570 PTEBYLOGBAPHY OF GOAT-SUCKEBS AND OWLS—CLABE. vol.xvii. 



wliile tlie owls liave four. The number of secoudaries varies in the 

 different genera, but it is almost always greater in the owls. It will be 

 noticed that these differences are all numerical and that the smaller 

 number is always the characteristic of the Caprimulgi. It is well 

 known that in the evolution of birds there has been a distinct tendency 

 towards a reduction of the number of remiges and this tendency has 

 been very marked in the specialization of many groups of small land 

 birds. This reduction takes place not only at the upper or inner end 

 of the forearm, thus decreasing the number of secondaries, but also 

 at the outer eud of the hand, thus decreasing the number of j^rimaries. 

 The former is much the more common and extensive method, so that 

 the number of secondaries may vary between six and forty; while the 

 latter is limited to one or two feathers only, the number of primaries at 

 least in Garinate birds, varying between ten and twelve.* We thus see 

 that in the structure of the wing, the Caprimulgi show a greater spe- 

 cialization than the Striges, although the arrangement of the coverts, 

 the absence of the fifth secondary, and the comparative lengths of the 

 primaries would seem to indicate that the original plan of the wing 

 was the same in both gronjis. This is in line with the conclusion to 

 which our examination of the general pterylosis bad brought us and 

 the consideration of the remaining characters may throw still more 

 light on the subject. 



TJie presence of qftershafts, oil gland, and doivn. — In the general 

 structure of the plumage there is a sui^erflcial resemblance between tlie 

 owls and goatsuckers, but a careful examination does not altogether 

 bear this out. The greatest difference lies in the presence of an after- 

 shaft on the feathers of tbe Caprimulgi which is entirely lacking in 

 Striges. Although stronger in Nyctidromus than Chordeiles, it is, even 

 in that genus, very weak, although I have always found it preseiil. 

 In the owls, however, it is uniformly absent and I have found no trace 

 of it in any of the si)ecimens whicb I have examined. This difference 

 can not, therefore, be easily explained away, at least not until we know 

 more of the origin and function of the aftershaft and are better 

 acquainted with the pterylograpby of all the genera. I have already 

 spoken of the i^ecnliar length of the calamus in the large wing and tail 

 feathers of the Caprimulgi, but I did not find the same structure clearly 

 shown among the owls, except in Glaucidium, where it was as evident 

 as in INyctidromus. Down feathers are very rare in both groups, if 

 present at all, and I found no trace of an oil-gland tuft in either, 

 although Nitzsch speaks of finding a trace of it in Strix. 



Number and length of feathers in the t<(il. — What has been said above 

 in regard to the reduction of the number of remiges in birds is also 

 true of the rectrices, although the evidence is less satisfactory. How- 

 ever, it will hardly be disputed that the presence often rectrices in the 

 Caprimulgi indicates greater modification than the presence of twelve 



* See Dr. Gadow's interesting article in Proc. Zool. Soc, 1888, j). 655. 



