Yet though these nests have been invariable in material they have varied otherwise. Some have been so firmly woven and the material so stoutly packed as to defy the storms of a winter or two. Others have been so frail as hardly to be found when the leaves are off. Perhaps these slight nests are made by birds that were nestlings of the previous year and have not yet learned the complete art of nest-building.
Once I found one whose makers were skilled indeed. Instead of placing it firmly in a crotch and building up with the fern wool within a netting of fiber wound from twig to twig, as is the usual method, these had launched boldly into a new architecture. Perhaps they had neighbored the year before with a vireo. Anyway, they took the vireo’s plans and built a yellow warbler’s nest on them, hanging it from a nearly horizontal barberry fork, and finishing a fine, firm, pensile nest, vireo style, out of yellow warbler material. I never found this nest’s successor, and I am not sure whether, having found they could do it, they abandoned the type for the old home style, or whether something happened to the birds, and thus the warbler world lost budding genius.
Only one other nest have I found that seemed to be in any way abnormal, and this, unlike the pensile nest, seems to have had a very definite reason for its abnormality. The hollow part which had contained the eggs and young was in no wise different from that of all other warblers’ nests. It was the depth and firmness of the foundation which surprised me. This was built up to the height of an ordinary yellow warbler’s nest before the real nest began at all, and (the young had flown) I promptly took it home and dissected it.
Then the murder was out. The extra height had been added to the structure to circumvent the villainy of a cowbird. The cowbird lays her eggs in nests of birds that are smaller than herself and there leaves them to be hatched. She is partial to yellow warblers’ nests because the eggs that belong there are much like hers in coloring, though smaller, and the fraud is less likely to be detected. When hatched the young cowbird is so much larger and stronger that it starves out the other nestlings or crowds them out. The nest-builders in the main are foolish enough to bring up this murderous changeling; hence cowbirds are perpetuated. Perhaps these warblers had had one experience.
Anyway, finding the cowbird’s egg in their nest, they had promptly roofed it over with fern wool and fiber, built up the sides to correspond to the addition, and gone on with their housekeeping. Here was evidence of prompt action in an emergency in nest-building. I do not think it possible for the birds to have lifted the cowbird’s egg over the side of their nest and to have dropped it on the ground, which would have been the quickest way of getting rid of it. A yellow warbler’s nest “tumbles home” a bit at the top, as does the hull of a yacht, and I do not think their slender claws could grasp the egg and get it over that lip. Instead, they had done what they could,—imprisoned the intruder egg where it could not hatch.
I found it there, addled and nearly dried up within, and I rejoiced. The cowbird is a light-o’-love and abandons children on other people’s doorsteps. All such should be put in a pie. Since English sparrows became so plentiful the cowbird has shown a decided partiality for their nests for its abandoned offspring. I found a cowbird’s egg with those of an English sparrow that nested in a crevice right over my front door last spring. If cowbirds must behave in this nefarious manner it is not so bad to find them choosing the English sparrows for their dupes. The surprising part of it is to find the cowbird with sufficient courage to come in under the porch.
I’d like to watch a young cowbird growing up in a nestful of young English sparrows. The tender nestlings of the yellow warbler have no show, but I have an idea that here Greek would meet Greek, and after the tug-of-war the cowbird would be among those not present. Perhaps in the falling out both would fall out, at which most of us who love birds would not grumble.