Spring is springing, and birds are thinking of love and babes. Migratory birds are starting to show up, looking for mates and suitable nest sites. About 90 per cent of our resident and migratory bird species are monogamous (well, there might be a little promiscuity sometimes).
What I did not know until recently is that one of the most common sandpipers in our temperate climate – the Spotted Sandpiper – is polyandrous. This means a female searches for and copulates with more than one male. Then she deposits eggs, at least one in each nest made by the males she has mated with. She then moves on, leaving the males to incubate the eggs and raise the broods (called sequential polyandry).
The males tend to arrive early, establish territories, and make nests. When the females show up a little later, the males do a broken-wing display to attract their attention. There are always fewer males than females, and female–female aggression is intense. The broken-wing display is a test of the male’s fitness, and if she likes him, she allows him to copulate with her.
The female lays her eggs over five to seven days, one per day in each of her partner male’s nests. Studies have shown she rarely deposits all her eggs in one nest. On average, the number of mated males per female is four. The female plays no part in rearing her offspring after laying the eggs.
Over the next 20 to 22 days, the males incubate all the eggs, each protecting his nest even though some eggs may not be his. He feeds and cares for the chicks in the nest over the next 25 to 35 days until they fledge. He then broods them for another week or until they become independent.
What is the advantage of this system? The female can lay more eggs over the breeding season than she would be able to if she were in a monogamous relationship. At any one time, she can carry up to four eggs, but more would be too heavy for her to fly. By breeding several times, she can spread out her total egg clutch over the season. In addition, there is a better chance that at least one of the various nests will be successful due to the quality of parental care and/or abundance of food.
Because the female lays eggs from one clutch in different nests, at least some of the male’s genes will be passed on in the event of nest failure due to predation. In monogamous pairings, nest predation often means the loss of all the offspring of one mated pair.
This system seems to be a successful adaptation. However, once a male starts incubating eggs, he is essentially removed as a potential breeder. Consequently, a shortage of males means that only eight eggs per female have been successful in most studies to date.
