be in the nest box (wood ducks are cavity nesters) between laying bouts, which take 30 minutes or less. The eggs maintain viability until incubation begins. This assures that the eggs have a good chance of all hatching on the same day.

Once an egg was laid, the resident hen would be back on the water doing duck things. That would give the parasitic hen the opportunity to enter the box and lay her egg.

Wood duck eggs typically hatch after 28 to 32 days of incubation. The entire clutch hatches in less than 24 hours.

Egg dumping is most common among precocial birds, those species that hatch young ready to roll upon egg exit. No intensive post-hatch care is necessary. Precocial species tend to lay large clutches. Care of self-sufficient hatchlings is much easier than care of the naked, blind, helpless nestlings hatched by songbirds.

Why do birds dump eggs?

Studies have determined that parasitic hens tend to be young, unmated birds (bred, but unpaired), females that have lost their nests, or females that cannot find suitable nesting cavities.

Strangely, offering several nest boxes close together, as we have done, will stimulate dumping. I worked on the theory "the more the merrier" last fall when I expanded nesting opportunities from three boxes to six. In previous years, three boxes held two nestings, one wood duck, one hooded merganser.

I have since learned (thanks to Google) that more boxes can stimulate this dumping backfire. Why? I couldn't find an explanation. But three of our boxes will come down.

The box used this year, by the way, is the oldest box, here when we bought the house 13 years ago. It has been used every year since. The same hen? Well, she looks like it.

Read Jim Williams' birding blog at startribune.com/wingnut.