The Homing Instinct

by Bernd Heinrich

  Of locally common species of “silk moths” in North America, I’m most familiar with the luna, promethea, polyphemus, and cecropia moths. The luna and polyphemus caterpillars prepare for their nine-month pupal stage by wrapping leaves about themselves and then within that cavity surrounding themselves with a layer of silk. The silk layer is flimsy and papery, but it holds the leaves together, serving as camouflage. The cocoon falls to the ground when the tree sheds its leaves, and then it becomes buried under the autumn leaf fall and overwinters under the snow. The promethea moth caterpillar also starts with a leaf wrap but makes a much harder, more leathery shell-like cocoon that is much smaller, for a tighter fit around the pupa. The promethea caterpillar usually wraps a single leaf, and at a sock-top-like exit hole it creates an extension of the home in the form of a thick strap of silk laid down along the petiole of the leaf and solidly wraps that around the branch. As a result, these cocoons remain attached to the twigs when the tree sheds its leaves. They look like the old wrinkled leaves that sometimes remain on trees all winter. The cecropia moth’s strategy of a safe home appears to rely on a psychological trick on potential predators such as squirrels, mice, and birds that prey on pupae. It is a very large moth, with a pupa that could be (and is!) quite conspicuous. Indeed, the cocoon looks balloonlike. It has an outer shell and a second inner shell within which the pupa resides. But there is a wide empty space in between. Presumably a predator that penetrates the outer shell would find what appears to be empty space after penetrating it, not realizing there is a second room beyond the first.

  Although the preceding cases exemplify homes made by solitary juvenile animals that serve as protection for themselves or a future stage of their life cycle, other solitary animals build homes as adults for their multiple larvae, which they then provision with food. In solitary wasps, the larval food is usually paralyzed insect and spider prey that is carried into either a natural cavity or one that is excavated to become the nest. My father told me of an unusual one in Angola, Africa. He had, as always, hung up his shotgun outside the tent when he noticed one day that the barrel was partially filled with bits of charcoal. Discharging the gun with a plugged barrel might have resulted in an explosion. He suspected one of his African helpers was up to no good but then discovered that the culprit was a blue wasp, busily carrying in the charcoal. It also brought pill bugs (a terrestrial crustacean). The wasp separated different individual pill bugs with layers of charcoal, and thus each larva had its own compartment with food. Potter (also called mason) wasps (genus Eumenes) may burrow in wood or use containers such as those created by a wood-boring beetle larva, or they make a narrow-necked round pot from earth and regurgitated fluid. The form and mechanism of making such pots could have been a model for those made by Native Americans.

  Some bees, called mason bees, genus Osmia, also make similar clay pots. But these are provisioned with pollen as a protein source for the growing larvae, instead of paralyzed prey. Other mason bees nest in preexisting tubes. Such bees are used for crop pollination—they are provided with homes that attract them. These are simply blocks of wood drilled full of holes and set near a pollen source, such as an alfalfa field or a fruit orchard. Some of these bees and their homes are now commercially available, but I simply provide a debarked old pine log full of holes made by larvae of the long-horned beetle pine borers. Give them homes, and they come.

  Diagrammatical views through cuts of trees to show nest cavities of a yellow-bellied sapsucker (center), and two of black-capped chickadees. Sapsuckers prefer to use poplar trees whose centers are softened by the hoof fungus. They do not reuse their nest holes, but other birds may use them, such as chickadees. Chickadees may also hammer out their own nest holes. The two shown here were in dead, partially decayed quaking aspens. One nest hole (at right) was left unused after the birds excavated all around a hard core and then abandoned their effort. Woodpeckers do not put a nest into their nest holes, but the various species of birds that use such abandoned cavities do.

  The homes of birds function in the majority of cases for one-time use to hold the eggs, incubate them, and rear a clutch of young. But some serve also as dormitories or shelters in the winter. Bowerbirds produce a variation of these homes as display objects to attract a mate. Some woodpeckers, such as the downy and hairy woodpeckers in North America, hammer out cavities in both spring and summer, the first to rear young and the second as a shelter in which to overnight.

  The nests of mammals parallel those of birds but usually serve more than one function at the same time. Those of deer or white-footed mice, genus Peromyscus, are used to raise young in the spring and summer, as a granary for storing seeds in the fall, and as a place to huddle with others to keep warm in the winter. One mid-October as I was showing students a season’s discarded catbird nest in a bog, we saw two of these mice jump out. I was surprised, because a catbird nest is a cuplike structure of roots and rough bark on a platform of twigs. It would not be a shelter for a mouse. But a closer inspection revealed that this pair of mice had done some renovations. They had separated the nest lining of roots from the base of twigs and had then stuffed the space created in between with the white fluffy down of milkweed seeds. I discovered another of these mouse nests in the glove compartment of my Toyota pickup truck, which was parked by the house in midwinter. I had opened the compartment to search for my registration, only to find that all the papers in there had been shredded to make an insulated mouse nest. Still a third deer mouse nest was in the (unused) paper delivery box on a pole next to a mailbox. Another one, on top of a chestnut-sided warbler nest, was filled to the brim with black cherry seeds and covered with the down of Clematis seeds.

  The home-making of birds and mammals is separated by two main life strategies: those of the “altricial” or helpless young, such as most songbirds’ that are born blind and helpless except for being able to beg for and take in food and excrete waste; and those of the “precocial” young, such as those of ducks and chickens that are mobile from birth and don’t use a nest. In the precocial strategy, the main solutions that help reduce predation are behavioral, such as guarding by the male mate (in the case of large birds such as cranes, swans, geese), deceiving a potential predator by feigning injury and luring it away from the young, and/or hiding responses of the young (in the case of grouse and sandpipers). Having a nest, however, has many potential advantages. First, it provides other options beyond just hiding. They include placement of the young into inaccessible places, such as over water, high on a cliff, onto the ceiling deep in a cave, at the end of a thin twig, and in uncountable possible hiding places—provided the adults can have access to feed their young. Very specific building materials and construction techniques are required.

  Instead of excavating cavities in trees (or the ground), some birds enclose space by the use of mortar. The nests made inside these structures consist of feathers and fine plant material. Those of cliff swallows, illustrated here, are often next to each other in colonies where one nest may serve as an attachment point for the next.

  Other birds make relatively inaccessible nests by weaving them and hanging them from the tips of branches. Here, from left to right, are four examples: The Eurasian penduline titmouse, Remiz pendulinus, makes a feltlike bag nest. The Asian tailorbird, Orthotomus sutorius, pokes holes into a large leaf and, using its bill as a needle, threads fiber through the holes to bind the two leaves or leaf ends together. The African oryx weaverbird, Euplectes orix, weaves grasses. North American Baltimore orioles, Icterus galbula, in New England commonly extract fibers from year-old dead milkweed plants to weave with.

  The “inventions” that we see in birds are seemingly endless and sometimes border on the bizarre, but they are always beautiful and sometimes astounding. Simple nests have the virtue of being “invisible,” and thus a mere scrape on the ground may suffice. Others, however, involve skills to construct that challenge our imagination. My own favorite examples of marvels of construction are the hanging nests, those
that have evolved the furthest from scrapes on the ground. Consider the tailorbird, Orthotomus sutorius, which makes holes in leaves on a plant and then finds fiber to poke through these holes and stitch the leaves together as a surgeon might suture skin. The bird then builds its cozy nest inside the leaf bag it has created. A variety of swallows create a similar home cavity within which they then build a nest, only they follow different options (depending on the species), such as finding an existing cavity such as one made by a woodpecker, excavating one themselves in the ground, or, as most do, mortaring one together out of clayey mud. Nest cavities are also created on the designs of clever baskets. Many weaverbirds expertly weave fibers of grass into retorts that are hung from the tips of thin branches where they are out of reach of many predators. They have a long, narrow tube at the bottom that acts as a secondary door which restricts access as well. On the same principle, there are also designs, such as in one titmouse, where there is a door flap that is closed when the bird leaves the home, and still another has a false entrance. Nest location is important. Hardly any space would be safer than the ceiling in a cave, and some swifts have achieved the occupancy of that perhaps safest of niches. They have built nests out of their own saliva by adding one bit at a time, which hardens and extends a shelflike projection on which they can deposit an egg. Many swallows achieve somewhat of the equivalent by using wet mortar that they collect.

  A fortress, if it is impregnable, does not need to be hidden. Cavities, especially those in trees, are one of the most common ready-made fortresses. There are few major bird groups in which some members do not avail themselves of this resource. The problem is that it is often a limited resource, and depending on this scarce resource may severely limit reproductive opportunity. Therefore, the ultimate in home-making for hole nesters is to excavate their own home, to their own specifications. Woodpeckers, perhaps because they are pre-adapted for it because of their foraging for wood-boring grubs, are the experts at it.

  Woodpeckers excavate their own home cavities out of solid wood, with the entrance reduced to a minimum size that excludes almost all predators larger than they are. Small predators that might be able to squeeze in can likely be repelled with the same tool used to make the home, the sharp and powerful bill. Larger predators that can’t enter might still be able to reach in through the hole, but if the cavity is made deep enough, they can’t reach the eggs and young on the bottom. Woodpeckers may expend what appears to be extraordinary effort to make a home, but the effort is worth it, because they suffer virtually no nest predation.

  Woodpeckers make no secret of where their homes are. In June when they have young, I can locate a woodpecker nest from within a hundred meters, whereas I may step over any of a variety of other forest bird nests without noticing them. A woodpecker comes and goes to its nest at frequent intervals, and the young in the nest make a nonstop din that increases in volume whenever they sense something landing on their home tree. All other forest bird babies stay silent until a parent is almost directly at the nest, and then the babies peep only faintly and only the moment the parent arrives with food. They shut up the moment it leaves. There is never (in most species) a single fecal droplet in or near the nest due to a complex interactive behavior between the parents and their offspring. In contrast, with the woodpecker nest, after the older young start to monopolize the nest entrance, feces accumulate and are simply trodden down into debris that collects on the floor. There is, it appears, a downside to almost everything, and each solution is a balance of trade-offs.

  Similar scenarios of homing apply in mammals as well, and the two divergent strategies of altricial versus precocial young are exhibited in closely related species. Those of the hares and rabbits, Leporidae, and apes and humans, Hominidae, are the most interesting and instructive examples.

  Hares and rabbits look very similar to our eyes but have vastly different life and home strategies. Hares, Lepus, such as jackrabbits and varying or snowshoe hares, bear their young into a simple bare depression on the ground. They have no protective home but, like precocial birds, are born with insulating and camouflaging covering. Like baby sandpipers, ducks, chickens, deer, and antelope, they are able to run from birth. Rabbits, on the other hand, are born blind and naked into a fur-lined nest (the hair the mother has plucked from her body). One species, the European rabbit, Oryctolagus cuniculus, is the ancestor from which all domesticated rabbits are derived, and in the wild, this species digs networks of tunnels to create what is called a “warren.” One might suppose that living in an excavated cavity is safer than having the young deposited in the open on the ground. It could be, but an earthen tunnel is not impregnable. Some predators have evolved to enter tunnels. A rabbit warren must have many potential escape exits. But it has an additional design feature—a nursery chamber with the nest and babies in a side tunnel that is without an exit. This is not a design flaw but a “clever” safety feature, since the helpless young could not escape in any case. Rather than allowing a predator that searches within a warren to reach the nest from two directions, there is only one. Furthermore, this entrance is plugged most of the time, and the doe (female rabbit) reduces the time window when the tunnel to the nest is open to a bare minimum—the three or four minutes per day, at almost exactly the same time every twenty-four hours, that she visits to nurse them. The young learn when to expect her and get ready by uncovering themselves to be ready to nurse the moment she comes in. After having nursed her babies, she leaves immediately and plugs the nest entrance behind her.

  It is probably not incidental that this species of rabbit is social as well. (In contrast, cottontail rabbits of the Americas, genus Sylvilagus, instead have extremely well-camouflaged ground nests.) An excavated home with many tunnels represents considerable investment, and it is an asset rather than a liability to have social tolerance for helpers with an equal stake to make, enlarge, and maintain it (as also in beavers).

  Few if any home-making feats are as impressive for sheer size, sophistication, and ecological impact as beavers’. Beavers don’t just make their nest, called a “lodge,” but also create their own specific habitat all around it by clearing trees. Felling trees up to half a meter in diameter with their teeth may sometimes require days, and the rewards from the work come only from the branches near the top. The twigs’ bark is their main food, and the peeled remains become one of the main materials for making the lodge, as well as the dam that then creates the moat around the lodge, the safe haven over the winter and where their kits are born.

  The dam, which creates a pond that serves as a moat, permits the home entrance to be underwater and keeps out unwanted visitors. In the winter this home has a roof and walls that freeze as solid as concrete. As long as the dam holds the water and the frost hardens the lodge, the beavers’ home is impregnable to bears and wolves. This lodge is no ordinary rodent nest, as John Coulter undoubtedly surmised. (Coulter, considered the “first mountain man,” lived for months alone in the wild and was the first white man to see the Yellowstone area and the Grand Tetons. He once escaped from pursuing Blackfeet warriors in the Yellowstone in 1809 by hiding in a beaver lodge.)

  Beavers’ engineering skills are most evident in their dam building, which may impound acres of pond and raises water levels one or more meters to make the safety in their lodges possible. The results of their collective work, often performed over generations and presumably hundreds of years, can be stunning. One beaver dam in Wyoming, though only ten meters long, was five meters high. Another on the Jefferson River in Montana measured 650 meters long, and one in Wood Buffalo National Park in Alberta was slightly more than 850 meters long. Dams break not infrequently, and their upkeep, repair, and replacement are almost year-round tasks. In the last spring rains, one sixty-meter dam backing up our pond of several acres was breached. The water rushed out, leaving a large mudflat. But within one week the beavers had repaired the dam. Close to that same time, in the nearby town of Adamant, Vermont, a beaver dam broke and seven houses had t
o be evacuated because of the flood. Beaver home-building reminds us that home doesn’t stop at the door of the dwelling. It includes the area from where we secure our resources to live on. The larger the area of their pond, the farther the beavers can range to reach more food in safety. The more the animal can do to make its place habitable, the larger the range of habitats it can use.

  A typical beaver colony starts out with an adult male and female, who have several kits in early spring that grow up by fall to help the parents in dam and lodge building and in harvesting the food stores required to overwinter. By the following spring the parents have another clutch of pups, and the colony then contains perhaps up to seven or eight individuals. The previous set of young then effectively helps the second. But by the following spring, before the pair has its next set of pups, they expel the two-year-olds if they don’t leave on their own, and thus the colony size stays usually below seven or eight individuals (although a theoretical fourteen is possible). Those that leave home face predators, which traditionally were wolves and coyotes, but judging from the dead ones I see every spring along roadsides, it’s now cars.

  The better a home the animal can make, the more needs are satisfied directly in its home area and the less it needs to roam. We are dependent on our homes, yet we often simply take them for granted—until we are left without one. A trip into the wilds of Suriname gave a party of us from New England homes a demonstration of where the limits of adventure abut our abilities of home-making.