THE Swallows appear to us to be such a well-marked and isolated Family of Passeres, that, in the absence of any detailed account of their anatomy and general structure, which, so far as we know, has not been attempted, there remains little for us to say. By Linnaeus and the earlier writers the Swallows were united with the Swifts and all were placed in the genus Hirundo. Then in course of time they were recognized as a separate Family, but they were not allowed to escape from their ancient entanglement, and, side by side with the Swifts, they flourished as members of the wide-gaping birds, or “Fissirostres.” When the “rostral” system became somewhat exploded, they were ruthlessly separated from their former companions, until now, in due course of re-action, they seem to be recognized as Passerine Birds of a somewhat Cypseline tendency, or perhaps, one might more truly say, the Swifts are Cypseline Birds with a Hirundinine tendency. For the nine years in which this book has been in publication, we entertained the hope that some competent Anatomist would enact such a comparison of the characters of the two Families that we should have been able to summarize the results, and tell our readers exactly how the Swallows may be differentiated from the Swifts in every point of structure. In default of this much-needed exhaustive essay from our more expert brethren, we venture to define the characters of the Hirundinidoe as follows:—
A. Passerine Birds with nine primary quills.
B. Spinal feather-traet bifurcated.
C. A single moult in the year, executed, so far as we know from the migratory species, during their residence in the winter home.
NO Passerine Birds, we believe, present this triple combination of characters, but further information is, of course, desirable.
In 1891, in the ‘ Catalogue of Osteological Specimens in the Museum of the Royal College of Surgeons,’ we placed the Hirundinidoe at the end of the series of “Passeres Norm ales,” and in our ‘Classification of Birds’ we further separated them from the ordinary Perching Birds or Passeriformes (Review Classif. B. p. 89, pl. xi.), placing them at the end of the series, and illustrating their position in the natural system as Passerine Birds without any very close allies. We have seen no reason to depart from this conclusion, and until the study of the anatomy, style of moult, and other features of certain Muscicapine genera—such as Artomyias and Hemichelidon.—tend to bridge the gap, we shall continue to regard the Hirundinidoe as peculiar Passerine Birds, standing well apart from the rest of the existing Passeriformes.
Dorsal view of Hirundo rustica, to show the pterylosis, with the spinal feather-traet forked on the lower back. (From the ‘ Catalogue of Osteological Specimens in the Royal College of Surgeons.’ by the kind permission of the President and Council.)
The Hirundinidoe were divided by us in 1870 into two Subfamilies—Hirundininoe (True Swallows) and Psalidoprocninoe (Rough-winged Swallows).
The Swallows are represented by twelve genera, which might almost be characterized by the peculiarities of their nesting-habits as follows :—