by Tina Yerkes, Ph.D.
Why in the world, I've been asked many times, would you spend years in a dilapidated trailer in prairie Canada with no water or heat, studying redheads? Because, I've invariably answered, they are cool ducks, and don't you know why redheads are so interesting?
At that point, I get a lot of blank, strange looks, and then I proceed to tell the story of a day in the life of the parasite queen: the hen redhead.
Most female ducks arrive on their breeding grounds with a simple decision to make: lay eggs or don't. Female redheads, on the other hand, have several different paths available to them: don't lay eggs; lay eggs in their own nest; lay eggs in another female's nest; or lay eggs in another female's nest and lay eggs in their own nest.
When females lay their eggs in another duck's (the host's) nest, it is called brood parasitism. There are two types of parasitism: when females lay eggs in nests of their own species, and when females lay eggs in nests of other species. Female redheads do both.
Dual-strategy females, those that parasitize other ducks' nests and then lay eggs in their own nest, always parasitize first and then build a nest, lay eggs, and care for their young. (Parasitic eggs receive no care from biological mothers, but instead rely on the charity of host hens.)
To make this even more complicated, individual female redheads often do different things from one year to the next. One year they may only parasitize, and the next they may lay in their own nest. At this point in the story, I get astonished, puzzled looks and more questions.
How do redhead females decide what to do each year? What are the benefits of parasitism to redheads? And doesn't it harm the host female? Now I have my audience hooked, and I know I'm creating a future redhead fan.
Making a decision is never an easy task, and redheads have many options to consider. The consequences are high, since a bad decision may result in no ducklings produced.
A female redhead assesses many factors upon arrival on the breeding grounds: What are the wetland conditions? How much food will there be? How fat am I? How old am I? And, are there many hosts out there? Poor wetland conditions, for instance, often translate into low food availability.
This is a bad situation for female ducks trying to raise a brood. When wetland conditions are poor, many redheads parasitize, some lay eggs in their own nests, but very few become dual strategists. In good years when potholes are full of water, there are many dual strategists.
Body fat is important in determining strategy choice because a hen requires fat reserves and abundant local food to lay eggs and incubate them. Skinny females may not have the ability to both lay a clutch of eggs and incubate them, so they become parasites. This is sometimes called making the best of a bad situation. On the other hand, a fat hen redhead can afford to lay many eggs and still incubate her own nest.
The age of a hen is also important. Older females may have few years of life left, so they will often take greater risks and even nest in marginal years, whereas young females can be conservative by laying parasitic eggs and waiting for a better year to build and lay in their own nest.
Finally, host availability is another consideration. Redheads favor canvasbacks and other redheads as hosts, but will also lay eggs in the nests of coots, mallards, scaup, gadwalls, and many other duck species. It is believed that redheads find their hosts by sneaking around behind them, watching their activity, and then moving onto their nests to lay eggs during the hosts' absences.
Sometimes, however, female redheads may actually tunnel their way under a host female on her nest and push her out of the way to lay an egg. If few host females exist in a particular year, however, parasitism may not be a very good choice of reproductive techniques.
Although all of these variables interact, research suggests that host availability and body fat, followed by habitat condition and hen age, are the most important aspects that determine the intensity of parasitism in a given year. For any female duck, the name of the game is producing as many ducklings over her lifetime as she can. Female redheads are influenced by many variables when they reproduce.
Parasitic eggs are less successful (fewer hatch) than eggs laid in their own nest, so how can parasitism pay? If a female is too thin, she may choose to lay parasitic eggs rather than no eggs at all; at least she might then produce a few ducklings that year. If conditions are good, females may also parasitize a host nest and then lay in their own nest. These females enjoy a higher production of young than females that only lay a more typical nest.
In some duck species, the host female does incur a cost for having her nest parasitized. The main negative effect is the displacement of her eggs from the nest when a redhead forces her way onto the nest. This reduces the number of host eggs that are incubated and hatch.
In some cases, the host may actually end up incubating more eggs from other hens than those she lays herself. A day in the life of the parasite queen is amazing because it is so different from other female ducks. The redhead prevails as one the most interesting waterfowl species in North America because of its adaptive, parasitic behavior.
Redheads aren't the only crafty ducks out there Brood parasitism is more common than you might think. In North America, wood ducks are notorious for egg dumping. Wood duck nests have been found with up to 50 eggs in them laid by parasitic wood duck hens. In most wood duck populations, up to 50 percent of the nests contain parasitic eggs.
Other North American duck species that frequently parasitize include black-bellied whistling ducks, goldeneyes, buffleheads, mergansers, and ruddy ducks-all of which are cavity nesters except for the ruddy. However, none of these species exhibits the dual strategy demonstrated by redheads.
When parasitism is the only option
The black-headed duck of South America is the only waterfowl species that is an obligate parasite: Black-headed duck females must lay their eggs in other nests, as they never build their own nests or incubate their own eggs; instead, they leave all maternal duties to unsuspecting hosts.
Favorite hosts include coots and rosy-billed pochards, but black-headed ducks' eggs have also been found in nests of raptors and predatory birds.