The Pneumatic Fossa of the Humerus in the Passeres

ONE of the greatest hinderances to the successful unravelling of the interrelationships between passerine families is the shortage of morphological features varying sufficiently between these groups to be useful as taxonomic indicators. The search for potential taxonomic characters has concentrated largely on the skeleton, which is, in spite of other shortcomings, the easiest and most convenient anatomical material to collect and study. Except for some features used to divide the perching birds into suborders, almost all of the past anatomical-systematic work was based on skeletal characters, and indeed this work has rested almost entirely on cranial features. But, although the skull can and does provide many good clues to relationships between passerine birds, a shadow of doubt always hangs over the conclusions because of the convergence hazard resulting from similar feeding habits in unrelated groups. No matter how many different cranial features are employed, all may be functionally associated and thereby are under the influence of the same selection force; hence they should be considered as members of a single character complex (= functional complex or functional unit). Evaluation and comparison of the individual components of a character complex in taxonomic work is exceedingly difficult, and even when the greatest care has been used, the results may still be subject to question. In systematic work it is safest to consider a character complex (e.g., the skull and associated structures) as a single unit or taxonomic character. It would, therefore, be most desirable to be able to check taxonomic conclusions based on cranial features by the use of postcranial skeletal characters; these latter structures are, in most instances, not directly influenced by feeding habits and hence do not belong to the same character complex as do the cranial features. Unfortunately, the postcranial skeleton is highly uniform throughout the Passeres except for differences in proportions and other minor features, such as variations in the number of the ribs and vertebrae. These proportional and numerical differences have little, if any, value in showing affinities between groups of perching birds. Any feature of the postcranial skeleton varying between passerine families is consequently of great importance to the student of avian classification as a potentially valuable taxonomic character. The pneumatic fossa of the humerus is one of the few postcranial features exhibiting marked variation within the passerine birds. Although this variation was already known in the last century (Shufeldt, 1888), it has received little attention from avian anatomists. For example, it is not mentioned in the standard texts of Fiirbringer, Gadow, and Beddard, nor