114 



the summer before. Those weak males, which at the end of April 

 still are in winterdress, even then have testicles of no more than 

 1 to 1.5 mm. width; the stronger males however, which at the end 

 of April are in transitional-plumage, have testicles of about 2 mm. 

 width; w^iereas the earliest moulters which in those same days 

 are in full breedingdress, have testicles of 13 to 15 mm. long and 

 2.5 to 3 mm. broad ')! So that the testicle-measures of males in 

 their first spring, which don't come to breeding that summer, are 

 larger than those of the same males ip. autumn, when they have 

 passed the summer in idleness! Though first-year-birds don't 

 breed, their genitals swell in spring and decrease 

 again in autumn! 



Measures: The foregoing list gives the billmeasures of 201 

 Guillemots examined, received between 29 October 1920 and 4 May 

 1921 and between 12 September and 21 December 1921. Besides 

 the billmeasures of a small number of birds, received at the end 

 of April 1922, have been added. 



What do we see now? Whereas the height of the bill of the 

 two years old male in breedingdress reaches no more than 15.2, 

 this measure for the old male in breedingplumage is abnormally 

 low, it reaches generally up to 16.—. Something like that we see 

 in the female. The now mature and apparently fullgrown bird is 

 not fullgrown and the maximum-measure of 18 two years old birds 

 (males and females) in nearly complete or complete breedingdress 

 remains considerably under the maximum-measure of 30 so called 

 old ones; their measures even are not any nearer to those of the 

 older than to those of the one year old birds. 



At the same time we notice that the growth of the bill takes 

 place during the moult into breedingdress, a fact which strikes us 

 most in the old birds, partly because of the latter more specimens 

 in breedingplumage have been examined than of the younger ones : 

 whereas the bill of the old bird in winterplumage in neither of the 

 23 males examined, reaches up to 16.— mm., 10 of the 13 old 

 males in breedingdress show a height of 16. — or more! So we 

 may say that the heavy bill completes the breedingdress; in the 

 summe rmonths it lowers again and even during the autumn moult, 



1) A male received on May 5th, 1922, 'without doubt hatclied oummer 

 1921, in breedingdress, with a naodérate number of growing feathers on the 

 inside of the skin of the neck, has testicles measuring 5 X 20 ! 



