The Homing Instinct

by Bernd Heinrich

  I have observed the same nest-emptying phenomenon in “my” phoebes in my shed on a couple of occasions also, and although I didn’t sit and wait to see how the babies left the nest, I did know that the cause was mites. In both of my cases the nests were crawling with hundreds, maybe thousands, of dust-particle-size mites. The baby birds had been captive in the nest. The mites are so tiny that it would be impossible to pick off the individual ones, and their numbers were overwhelming in any case. The babies may have either hopped out on their own or been thrown out by the parents in a case of “throwing the baby out with the bath water.”

  Getting rid of small nest parasites might involve preventive home maintenance measures. No cases are known where birds respond directly by applying miticide to their babies or nests, but some birds have been thought to incorporate aromatic greens into their nests that could discourage nest parasites. But this is at the time when the nest is being built, not afterward in response to mites and/or other parasites as they appear. Nevertheless, home maintenance behavior is indeed well developed in many birds, and it takes some fascinating turns. It starts with nest hygiene.

  Nest hygiene starts at nest building. A ground-nesting bird may scrape the ground, clearing it of debris. A woodpecker, nuthatch, or chickadee excavating a nest cavity picks up the loose chips on the bottom of the nest hole, carries them out, and drops them. In one recent comic twist of this behavior that made the news, a car wash owner in Frederick, Maryland, was losing “significant money” each week from one of his machines. He suspected his employees of having a key to the change box and stealing his money, so he installed a camera to catch the thief. Photographs showed a bird sitting on the change slot, with three quarters stacked on top of each other in its bill. There was no apparent explanation, and several people e-mailed to tell me the apparently puzzling story. Like almost all the amazing YouTube postings where the relevant details are left out, this one was not puzzling at all; a pair of starlings had found the loose-change cavity a convenient site in which to build their nest, and they were cleaning out the “trash” to make space to build it.

  Nest hygiene is the behavior of removing foreign or harmful material from a nest. Phoebes, for example, carry off the hundreds of fecal droplets that their babies produce in the nest. They usually “catch” them immediately after their baby offers one, an event conveniently programmed to occur when a parent is present, namely, immediately after the baby is fed (or induced by the parent palpating the nestling’s cloaca with its bill, as I observed in flickers). I have watched fecal pellets accumulate under phoebe nests only when it is late in the second clutch’s development and when there have been two clutches in the same nest in a single season. Such nest hygiene is practiced in most small perching birds, and it probably functions to reduce the disclosure of the nest location to potential predators, as well as to prevent the soiling of feathers. Almost all warblers and other birds with open nests practice such nest hygiene, although those nesting on cliffs or in hollow trees that predators seldom breach practice it less or not at all. Those encased in solid fortresses or safe locations may, instead of packaging their feces in easily grasped and transported packages, projectile-defecate liquid “mutes” instead. That is, they literally “shoot” their feces out and away from the nest.

  Nest hygiene functions to maintain the welfare of the young, but that also involves something much more important than cleaning up wastes, namely, distinguishing rightful residents from the unwanted ones that insert their young (in the egg stage) into their home. Since many birds lay their eggs into the nests of other individuals, the parasitized parents must discriminate against foreign eggs—against those that pawn parenthood onto them at the expense of their own reproductive effort. Foreign eggs may (as in cuckoos) result in the outright murder of the whole clutch, or (as in cowbirds and in birds that lay their eggs into the nests of others of their own species) reduce an unsuspecting pair’s reproductive output or at the very least increase their workload.

  Nest hygiene, in the form of destroying or expelling foreign eggs, has evolved in many birds as a counter-strategy of others’ egg dumping. Elaborate strategies of color-coding the eggs have evolved, where the colors and color patterns of parasite eggs match the host’s eggs ever more closely, and hosts make ever more differently colored eggs and ever finer discriminations in an ongoing arms race. Mistakenly accepting parasitic eggs can potentially cost the hosts their young through parasitism, whereas too-fine discrimination of differences could result in them ejecting their own offspring.

  Whether or not it is appropriate to throw an egg, one that appears to be “different,” out of the nest depends on the balance between the likelihood of being parasitized and the cost of accepting a parasite egg. Chickens, for example, readily accept almost any foreign egg because there is little cost in having more babies, since the adults don’t have to feed them. Ravens, I found out by experiment, accept almost anything, including red-painted chicken eggs, potatoes, and flashlight batteries. Their tolerance is not based on inability to distinguish these items from their own eggs. Instead, it is because they are unlikely ever to be cuckolded; their large size, high vigilance, and strong territoriality make it easy for them to detect strangers coming near the nest, so the benefit of acquiring nest-content-ejecting behavior is less than the cost.

  Consider now the very consistent and potentially high cost of accepting practically invisible parasites that can weaken and kill. Such parasites are no problem when they are few, but those few can become the seeds of destruction if they multiply fast enough, until it is too late to combat them by direct one-to-one encounters. They are the aforementioned fleas, bedbugs, mites, and lice. Discrimination is not always possible; maggots of certain species of flies may either be harmful if they suck the blood of the young, or helpful if they eat their wastes, as in some woodpecker nests.

  Problems arise when the home is used for a long duration for each of the deposited parasite eggs to grow to an adult and for those adults in turn to stay and reproduce for multiple generations. That is what the bedbugs, mites, and fleas do, because the larvae and the adults feed on the same thing: the blood of the ever-present living occupants. In the arms race between these unwelcome houseguests and their hosts, it would appear that the guests have the upper hand. Often they do, but being too successful would guarantee their own eventual extinction, because they would eliminate precisely that which they need to survive on. They exist because they are held in check, perhaps since their hosts have evolved defenses such as fast growth rates of the young and/or frequent change of homes.

  Biochemical immunity and/or biochemical defense derived from medicinal plants are also potential options against the often invisible home crashers. Many insects, such as ladybird beetles and monarch butterflies, have evolved to incorporate toxic compounds from plants into their bodies that make them distasteful to potential predators. We have learned to use some of these compounds as repellents, and it would seem natural that some birds protect their nests with them as well. But, as with most things, in practice it is far from straightforward.

  Starlings are one bird species that appears to exploit plant chemicals in nest defense. Recent studies by Helga Gwinner and colleagues in Austria have demonstrated that the European starling, Sturnus vulgaris, a cavity-nesting bird, may incorporate green plant material into a nest. Dry material is usually a preferred nesting material for most animals, because it is the best insulation and prevents rot. But since these birds did incorporate green aromatic herbs, it was thought that their potentially negative effects are counterbalanced by medicinal value, specifically as a miticide. Gwinner’s studies were aimed to test this hypothesis, and various other possibilities and nuances were evaluated.

  Perhaps the first surprise was that, although in starlings females are the main nest makers, it was the males who brought in the greens, and only near the end of nest building when mating occurred. Proximally, the greens functioned as sexual attractants—they were thought to
attract the females to the males who provided them. However, as with most sexual attractants, for them to be effective it helps if they have value. It turned out that, although the greens that the males brought did not affect mite populations in the nest, they did affect the health of the young, enhancing their immune function and increasing their growth rate.

  A just-hatched broad-winged hawk on a freshly inserted green fern frond. No greens are added during the month-long incubation, but after the young hatch, the greens are brought almost daily until the young fledge.

  I have on occasion looked into starling nests and had dissected one nest into its component parts and counted the number and color of the feathers it contained, to contrast them with those in a nest of tree swallows in an adjacent nest box (the starling had mostly brown feathers, the swallows almost exclusively white), but I found not a single green sprig in it. On the other hand, I found huge fresh green ferns and cedar sprigs lining the nest of the broad-winged hawk. These greens were so big, so prominent, and so fresh that they “knocked my eyes out” in their conspicuousness, yet I knew of no studies that examined the phenomenon of covering the entire lining of a nest with greens.

  “My” hawk nest was located about ten meters up in the triple crotch of a sugar maple tree. Broad-winged hawks build a rough nest structure of sticks, and they line the nest mold with chips of dry bark. Finished with this lining, these birds, like most others, do nothing further at the nest after laying their eggs except incubate for about a month. However, in the hawk nest that I climbed up to examine almost every other day, there was something unusual: it was only after the young hatched that the nest contained the foot-long fresh green fronds of ferns and cedars. Even stranger, the hawks continued to bring fresh greens about every day or two for the next month until the young fledged. Were these greens some kind of remedy to discourage bugs?

  The chronology of the insertion of the greens alone precludes any likely involvement of the various hypotheses previously proposed. Furthermore, in comparison with the amounts of greens incorporated in the starling nests, those in the hawk nest were massive. Since the greens were routinely and consistently replenished only after the young hatched, it had to do with the young. But what? My so-far-untested hypothesis is that the greens serve to provide a clean surface for the meat that the hawks bring in for food.

  Hawks routinely bring a surplus of food into the nest for the young and leave it on the nest platform for them eventually to dismember and eat. This behavior would be comparable to a human parent routinely lugging in a hunk of beef and dropping it onto the floor of the home to then let the kids scrabble over it. Broad-winged hawks have their young in the hottest part of the summer, and meat that is not immediately eaten can spoil quickly if infected with bacteria. Uneaten meat scraps and a soiled “floor” seeded with bacteria that had multiplied from previous meals would hasten spoilage. A layer of greens, replaced frequently, would prevent debris and soiled material from accumulating and so retard food spoilage.

  We too use herbs, chemical sprays, and swatters to keep the bug crowd as well as bacteria at bay. I didn’t do that in my home and as a result inadvertently adopted an unusual and perhaps uncommon houseguest. It was of the eight-legged kind whose specialty is catching the six-legged winged kind. I gladly let her stay, and she entertained me for two years, to then spark a scientific project.

  Charlotte II: A Home Within a Home

  There are spiders living comfortably in my house while the wind howls outside. They aren’t bothering anybody. If I were a fly, I’d have second thoughts, but I’m not, so I don’t.

  —Richard Brautigan, The Tokyo–Montana Express

  A LOUD BUZZING ALERTED ME, AND LOOKING UP I SAW A large bristly fly caught in a spider web that I hadn’t known was there. As the fly continued to buzz, a large orb web weaver dropped from a ceiling beam and pounced on the fly. In seconds the black fly was wrapped in silk and looked like a mummy in a white sack. The spider then attached the fly by a short strand of silk, and touching it with one of her hindmost pair of legs, which she held out stiffly, she then casually walked back to the log on the ceiling from where she had come, turned around, and while holding the fly by its front pair of legs, pulled it to her mouth in what looked like an embrace, and followed up with a very prolonged “kiss” with her fanglike chelicerae. Five hours later the empty hulk of the fly dropped onto my desk. This July 11, 2010, meeting was my first introduction to what would continue to be a housemate for the next year, and through the next summer as well. She had made her home in mine, and in honor of the fictitious spider E. B. White made famous, I called her “Charlotte.”

  Charlotte, while on the underside of her slanted web, has wrapped a moth in silk and attached it to a short thread, which she uses to carry the prey up to her lair above the net. She uses a hind leg to hold the prey away from her web while carrying it.

  E. B. White featured an imaginary orb web spider, possibly Araneus cavaticus (which he named Charlotte A. Cavatica, or Charlotte for short), living in a barn in his famous children’s novel Charlotte’s Web. The orb web spiders, genus Araneus, spin famously intricate and beautiful silk webs for catching insects. These webs are not just their hunting tool. They are also their territory to which they attach a home, which is usually a crevice, or a hoodlike roof they create by bending a leaf and holding it together with silk strands. The webs consist of sticky “catching threads”; radial “spokes” for holding the sticky threads; “bridge threads” like guy-lines for holding the net up; “signal threads” that inform the spider, through vibrations she feels in her legs, if prey is struggling in the net; “drag lines” for access into the web from her simple home; and still other threads for wrapping prey and eggs and transporting prey. Orb web spiders even employ a temporary silk; while beginning to make their web, they insert spirals of silk into the center of their orb that help to stabilize the spokes while they attach the sticky catching threads. They remove the central stabilizing threads (by eating them) before they finish adding the remainder of the sticky threads.

  The silk for all these constructions is stored as liquid, which the spider extrudes according to use from six apertures (“spinnerets”) at the tip of her abdomen, and this liquid hardens into the silk thread when it is exposed to air. How a spider keeps all her threads in order to produce an intricate orb web is a miracle beyond my comprehension, especially after every step of the process is enumerated, which makes it only more incomprehensible rather than less. Although E. B. White touted Charlotte, his “extraordinary” spider, for imaginary web-spinning prowess, she lived like many do, in a barn for a summer, and she died in the fall like all northern orb web spiders are reputed to do, namely, at the age of one year, after they have laid their huge clutch of eggs and wrapped it in a silk sac for safekeeping through the winter. (I’ll have more to say on orb web spider life span later.)

  Adult females make silk egg sacs in the fall and guard them for their remaining short life. In the spring, the dot-size spiderlings exit the sac and extend threads that catch the breeze to waft them aloft until they eventually settle far from their origins. I am familiar with several local species of this family, the Araneidae, and my “Charlotte” was, as I later verified, indeed the one named “barn spider,” Araneus cavaticus, the one featured by E. B. White. Another of the local large orb web spiders, the pale white and yellow six-spotted orb weavers, Araniella displicata, weave leaves together to create an envelope-like shelter, leaving an entrance at the bottom through which they exit to their net below that is strung across a small clearing in the brush. The common garden Argiope spiders, Argiope aurantia, perch usually directly in their webs on the milkweed and goldenrod in my field. A marbled orb weaver, Araneus marmoreus, has for the past two years made its web at the same spot at the front of my outhouse. It uses the roof to hang its web and spends most of its time poised to pounce from a cleft under the roof. Spiders, though, don’t always have such discriminating home specifications. In many other kinds of spid
ers, such as nursery web spiders, Pisauridae, their homes become the place where the eggs hatch and then become the “nursery” of the small young where the adult female stays to guard them. Others, such as wolf spiders, Lycosidae, carry their egg sacs until the eggs hatch, and then continue to carry or guard the young after they hatch. They carry their young on their back in a tight backpack-like ball. Thus, baby carrying is not a strategy of only some Hemiptera bugs, marsupials, and primates, all of which (except for one exception we will examine later) do not make a home.

  White’s Charlotte was deemed “extraordinary” because she saved her friend, Wilbur the pig, from being turned into bacon by spinning words such as “Some Pig” in her web directly above Wilbur in his pen, to be read by the amazed farmer. I’m more fascinated by the story of live spiders and was wondering what they might have to say by their own behavior, rather than what they thought about pigs. But, as with the famous imaginary spider, the skills that made my Charlotte amazing also related to her web.

  There is no way of knowing how long she had been in my home, but I suspect she had lived elsewhere before I first met her, because she was already huge then. By three weeks later, in August, more flies had made the mistake of flying to the screened window, trying to get out, and then instead getting into her net. I suspected she would soon convert her food into eggs rather than growth.

  One day, at 7:30 a.m., a little before my breakfast, I decided to feed her first. I ventured out into the warm sunshine and grabbed a bumblebee from among those that are at this season numerous on meadowsweet blooms. Back inside, I threw the bee into Charlotte’s web. She almost instantly “dropped” along a thread the seventy-five-centimeter distance from her home, the lair on the ceiling, into the center of her web. The bee remained motionless. Charlotte stopped dead in her tracks. But only for a second. Then she held her front feet to some threads, jiggled the web with them, and almost immediately turned directly to the bee, which was ten centimeters beyond her. She grabbed it and in the same motion began to twirl it with seemingly most of her eight legs, as a skilled juggler on her back might rotate a ball on her feet. Silk spewed from the spinnerets and in ten seconds her prey looked like a white mummy except that it was buzzing and I could see the bee’s barely moving legs laboring inside.