EXTERNAL FEATURES 31 



The testes of the mallard are active only during the winter 

 months and in early spring, and inactive in summer, but 

 there is no precise coincidence between the changes in the 

 testes and those in the plumage. Yet the normal passage 

 of the mallard from the full winter (breeding) plumage to 

 its dusky summer (eclipse) plumage is delayed if castration 

 is effected during the months whilst the gonads are assuming 

 or have attained activity. The delay has its analogue in 

 the well-established fact that if a colt is castrated when 

 shedding its winter coat, the shedding is for a time arrested 

 and thereafter proceeds very slowly. Further inquiry may 

 show that the change of plumage is in some measure 

 dependent on a testicular hormone ; it has to be borne in 

 mind that a small fragment of testes left during the operation 

 may regenerate the entire organ. But the general vigour 

 of the body is probably another factor. 



One of the special cases of plumage change is what 

 Charles Waterton called the " eclipse " of males among 

 ducks. " The male, as soon as his expectations of a family 

 are realised, that is to say, in June, doffs his coat of many 

 colours and puts on a livery very closely resembling that of 

 his spouse, and so closely that it takes an expert to dis- 

 tinguish the sexes at this time " (Py craft, 19 10, p. 277). 

 This " eclipse " used to be regarded as an interpolated 

 plumage giving the male a" protective inconspicuousness, 

 particularly important since the moulting duck is unable 

 to fly, but it has been shown that the " echpse " plumage 

 corresponds to the " winter " or post-nuptial plumage of 

 such birds as the plovers. An evolution is in progress 

 among the ducks towards bright plumage, and the eclipse 

 of the drake is a somewhat premature setting in of an evane- 

 scent dull phase. It lasts for a short time and gives safety ; 

 it is then succeeded by the brightly coloured plumage. As 

 is usual in such cases, the facts are somewhat intricate, and 

 it is difficult to avoid a false simplicity ; but the biological 

 gist of the matter is that we are face to face with a temporal 

 variation, a shortening down of one part of the seasonal 

 cycle and a lengthening out of another part. 



