The main requirement for building a certain style of bird nest is to have the bird that builds that kind of nest do the job! There are many kinds of nests and nesting places, and each is mainly specific to a specific bird species. Often, if a nest builder’s habit is known, a specific bird can be associated with that nest and/or nesting place. On the other hand, as much as bird species are true to the “code”, there are deviations, but most often in the place chosen for a nest.
A four-inch nest built mainly with grass and mud and lodged in a tree can be said to be a robin nest. A similar-sized nest lacking the mud would have a different builder. However, for those adaptive robins, the nest location can be something different than a tree branch, crotch or bark ledge, especially when there are, say, suitable man-made structures around. They will nest on building beams, ledges or other flat spots that are usually similar to their tree nesting habit in that they are eight or more feet from the ground. Robins appear to be “smart” birds, but at other times not so smart. Robins, when nest constructing on a beam, sometimes forget which beam they chose and, thus end up working on several nests none of which get completed. Too many projects!
Then again, choosing an unusual nest site doesn’t always end up in failure. I once heard of a robin that was seen sitting on a nest on a caboose that had just tailed a train into the railway yard after travelling many miles across the country. The train crew, when discovering the nest, set the caboose off on a siding and attached another caboose to the train. Train travelling wasn’t so bad, after all, for the nesting venture was successful.
Juncos, very consistently, build the same type of nest, using the same kind of materials and building it on the ground or at nearly ground level, almost always choosing a natural location. Even though I know that juncos stick to the junco building code, I was struck by the similarity of nests in two different photographs taken miles apart and taken at different elevations. A bit confused, I checked my notes to see which photograph was taken where. Juncos may pick a small clump of grass or other vegetation, the shelter of an overhang at the top of a bank, or the base of a small, thick conifer for a nest site. Next summer, if you run across an agitated junco, it could be that you are near its nest, so take a look around at sites that suit junco building code, at the same time being careful not to make a trail in the vegetation that predators might follow.
The two very small, round nests of soft grasses with, of course, identical eggs, looked very much alike, except the one found at 7.000 feet on Sphinx Mountain had six eggs in it while the other, found at 4,800 feet on Rolf Mountain, had five eggs. I thought, like some birds, the one at higher elevation would have less eggs because of the shorter season. But, then, juncos are seed eaters, so their food would be available late into autumn. And when snows came, they would just migrate downward.
Red-winged blackbirds use coarser material that makes for a much stronger nest, which is often built off the ground or over water and attached to cattails or sedge grasses. I once located a red wing nest taht appeared to be two colours, well, shades, of brown. On closer inspection I discovered that the top portion of the nest was wet while the lower part was dry. It was then that I realized the builder gathers wet material, which would be easier to form for the nest, and to weave around supporting cattail stems. The use of dry materials would have produced a pretty bristly and large, haphazard nest. Waxwings do that in a small way. The nest will hardly hold the eggs.
Birds follow their respective building codes, yet some species are opportunistic and versatile in how they interpret the code. Eagles may nest on cliff ledges or non-accessible rocky prominences where no trees are available. Where no trees are available, ravens are also known to nest on cliff ledges and, even where trees are available, on ledges in railroad tunnels. Great horned owls may choose large burrows in clay cliffs in which to raise their young. And, if you lived on prairie landscape, you probably learned that mallards, normally ground nesters, will nest high up in thick evergreen trees. Where else have you seen bird nests? For us humans it is best to stick to the codes!
Ed McMackin is a biologist by profession but a naturalist and hiker by nature. He can be reached at 250-866-5747.