In the January-February  issue, Wayne Mones reviews Bernd Heinrich's new book, The Nesting Season: Cuckoos, Cuckolds, and the Invention of Monogamy .
Mating behavior has preoccupied us from the time we first diverged from apes and stood erect on the broad African plain. From our first gatherings around the communal fire, mating behavior has drawn the attention of shamans, tribal elders, priests, Talmudic sages, matchmakers, anthropologists, sociologists, sexologists, psychologists, courtesans, and presidents.
It is also a hot topic in biology, because it is one of the best examples of problem solving by evolution. Evolution, in its sublime indifference, confers selection advantages only on those behaviors that best perpetuate all species, from the simplest to the most complex. In his newest book, The Nesting Season, renowned naturalist Bernd Heinrich draws heavily on his personal observations as he explores this most compelling topic in birds.
Heinrich’s father was a field biologist. His parents deposited him and his sister in a school for disadvantaged kids so they could go off to Africa and Mexico to collect specimens. Having inherited his father’s passion for biology, Heinrich has devoted his life to drawing, photographing, collecting, keeping journals, and teaching and writing. He is professor emeritus in the biology department at the University of Vermont and author of more than a dozen books on a broad array of subjects, including winter survival strategies, animal intelligence, bumblebee economics, and long-distance running. (At 70, he still runs ultramarathons.)
When it comes to choosing a mate, males typically pursue and persist, but females ultimately get to decide. They make their choice, Heinrich explains, based on traits that serve as a sign of health and vigor. Depending on the species, selection might be based on an elaborate vocal repertoire. Showy behavior, colorful plumage, or architectural genius also come into play. Female song sparrows, for instance, opt for males that sing a variety of songs—some have up to 15. And in the case of the penduline tit, the male’s intricate, pear-shaped nest is the sexual magnet. “A female inspecting nests compares males indirectly,” Heinrich writes. “She chooses for herself a resource she needs for reproduction and also indirectly assesses the male’s potential vigor and industry.”
The core of The Nesting Season is Heinrich’s exploration of those mating strategies that work, how they work, and how conditions determine what works and what doesn’t. Since humans find monogamy to be an attractive trait in birds, Heinrich explores it early in the book. “Pairwise parenting is rare in mammals, insects, crustaceans, fish, reptiles, and amphibians, although it occurs sporadically in all of them,” he writes. “It is common only in birds. Birds routinely team up into one-on-one male-female partnerships, and such pairs are so obvious and conspicuous that we take monogamy almost for granted.” Yet monogamy, like all mating strategies, incorporates a rich palate of behaviors and rarely results in complete lifetime sexual fidelity to a single mate. It’s advantageous only when nesting and parenting responsibilities demand an extraordinary investment of energy by both parents. In fact many birds that we “know” to be monogamous are like other animals (including humans) in that they frequently engage in extra-pair copulations as conditions allow.
What good is monogamy anyway? The answers depend on conditions. Heinrich cites the example of screech owls, which are typically monogamous throughout their lifetimes. But when food is plentiful and nest density is high, males can easily provision more than one nest, so they become polygynous. And when food is readily available, females need less parenting investment from males. On the other hand, when food is widely scattered, so are nests and females. In these instances, females benefit most from increased help in parenting, which means males can maximize their reproductive success only by devoting themselves to a single mate and a single nest.
Although polygyny appears to be a kind of captain’s paradise, having more than one mate, like all behaviors, has its pluses and minuses. Heinrich recounts the discovery by Swedish biologists that the primary female mates in certain populations of reed warblers suffered three times more nest predation than did the secondary females. The team devised an experiment to determine why and discovered that the secondary females were the culprits. They were destroying the eggs of the primary females, presumably to obtain increased parenting help from their male partners for their own offspring.
Since predation is often the main limiting factor to reproductive success, birds must develop strategies to reduce its toll. One approach is to hide the nest and develop behaviors to avoid tipping off possible predators to its location. Another is to build nests in colonies, where there are many neighbors to ward off predators. One of the seemingly strangest is to build a nest in or very near the nest of a raptor. Heinrich cites examples of house wrens and common grackles that build theirs in the interstices of active osprey nests, and of kingbirds that build theirs in active golden eagle nests. A raptor nest would seem like perilous digs for a songbird, but all birds, it turns out, have an innate reticence to attack or destroy anything in or nearby their own nests—a reticence probably stemming from a protective behavior that keeps birds from devouring their own offspring. As with all real estate, the three most important things in determining value are location, location, and location. Build your nest as far as possible from a raptor, and risk having raptors and other birds snack on your eggs or chicks. But build your nest in a raptor nest, and you will be assured of formidable guardians that are very good at keeping undesirables out of the neighborhood.
Reproductive success depends on a huge suite of behaviors and strategies that must change as conditions demand. This book is a rich and detailed account, and as with Heinrich’s other works, it’s illustrated with the author’s own drawings and photographs. The Nesting Season is one to read and read again.