From time immemorial it has been the current popular belief that birds of the same species never varied their style of architecture, but constructed the same form of nest, and out of the same materials, as their remotest progenitors did, instinct being the principle by which they were guided. This opinion, though long since exploded by scientific research, is still, I am sorry to say, entertained by persons who should know better. An examination of nests from different and widely-separated localities affords evidence of the most convincing character of its erroneousness. Most marked differences will always be found to exist in composing materials, as these are sure to vary with environment, and in a wider degree in the nests of some than in those of other species; even configuration, which is less prone to change, is often influenced by circumstances of position and latitude.
Among the Thrushes, the nest of the Robin is the most addicted to variation, and this is not wholly restricted to the constituents of its usually mud-plastered domicile, but is quite frequently observed to occur in the arrangement of materials, and in contour and position as well. Where low marshy woods abound on the outskirts of towns and villages, as is the case in Southern New Jersey, nests of this species have been taken that contrasted in a most wonderful manner with those one is accustomed to see in more northern localities. The great masses of grayish-green fibrous lichen, which depend from shrub and tree in sylvan marshes, are most freely used, and from its very nature to mat when pressed together all necessity for mud is precluded.
But the most curious nest I have ever met with was built upon a railroad cutting, where the ground had a slope of more than forty-five degrees. Such a position for a dwelling of the kind the Robin is known to build, to one not conversant with the facts, must appear incredible. But that it was accomplished, the nest itself was the monument of the builders’ thoughtful skill and labor. A semicircular wall of mud, eight inches in diameter and five inches in height, constituted the groundwork, and within the cavity thus formed was reared a coarse, substantial, bulky fabric, that was entirely composed of the stems of grasses, leaves and roots, loaded down and held in place by pellets of mud.