Why are mallard drakes so aggressive?

The Short Answer: Michael, these are great questions and I hope I can convince you that there are actually logical answers. But I warn, as with the previous question about forced copulation in ducks, that animal behavior sometimes isn’t “nice” and we shouldn’t judge animals as if they are being cruel or immoral. With that warning out of the way, here are some things to think about …

1) The drake actually has a great incentive to harass and try to mate with a hen who has ducklings. Mallard hens will often lay a second clutch of eggs if they lose the first nest of eggs or ducklings. So any drake who is not the one who fertilized the first clutch has an incentive to breaks eggs or scatters ducklings if it means he gets a chance to be the father of at least some of the second batch.  Drakes who do this successfully may be more likely to pass on their genes than drakes who do not.

This kind of infanticidal behavior, by the way, is not unique to ducks or to birds. Male lions who take over a pride will often kill existing cubs, because this usually causes the lionesses to come into heat sooner, allowing him to father cubs and pass on his genes. It also focuses the pride’s energies on raising his offspring instead of the previous male’s. Male lions are constantly being challenged for their position and they don’t stay on top long. On average, males who kill their predecessors’ cubs may father more offspring than males who help raise the previous male’s cubs while waiting for their own chance. The same behavior is seen in some primates. It can be pretty hard to witness, but it makes evolutionary sense.

2) There are a couple of issues in your second question. The first is that unlike fish or reptiles, birds don’t carry a bunch of eggs at once. In fact, there is usually no more than one or two eggs in the developmental process. This is probably an adaptation that keeps birds lightweight so they can fly. So most birds develop an egg, and then lay it. And then develop the next egg and then lay it. Mallard drakes are mating with the hens during the time of egg laying because every day, the hen may be developing a new egg that can be fertilized by whatever male successfully introduces his sperm during the fertilization window. The same is true in most birds, which is why male birds that defend mates do so most strongly during the time the female is laying eggs.

So when you see a mallard hen who is “very pregnant, as in, can barely fly” you are not seeing a hen who is full of eggs. You are almost certainly seeing a hen who is molting. Because feathers need to be in good condition for flying and insulation, all birds lose and replace feathers, usually at least once a year. In mallards the molt of flight feathers takes place sometime between May and November, with most birds molting heavily in June and July, soon after laying. During the time when they have dropped their primary feathers, mallards can’t fly well. Usually, this is not a problem for them because they are safe from predators in the water and at that time of year, food is generally plentiful, so they don’t need to move to another body of water.

What you are seeing is probably a hen who is molting at the end of laying a clutch of eggs. Drakes will attempt to mate with them during this period, in case the female has begun to develop new eggs in preparation for a second clutch.

Of course, it’s not as if drakes know the egg laying status of every hen. Probably the default behavioral instinct is, if they see a hen unattended by her mate, they attempt to mate with her. If they’re wrong and she is not in the process of developing eggs, then they have wasted some energy. But if they are right, the payoff is increased offspring. From an evolutionary standpoint, this may be a productive behavior.

Sources: The Birds of North America Online. From the CORNELL LAB OF ORNITHOLOGY and the AMERICAN ORNITHOLOGISTS’ UNION. Subscription required. http://bna.birds.cornell.edu/bna/species/658/articles/introduction.