82 DWIGHT 



been masters of microscopic technique have sometimes signally 

 failed to grasp the rudiments of wear, let alone those of moult. 



It would seem to be an easy matter to determine the age of a 

 feather by the amount of wear, but as a matter of fact it is not. 

 All of the factors I have mentioned must be taken into consid- 

 eration. Minute and careful study, not only of single feathers, 

 but of many feathers, the whole plumage in fact, is necessary 

 in order to reach conclusions. Even then, in some cases, one 

 must make comparison of many birds in order to eliminate in- 

 dividual irregularities. It is not difficult to say that a feather 

 is not new, but without some corroborative evidence, aside 

 from the feather itself, it is not easy to estimate whether it has 

 been worn, let us say a couple of months, or perhaps three 

 times as long. A dark feather growing at the side of a light 

 one shows far less wear in a given time, and in the same way 

 remiges and rectrices of young birds, compared with those of 

 adults, show much more wear, but it is only possible to prove this 

 by knowing that all of these feathers grew at the same time of 

 the year. Hence the importance of knowing the autumnal plum- 

 age of both young and old birds in order to estimate wear. On 

 a correct estimate often hinges the question of a moult that may 

 have occurred in southern latitudes during the winter months. 



It is, however, quite possible to reach intelligent conclusions 

 in many cases without other aid than the naked eye, although a 

 lens magnifying ten or fifteen diameters achieves better results. 



II. PROCESS OF MOULT 



The moult of a bird is a physiological process, whereby new 

 feathers grow periodically to replace the old ones. The whole 

 plumage may be renewed or only a part of it and the moult 

 periods must not be confounded with occasional new growth at 

 any time and anywhere to replace feathers accidentally torn out. 

 There are two seasons of moult peculiar to the adults of most of 

 our Passerine species, one in all species which is complete fol- 

 lowing the breeding season, and one in some species, which is 

 usually incomplete, preceding it. The first, the post-nuptial, re- 



