Perhaps nothing speaks to the cleverness of birds more than the variety of their nests. Driven primarily by predation, birds construct and locate nests to produce maximum survival of youngsters. Nests also favor efficient heat transfer during incubation.
Nests vary from a hummingbird’s small cup, only two centimeters in diameter (about three-quarters of an inch), to an eagle’s six-foot-wide platform, and from the scrapes left in the ground by shorebirds to the massive piles of vegetation heaped up by megapodes, birds found in Australia. Their nests can stand more than 15 feet tall.
Birds find nests in natural and manmade structures, build them from scratch, or take them over from previous owners. Nests are generally hidden, inaccessible, or camouflaged. Nest materials typically include twigs, grasses, leaves, rootlets, fungal threads, spider silk, mammalian hair, feathers, and mud.
A few birds do not build nests. Whip-poor-wills and nighthawks, for example, lay their eggs directly on the ground without moving a leaf. Auks and murres lay theirs on narrow ledges on the face of cliffs. The seabirds’ eggs are pointed on one end. If bumped, they roll in tight circles rather than over the edge.
Tree cavities are occupied by many birds, including ducks, owls, flycatchers, chickadees, and woodpeckers. Some cavities are natural. Most of the others are excavated by woodpeckers, and many of these are later re-used by other species.
Mud adds strength to nests. American Robin commonly mixes it with grass. Barn and Cliff Swallows gather it by the mouthful. Each mouthful dries as a small brick. Ovenbirds of Central and South American use clay to construct covered nests that resemble early Spanish ovens. When they bake in the sun, they become nearly indestructible.
Shallow scrape nests are common but seem irresponsible places to entrust eggs, especially when located in the open. Eggs laid in most scrapes are patterned, not plain, which helps camouflage them. Gulls, terns, and other birds have visible nests, but they usually assemble in large colonies where group protection seems to compensate for the exposure.
Most songbirds build cup nests from small twigs, grasses, other fine plant material, and spider silk, and locate them either on the ground or in trees. Hummingbirds use proportionately more spider silk than other species and typically cover the outside of their nests with lichens.
There appears to be a relationship between brain size (at least cerebellar development, because it controls fine motor skills) and nest complexity. Tailorbirds, a group of Old World warblers, are a good example. They poke holes in leaves and sew them together with grass, strips of bark, or spider silk, forming a small basket. Then they line the inside with grass and make their nest.
The most complex nests are woven. Orioles and other New World weavers fashion solitary, pendant nests that are a few inches deep, while oropendolas, large colonial blackbirds found in Central and South America, weave hanging nests that can be more than three feet deep. Their nesting tree is a spectacular sight.
Males of Old World weavers tend to build ovate nests with an entrance tube extending downward, opening at the bottom — a deterrent to snakes. During courtship, if a female weaver doesn’t accept a male’s nest, he tears it down and makes another.
While nests confer a reproductive advantage to birds that use them, there is a negative side: Nests also provide hiding places and food sources for parasites, such as mites, lice, fleas, and several insects. Nestling mortality can occur.
One way to combat the intruders is through chemical warfare. While a majority of nests are lined with plant material that simply serves as an egg chamber, some birds add yarrow, lavender, curry, and other plants that give off aromatic chemicals that tend to repel parasites. As the chemical concentration in the nest wanes, the birds add fresh vegetation.
Anecdotal observations of House Sparrow and House Finch nests in urban areas prompted studies that showed that nests containing cigarette butts have fewer parasites than nests that don’t. Nest mites were drawn more readily to heat traps with non-smoked cellulose cigarette filters than traps with smoked filters. The takeaway is that mites avoided the nicotine and other chemicals present in the smoked cigarettes.
This isn’t surprising, considering that the tobacco plant produced nicotine as a defense against plant-eating insects. It is astonishing, though, that urban birds utilize smoked cigarettes in the same way that non-urban birds employ aromatic-chemical-emitting plants. That birds have adapted to their urban environment so quickly is surprising. Their ingenious array of nests, nest materials, and nest sites is clearly amazing.
This article from Eldon Greij’s column “Amazing Birds” appeared in the July/August 2015 issue of BirdWatching.
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