Summer World_ A Season of Bounty - Bernd Heinrich [32]
Eighty million years ago, in the Cretaceous, the birds’ relatives, Maiasaurus, were hollowing out holes in six-foot scrapes to deposit their eggs and then care for their young in them. They nested in colonies, as many birds do now, probably because there is safety in numbers, and maybe also for defense. Undoubtedly, nest building by insects is at least twice as old.
Unlike dinosaurs, small birds had the option of building their nests in trees, and also of hiding the nests, which requires more finesse than digging a crude scrape. As with insects, each bird species makes a nest as distinctive as its plumage and just as circumscribed or encoded in its DNA. The goldfinch’s nest is wedged into a vertical fork and is made of fine grasses and plant down. The oriole’s is a bag made from the fiber of dead milkweed plants and is hung from the tip of a long limb on a spreading tree. The chestnut-sided warbler’s nest is hidden near the ground in dense meadowsweet or raspberry vines and is a flimsy affair made entirely of very thin grass stalks. The tree swallow’s nest in my bird box is made loosely from dried grass, and almost invariably the nests of this species are lined with feathers, preferably white ones. Robins build a hardened mud cup on leaves and debris and line it with thin grass strips. Wood thrushes commonly incorporate a snake skin into their nest. Catbirds line their nests with fine rootlets and use grape bark to garnish the exterior. Ravens and chickadees line their nests with fur. Often the specific items used in the construction seem to have little rhyme or reason, but the nests are always exquisitely “perfect” in functional design and constructed unerringly.
Many kinds of wasps make nests from clay or mud mixed with saliva, as barn and cliff swallows do: the mud hardens, and as long as it stays dry, it stays solid, like concrete. Like birds’ nests, wasps’ nests are shelters for their offspring. Solitary wasps, however, provision their nest not only with their eggs but also with food for the larvae after they hatch, and then seal the nest off to prevent parasites from entering. Some wasps, like the potter wasp, make a nest that closely resembles a narrow-necked jar.
The organ-pipe mud daubers I had been watching are another kind of solitary wasp that uses mud to make nests, but of a very different design from those of the potter wasp. The daubers fashion an upside-down tube with an entrance at the bottom. The tube is plastered against a wall (such as a cliff face or house wall). After making a first small section of tube, the female wasp (no male insects make or help make a nest, or provision one, or sting) collects spiders; jams the prey, still alive, up the tube; inserts an egg; and then makes a partition at the bottom so that the contents, the prey, won’t fall out. She does not have to be concerned that the spiders will crawl out, because after she catches them she injects them with a chemical that keeps them in a zombie-like state of suspended animation. As a result, they don’t struggle when carried, and they will still be alive and fresh when the larvae (which look like white grubs or maggots) need to feed on them days or weeks later.
Fig. 14. Nests of the organ-pipe mud dauber. Left: Exterior of a nest, with ridges resulting from successive loads of hardened mortar. Three adjacent nests have been opened, each showing three cells containing various stages of development, from egg placed on fresh spiders to larvae (“grubs”), and to pupae.
The vertical tube design is efficient, because to have a cell for the next potential offspring, the wasp merely extends the bottom of the tube. And so she may continue to make one cell below another in a lengthening