The central plain of Mexico is at a considerable elevation, ranging from 3,000 ft. at the frontier to 7,300 in the immediate neighbourhood of the City of Mexico. The air is, consequently, considerably rarefied, and one would suppose that birds would find it difficult to practise sailing and circling flight; but that does not seem to be the case, as, at any time during the day, numbers of turkey-buzzards can be seen sailing or floating and circling with only a rare flap of wing; and, as a proof that rarety of atmosphere does not seem to diminish their sailing powers, I have seen at Tres Marias, a station on the Cuemavaca line, as it crosses the hills bounding the southern limit of the plain, at an elevation of over 10,000, these birds sailing equally gracefully and without any appearance of labour. There can be little doubt that the air at this elevation has less supporting power, and, therefore, it must be compensated for by the greater speed if the upward current, and shows how wonderfully adapted the wing of the turkey-buzzard is for sailing flight, and that at a lower elevation the wing could support more than the weight of the bird.