The main part of the paper is devoted to a description of differences in the microscopic appearance and structure of the crown-feathers of the Marsh-Tit (Parus palustris) and the Willow-Tit (Parus atricapillus), amending and amplifying an earlier description by Dr. P. R. Lowe (1932), which appeared to indicate a remarkable degree of difference in the minute structure of the feathers of these otherwise remarkably similar species. It is shown:
- 1That although anterior crown-feathers of adult birds can usually be separated under the microscope without difficulty, the differences are much less than the earlier account suggested.
- 2That the apparent greater breadth of the Marsh-Tit barbule, which is one of the main features enabling the feathers to be distinguished from those of the Willow-Tit, is not due to any marked difference in the real width of the barbules, but to those of the former species being more strongly twisted at their bases, so that their broad faces are more nearly in the plane of the surface of the feather. This, in conjunction with a slightly closer placing of the barbulea on the barb, no doubt helps to increase the amount of light reflected from the surface of the feather, but the most important factor in this connection is the amount of pigment present.
- 3The segmented or nodal structure of the barbules is quite well developed in the Willow-Tit as well as in the Marsh-Tit, though obscured in an undisturbed feather by reason of the fact that the barbules in the former species are viewed edge-on.
- 4The characteristic specializations of the Marsh-Tit feather decrease from front to back of the black cap, the hinder feathers closely resembling those of the Willow-Tit. This difference between anterior and posterior crown-feathers is less marked in forms with maximum gloss, like Parus major.
- 5The juveniles of the glossy-crowned forms have crown-feathers resembling those of the non-glossy forms in microscopic appearance.
The demonstration that there is no real structural or other important difference between the feathers of P. atricapillus and P. palustris removes any support (which the previous description might seem to have afforded) for the view that they represent two distinct stocks which are converging rather than forms diverging from a relatively recent common ancestor.
The second section is devoted to an account of the microscopic characters of the crown-feathers of other black-capped Tits, with reference to various species presenting miscellaneous points of interest. In general it appears that the Marsh-and Willow-Tits may be regarded as representing two stages in the evolution of a strongly glossy blue-black cap from an undifferentiated brown one, the later phases entailing an accentuation of the characters which distinguish the Marsh-Tit feather from the Willow-Tit. The view that the Willow-Tit is a more primitive form than the Marsh-Tit is consistent with its geographical distribution.