Did You Know? Nesters in Cavities – Birds
Wood Duck, adult female © Glenn Bartley/VIREO
When the wood ducklings of North America are ready to leave their tree trunk nest in late June or July, they look down to the ground perhaps twenty to thirty feet away. Not hesitating, they come out of the hole in a rush and fling their tiny bodies outward as though they already know how to fly. But they fall, bounce and in a moment are ready to follow their mother, voicing her concern from a nearby thicket.
The wood duck is one of a number of waterfowl to use a cavity to build its nest.
Cavity-seekers are spread through almost every other family of birds: flycatchers, owls, penguins, auks, hawks, sparrows, starlings, jackdaws, woodpeckers and many others. The search for the right cavity becomes a study in the art of survival.
There are rarely enough cavities. Those birds that do not find suitable cavities do not raise families.
The bluebird of North America, which has long accustomed itself to seeking low cavities in hollow trees so that it would not be in such fierce competition with all the woodpeckers, now must contest the ubiquitous starling.
Cactus Wren, adult © Brian E. Small/VIREO
The cactus wren waits until a gila woodpecker has drilled a hole in a large organ-pipe cactus, then seeks to take over the cavity.
Elf Owl, adult and owlets in nest © Rick and Nora Bowers/VIREO
Owls, elf owls for example, vie with day birds for a chance at the hollow tree cavities or holes in saguaros.
Birds so well occupy every level of the environments of earth from the darkest caves to the highest mountains that it was inevitable that they would make such good use of holes in trees, or in caves, or in sandy and clay banks.
They have extended the possibility of their survival, not merely by accommodating themselves to natural holes but also by building their own, as do the many wood boring woodpeckers, the burrowing owls of the American prairies.
The bank-digging bee-eaters of Africa and Australia.
Bank Swallow, nest © Greg Lasley/VIREO
The bank swallows of North America dig long tunnels in soft banks, despite their relatively weak beaks and feeble feet.
The puffins of the North Atlantic, members of the auk family, dig deep burrows on offshore islands and so find sanctuary from prowling peregrine falcons, from jaegers and foxes. The urge to dig, to burrow, to drill wood, to seek complete concealment is an innate force of evolution as each species seeks the same result with different means.
The cavity-seekers are not united by species or in physical form or geography. They include many sea birds as well as owls, wrens as well as parrots, warblers as well as kingfishers. And there is no conformity to the types of cavities sought, or the methods used to nest in them, to build them, to acquire them or to commander them. Each species has conformed to the availability of cavities and has evolved accordingly.
Atlantic Puffin adult, breeding © Garth McElroy/VIREO
Puffins dig underground tunnels close together to create widespread underground refuges of nesting birds.
The rhinoceros auklet of British Columbia digs out individual tunnels, which may be as long as thirty feet, making it virtually impossible for any predator to reach the eggs buried so deeply in the ground.
The burrowers, it would seem, should be physically powerful, but this is not always true.
The bee-eaters are quite slight physically, and so are all of the smaller kingfishers. But this does not prevent them from digging with their beaks and kicking out the loose earth with their feet like digging mammals.
The woodpeckers are the master builders of the cavity-makers, as opposed to the cavity-seekers, and they are specially adapted to the work. They have heavy skulls, strong gripping feet and stiff tails to support them while clinging to the tree, powerful beaks with capability of making staccato blows-all of which enable them to dig into solid wood.
But not even physical adaptation is necessary to make a cavity-digger. Chickaees dig holes in trees but, unequipped with either heavy heads or strong bills, they search for trees already rotted and so are able to excavate as effectively as the woodpeckers can chisel.
After the chickadee come an array of birds that search for existing cavities, either naturally formed or made by others.
The nuthatches look for chinks in trees, formed by branches partially broken from the trunk and then healed into place. If the entrance is too big, they will partially seal it with clay, allowing just enough room for their slight bodies to slip safely.
The cavity-seekers and cavity-builders are studies in the force of evolution. They have grouped themselves according to the sophistication of their evolution.
Razorbill adult, breeding © David Tipling/VIREO
Those that nest in natural holes-fissures in soil or rock, apertures between boulders along the seashore, such as razorbills and guillemots, or in caves along the sea shore, such as Humboldt penguins-have not found it necessary to devise more complex shelters.
The next step in the evolution of the cavity nest is taken by those birds that use the burrow nest.
The small petrels, for instance, have weak, webbed feet and even smaller bills, but they somehow manage to dig deep burrows on the islands where they nest in offshore waters. Such burrowing is made even more extraordinary by the fact that is done at night-the petrels are totally day birds during the rest of their nonbreeding lives.
By digging a burrow the bird can be greatly extend its range of breeding possibilities.
Burrowing owls usually claims burrows that have been abandoned by prairie dogs or pocket gophers but is quite capable of digging its own.
Puffins are safe from gyrfalcons and peregrine falcons and eagles, which once constantly prowled their northern breeding islands.
Black-capped Petrel, adult © Harold Stiver/VIREO
The petrels are safe from their great enemies, the gulls.
Many birds of the Andes have been able to colonize the high mountains because of their capacity to dig in loose soil or under rocks, thus achieving not only safety from enemies but also more or less constant temperatures.
There the technique is modified yet further by the Andean flicker which, lacking trees for its nesting place, has taken to burrowing in to the ground.
The tree-cavity nesters include a more widely diffused group of birds.
The woodlands and the forest edges give more opportunities for breeding and feeding than do less diverse environments, such as open grasslands.
Thus many species have had the chance to exploit the opportunities if cavity-nesting. Here both owls and parrots find similar refuges, as do the hoopoes and the rollers, the pigeons and the toucans, the trogons and some of the ducks, the flycatchers and the starlings, the tits and the barbets.
In one sense, at least, the cavity-seekers are all more successful forms of evolution than the nest builders, despite the great skill of the weavers and the others that make complicated warmer constructions.
Only about half of the eggs laid in cavities are hatched out.
Cavity nesting is a fine point of evolution, and the cavity-seekers exemplify the ingenuity of the bird to use every part of its environment to survive in a world of enemies.