Popular Science Monthly/Volume 6/February 1875/A Short Study of Birds'-Nests
By CHARLES C. ABBOTT, M. D.
HAVING had many opportunities of examining the nests of those birds habitually breeding throughout Central New Jersey, during the past fifteen years, and so, familiar with the construction and location of such nests, I have, since the publication of Mr. "Wallace's essays on "Natural Selection," in 1870, endeavored to determine if the theory there expressed was applicable to the birds that are common to the locality we have mentioned.
In so studying birds'-nests, I have carefully avoided prematurely arriving at any conclusions that might influence my judgment when subsequently examining a series of nests, and therefore I believe the notes made concerning the construction of each nest, and the inferences drawn, are exact in the former case, and justifiable in the latter.
At the very outset, I found a careful study of the courtship of birds essential to a proper appreciation of their subsequent habits, and learned, not at all to my surprise, that marriage among birds, as among mankind, is not universal, but that both bachelor and spinster birds of every (?) species constitute a fraction of the ornithic population of our woods and fields.
I reached the above conclusion in this way: Having carefully gone over a given extent of ground, and noted every nest, say of the cat-bird (Galeoscoptes Carolinensis), I have then endeavored to learn about or precisely the number of individuals of this species frequenting the same extent of territory. As birds, during the breeding-season, do not wander any very great distance from their nests on the one hand, nor from the locality whereat they halt on their arrival in early spring, on the other hand, it is not very difficult to reach a very close estimate of the numbers of each species occupying a locality, any given season. Thus, during May and June, 1873, I found eleven nests of the cat-bird in a given area, and feel confident that I recognized twenty-seven individuals of this species. If this is correct, then there were five cat-birds not nesting, and, I should judge, all male birds. It will be remarked that I overlooked the nests of these "extra" birds. This I believe is impossible. By going over a given space—an acre at a time—prying into every nook and cranny, climbing every tall tree and searching over every small one, as well as bushes and brier-patches, it is hardly possible to overlook any nest, especially so large and conspicuous a one as that of the cat-bird. The habits, too, of non-nesting birds differ from those then breeding. They are much less restless, do not chirp and twitter, or exhibit distress when closely followed, as in the case of nesting-birds.
Having carefully examined a bird's-nest which seemed to agree most nearly with the published descriptions of such nests, I then noted each nest found and marked the amount of variation in the construction and position. Take, for instance, the nest of our very common robin (Turdus migratorius). Here we have a nest largely constructed of coarse twigs and grass, lined with "a cup-shaped fabric of clay or mud," this mud, again, being covered with finer grass, horse-hair, and occasionally a few feathers. This nest is an excellent one to study for degrees of variation in construction; and here note these differences. During the past spring and early summer, we found thirty-two nests of the robin in an area of about four hundred acres.
Of these thirty-two nests I will speak, principally, as to their construction, especially with reference to the care exhibited in the mudlining, and refer but incidentally to the positions of the nests.
Eleven nests were what might be called "typical," following the description given by Dr. Brewer in the latest work on North American ornithology. In the eleven nests the mud-lining was complete, extending to within about an inch and a half of the rim, or top of the nest. In fourteen, the mud-lining was more or less incomplete, although always extending over the bottom of the nest, i. e., so much of the interior surface as the eggs or very young birds rested upon. Without an exception, I believe, the fine grass and hair lining the interior of each nest were in greater amount in proportion as the mud-lining was imperfect; so that, in some instances, the mud being concealed, the nests were very similar to those of other thrushes. The remaining seven nests were altogether "abnormal," and, noticeably, each of these seven nests was in such position as a robin would not be supposed to select. A careful study of the surroundings, however, showed that there was always some outside advantage, such as proximity to abundant food, and this may have had some influence in the choice of location. As an instance, one of these seven nests was placed in a deep cleft in the trunk of an apple-tree. It had a southern exposure, was protected from rain by the trunk and branches of the tree, and, altogether was admirably located. But, as the tree itself had an abundance of branches, and for many summers had had nests upon it, there seemed to be some reason in the location now first occupied. What, indeed, was the cause of this change from the branches to the cleft, I could not discover. The nest itself was merely a few coarse twigs for extra support of the "clay fabric," which was placed so as to resemble a modified cliff-swallow's nest more than that of any other bird. If, now, young birds build nests through imitation, then the young robins reared in this nest will seek out somewhat similar situations for their own nests; but if such a locality did not suit the bird's mate, then a nest in a more exposed position would be built, but, I doubt not, with some of the peculiarities of the nest in which it was reared.
In comparing the eleven typical nests of the robin, it could not but be noticed that minor differences or peculiarities existed. These small variations were such as size, which was, in fact, considerable; in shape, some of the nests being rather oval than circular; in the choice of material for the interior lining, which, I am sorry to say was, in one instance, suspiciously similar to the lining of the nest of the chipping sparrow, and was probably stolen. Indeed, among robins, as well as all other birds, there are individual rogues, as well as cross-grained, scolding wives and husbands.
Taking a careful survey of the whole thirty-two nests, they suggested at once an ordinary village: there were handsome structures, such as opulence builds, and very modest ones, such as those in straitened circumstances are compelled to occupy; and, while the same causes for this variation in dwelling-places does not obtain among birds as among mankind, causes do exist among the birds, in many ways analogous. For instance, there are energetic birds and lazy ones. There are plucky birds that will overcome obstacles, and despondent ones that are easily cast down; and will not this of itself account for a great deal in the variations of birds'-nests? Can it be doubted that birds differ greatly in their temperaments? Who, that has kept canaries, has not noticed that, while some are cross, others are affectionate, others lively, and, again, others moody—that their dispositions were nearly as varied as in mankind? If it is admitted that variation in disposition exists among birds, may we not go a step farther, and claim also differences in mental ability—that, in plain language, the "smarter" bird will build the better nest? One reason why nests do not vary more than they do, simply, is—a mud-lined nest being best suited to a robin's welfare—that a bird reared in a poorly-constructed nest may be of greater ability and more energetic than its parents, and this, joined with the fact that the bird's mate may have been reared in a nest of perfect construction, of itself would tend to remedy, in part, the defects its partner might allow; these facts together would certainly secure an approach to, if not the complete attainment of, a "typical" robin's nest. So, as the years roll by, the nest of the robin would remain substantially the same, while the amount of variation that now exists would be perpetuated, and probably very slowly increased.
Why indeed, a robin should line its nest with mud, and a cat-bird should not, is probably past finding out, but, as changes gradually brought about by man's agency have already effected changes in the habits of some of our birds, so these same changes, ever in progress in the haunts of our robin, may cause these birds to gradually omit this lining of mud in their nests, and so make them more like the nests of other thrushes; just as the cliff-swallow, with us, no longer places a "bottle-neck" opening to its mud-built nests. There is an instability in the whole range of the habits of birds, going hand-in-hand with the undoubted tendency to variation in their anatomical details. Natural selection, or whatever may be the governing impulse that controls it, also indirectly causes the range of variation in the details of the construction of their nests, inasmuch as these variations of habit are the necessary result of changes wrought in the physical construction of the creatures themselves, for, stripped of the haze that metaphysics has gathered about it, as a bewildering gloom, we can see in the operations of the mind, in man and bird, only the curious results of the workings of those fatty atoms, intimately combined, we call the brain; and no argumentation can separate this brain and mind. They are just as interdependent, and parts of a single whole, as the eye and sight, the nose and smell, hearing and the ear, the circulation of the blood and the beating of our hearts.
A nest of totally different character, that of the well-known Baltimore oriole (Icterus Baltimore), was more carefully studied by the writer, inasmuch as it afforded more marked variations from what may be considered a "typical" form of the structure.
Mr. Wallace has shown that, where a nest is so constructed as to conceal the sitting bird, in all such cases, the birds are of bright, showy plumage, and would be easily detected by birds of prey, if not concealed when occupying their nests. Of the family Icteridæ, to which our Baltimore oriole belongs, Mr. Wallace says, "The red or yellow-and-black plumage of most of these birds is very conspicuous, and is exactly alike in both sexes. (This is not true of the Baltimore oriole, the female of which is much less brightly colored.) They are celebrated for their fine, purse-shaped, pensile nests." There are now two considerations worthy of attention, with reference to this bird and the character of its nest. In the first place, as the male bird is much brighter in the color of its plumage, would it not require a concealing nest if it assisted in incubation? Now, does the male bird assist in covering the eggs? It unquestionably does.
Secondly, if the bird-concealing nest, a "pendulous and nearly cylindrical pouch," as described by Dr. Brewer, is constructed solely with reference to the protection of the parent-birds, would it not be within the range of probabilities that, the danger no longer existing, the labor of constructing so elaborate a nest would be abandoned. Has this actually occurred? During the summer of 1872, I found nine nests of the Baltimore oriole within a comparatively small area; in 1873, I succeeded in finding seventeen nests in an area nearly ten times in extent; and during the present summer (1874) I found thirteen nests in an area of the same extent as that examined in 1873. These thirty-nine nests I classified as follows: Of the nine nests of 1872 that I examined, six were so constructed as to effectually conceal the sitting bird, and three were sufficiently open at the top to give a hawk, hovering above it, a view of the bird.
Of the seventeen nests of the oriole which I found and inspected during the summer of 1873, eleven of them were "bird-concealing" in their shape, and the remaining six like the three I found in 1872, i. e., open at top.
During the present summer, Baltimore orioles have been unusually abundant, and, of the thirteen nests I found, eight were open at the top, and five were long, pendulous pouches, that wholly hid from view the sitting-bird.
Bearing in mind the supposed reason for building a nest that would conceal the parent birds when occupying it, I noted down the exact location of each of these thirty-nine nests. In every instance those nests that concealed the sitting bird were at a considerable distance from any house, in uncultivated parts, the larger number on an unfrequented island, the others on elm-trees growing on the banks of a lonely creek. In both of these localities, sparrow-hawks (Tinnunculus sparverius) were frequently seen—they are nowhere so numerous as some seventy years ago—as compared with the neighborhoods selected for the building of the open-topped nests, all of which were in willow and elm trees in the yards of farm-houses, and in full view of the people continually passing to and fro beneath them. The conclusion drawn from the study of these nests was, that the orioles, knowing there was much less (if not total) absence of danger from hawks, therefore constructed a less elaborate nest—one which answers every purpose of incubation, and yet does not conceal them when occupying it.
Of the nests that did conceal the sitting bird, every one was really open at the top, and the bird entered from above. The weight of the. bird, when in the nest, appeared to draw the edges of the rim together sufficiently to shut out all view of the occupant. The rims of these nests that, when occupied, concealed the birds, were all much smaller and the nest itself deeper than in those nests where concealment was not considered in the construction, these latter being in every way much like the nest of the orchard oriole (Icterus spurius).
Originally, in all probability, when its enemies were more numerous, especially the smaller hawks, the nest of the Baltimore oriole w r as perfectly closed at the top, and with a side opening; but, of the many scores of this nest that we have met, we have never seen a nest of this bird so constructed.
The very fact of the Baltimore oriole constituting a partial exception to Mr. Wallace's supposed law of birds'-nests is, we think, here shown to be a proof of the correctness of his theory.