In the late 1980’s, Hooded Merganser (Mergus cucullatus) hens began to show up in my Wood Duck boxes in west central Minnesota. I monitor about 100 boxes, with strong wood duck usage.

In the past three seasons, from 8 to 17 percent of my boxes have contained hooded merganser (hereafter, merganser) eggs. Half of these have been mixed (interspecific compound) clutches of both merganser and wood duck eggs.

I’d like to pass along some things I’ve observed while monitoring these mixed clutches, with special emphasis on three boxes from the current 2005 season.

Background information: When both merganser and wood duck hens are laying in the same box, the right to incubate may be won by either hen, sometimes after a heated in-box battle. An edge seems to go to the hen that begins to incubate first, usually after she’s laid the most eggs. The incubation period for both species is known to be the same. Observations of mixed clutches and broods are made easier by the marked differences between the appearance of their eggs and ducklings:

Eggs: Merganser eggs are usually pure white. They are oval, but rounder than woodie eggs, and look like cue balls. They are larger, with shells about twice as thick as wood duck shells. Wood duck eggs are light tan or off-white, and are shaped much like small chicken eggs, with one end being distinctly fatter than the other.

Ducklings: Merganser ducklings have a white breast and are taller than wood ducks. They look like they have a brown stocking cap pulled down past eye level. They dive for food. Newly-hatched wood ducks have yellow faces, and their pale breasts are yellow-tinged. A brown “racing stripe” runs back from their eyes through the face, and the tops of their heads are dark brown. Woodie ducklings dip for food.

2005 Mixed clutches: 05’ has been a banner year for this study, since three of the four boxes containing in-box cameras on my home unit (FP#12, and FP#16) have held mixed clutches. The cameras and the proximity of these boxes allowed me to record observations and to make regular egg counts.

FP#2: Thirteen wood duck eggs were laid in this box by two hens. On three successive days, two new eggs were added, once when both hens were observed and recorded in the box at the same time. One of the woodies eventually assumed incubation duties. One merganser egg was laid early on. It eventually hatched and the duckling followed the hen to the water along with its wood duck broodmates. Only one of the wood duck eggs did not hatch.

FP#12: A total of 17 wood duck eggs were laid by two hens, with a single merganser hen adding three eggs to the clutch along the way. All three merganser eggs were hatched by the woodie, along with 14 of the wood duck eggs. As in FP#2, this was a compound clutch in two ways, with two laying wood ducks (intraspecific) in addition to a single merganser (interspecific). I also observed this mixed brood being led by the hen to the water.

FP#16: Here the story was more involved. A merganser hen began laying eggs, with the number initially accumulating at slightly more than the usual on every other day. After eight eggs were laid, a wood duck hen was observed exiting the box. She had laid a typical light tan egg and added it to the growing merganser clutch. The merganser spent that night in the box. When checked the next afternoon, there was one more merganser egg, and the wood duck egg had a ¼ inch diameter hole in its side, with yolk leaking out. This pointed to avian predation, and it seemed easy to blame the merganser. However, the damage was compatible with woodpecker predation, and a Northern Flicker was observed at the hole entrance the next morning. Four days later, the damaged egg had disappeared from the box, but a new wood duck egg had been added next to what became the final total of 12 merganser eggs and the one woodie egg. The merganser began full-time incubation at that point and went on to hatch them all.

On the morning of the jump, about 30 hours after the hen completed the hatch, the camera recorded a very unusual event: The hen reached down and grasped the lone wood duck by the neck! She proceeded to shake it with vigor –several times- before dropping it back beneath her.

The little wood duck survived the attack and finally jumped with difficulty from the box, to land amidst its merganser broodmates. However, it did not follow the hen to the wetland. It peeped and walked in circles below the box as the hen and all 12 of her merganser offspring marched away without pausing.

Discussion of egg removal: It’s been well established by observation and photos that ducks can, and do, remove eggs from boxes, carrying them with bills opened widely. Sometimes it’s clear that these eggs have been damaged and sometimes not. A couple years ago, Steve Straka monitored a compound mixed merganser/wood duck clutch being incubated by a merganser. Initially it contained seven merganser eggs and 19 wood duck eggs. During incubation, Steve found two broken wood duck eggs on his lawn beneath the box. A count done on the day before the hatch revealed that six additional woodie eggs had disappeared, but all seven merganser eggs were still in the nest. The next day the hen hatched six of the seven merganser eggs and nine of the 11 remaining wood duck eggs. They all jumped and were led away by the hen.

Before quickly assigning to the merganser hen the ability to recognize a wood duck egg as foreign and also having the will to remove that egg, it’s well to note again that wood duck shells are only half as thick as merganser shells. Thus, the woodie eggs were theoretically more prone to injury during the merganser’s vigorous incubation egg-rolling maneuvers. Perhaps they were removed because they were recognized as foreign eggs.

Discussion of duckling attacks by hen: In over five years of recording nesting with in-box cameras I had never, before this season, observed a hen afflicting anything even close to serious punishment or damage to a duckling. The attack in FP#16 suggested to me that it was species-directed, especially after reading Arnold Krueger’s letter last year, as reported in Newsgram #42: Three years ago Krueger monitored a mixed hooded merganser/wood duck clutch in a box equipped with an in-box camera. He wrote: The merganser hen took good care of the eight merganser chicks and the two wood duck chicks while they were in the box. However, after they all went down to the water, the merganser he shook the two little wood ducks to death.

Such merganser behavior is not universal. Several years ago I make a project of following a small mixed brood following the jump. They were also hatched by a merganser. This hen raised he mixed clan in the slough below our home. They were highly visible in the early weeks as the hen sought open water for diving (the woodies continued to dip for their food). The surviving ducklings (three mergansers and two wood ducks) continued to feed and roost tightly together until nearly fully grown, when they made their first attempts at flight.

Epilogue: The story from box FP# 16 has a happy ending: After the jump, I easily caught the abandoned and battered (but still game) wood duck duckling below the box. By lucky coincidence, it was also Jump Day in camera box FP#2, and the wood duck hen was about ready to call out her mixed brood (as described earlier). My wife suggested adding it to the box, which I did by walking up and tossing it inside through the exit hole. On later review of the tape of this event, it revealed that the hen acted appropriately surprised as the duckling hurtled in from the outside.

Thus it was that this duckling had the rare and eventually successful opportunity to climb out of the nest box twice on the same morning. The last I saw of this little bird, it was churning along with its new broodmates, as they took their first swim behind the adoptive hen Woodie.