For the Birds Radio Program: Dr. Ruth of Ornithology Pt. 2: How birds prepare their bodies for reproduction
This week I’m trying to live up to my reputation as the Dr. Ruth of Ornithology by answering some questions about the private lives of birds. Last time I talked about the reproductive equipment male and female birds each have, and now it’s time to explain how birds get that equipment ready to produce new little babies.
Before they can do anything at all, of course, birds need to be in the mood. Avian reproductive organs literally shrink to a tiny size after the breeding season, since being flying creatures, birds need to be as light as possible, and outside of the breeding season sex organs are nothing but excess baggage. Increasing daylength is normally all it takes for their hormones to stimulate their organs to start growing. But once the organs are back at functional size, they also have to start producing eggs or sperm. Some species get physiologically prepared for breeding very quickly and easily, usually in North America as a simple response to increasing daylength, but in some dry climates in response to water availability. Mallard drakes get so pumped up by this time of year that they’ll mate with anything that will hold still long enough to let them, which is why there are records of hybrids between Mallards and every other kind of duck, including species that aren’t even in the same genus. I’ve even watched Mallards try to mate with rocks.
But for females to be pumped up like that, it has to be easy for them to find a male nearby to mate with and quick and easy to build a nest during the time they’re ovulating. Mallard hens have an easy time finding cooperative males and building their simple nest. In contrast, female Bald Eagles nest in some isolation on large territories–they have to ensure that they won’t ovulate until their mate is back from migration too, and that any winter damage to their nest has been repaired. So their reproductive organs don’t get fully functional until the females have gone through courtship flights with their mate. Cranes stay with their mate throughout winter and migration, but their long, arduous journey has to be completed before the females can afford to ovulate. So female cranes don’t start dancing until they’ve built up a lot of fat reserves. For cranes, the courtship dances synchronize physiologically both males and females so by the time they reach their breeding grounds and make the nest, they’ll immediately be ready to produce fertilized eggs. A few other species have a long and complex courtship in order to be physically primed for breeding, but most bird species are somewhere in the continuum between ducks and cranes.
Many birds are stimulated by the act of nest-building itself to ovulate. This helps to ensure that females won’t ovulate and lay an egg before their nest is finished. Nest time, in our continuing series of questions answered by the Dr. Ruth of Ornithology, we’ll find out how exactly the male and female birds come together to produce that fertilized egg.