In The Nesting Season, Bernd Heinrich examines the myriad strategies birds have evolved to reproduce. Their diversity is mind-boggling, even to an experienced naturalist. As he says: “It’s all about the details, and that’s what makes the mating game of birds one of the hottest topics in biology since Charles Darwin spawned the pillar of biology, the organizing principle of natural selection.”
Love in birds is like and unlike love in humans (Heinrich makes a good case for using words such as “love” and “grief” in birds without departing from scientific orthodoxy). But the details of birds’ strategies are endlessly complicated.
For instance, Old World cuckoos, which are nest parasites, have different-colored eggs according to their hosts, and they imprint on them as nestlings so that they may seek them out as adults. Brown-headed Cowbirds have not yet perfected egg color-matching, because they have not been parasites for as long.
Ruffs, large Old World sandpipers, are polygamous with a twist. As in many polygamous species, the males gather in “leks,” dancing grounds, where they compete for females. But in Ruffs, each male is slightly different, though they are grouped into two general patterns. Darker males are territorial, holding possession of the lek. Light ones bounce from lek to lek but are allowed to associate with the others, because a dancing ground with both types is more attractive to females.
But there’s more! Researchers recently found a third type of male that looks exactly like a female, but has bigger testes than the other males. They sneak in “under the radar” and mate secretly with the females without drawing attention from the showier males.
Nesting diversity extends to more than mating strategies. European swifts “spend about six weeks in the nest and leave it about fifteen minutes after sundown…they ascend into the sky to 2000–3000 m in altitude to meet up with others, staying continuously and uninterruptedly airborne for two to three years.”
“Endless forms,” as Darwin said. All that we do—fidelity and monogamy, “adultery,” polygamy, and even polyandry—is found in birds, along with a much greater range of behaviors. We may indeed be a bit like birds, but they are older and stranger. Heinrich’s always-attractive drawings and watercolors are a bonus. ◆