102 



The testicles of the one year old males in the wintermonths are 

 poorly developed, those of males in their second winter (in the 

 same months) more strongly, those of older birds (from the same 

 months) are more or less swollen. As far as I know, it cannot be 

 made out anatomically whether the male in the preceding summer 

 was mature or not, but this has to be concluded from a comparison 

 of the measures of the testicles. And so it depends for a good deal 

 on the number of birds which have been examined, whether one 

 can recognise the bird as one or two years old or older. In spring 

 it can be done very well, in the wintermonths it is more trouble- 

 some. In the Razorbill the difference of measures often is so small 

 that it is better to go by the number of furrows in the bill (see 

 further on). 



Summary of Material. 



(~f ad, =: cf in third or following autumn, winter or spring. 



cT 1919 = cT hatched summer 1919. 



cT juv. ^ cT in first summer, winter or spring. 



In the Razorbill birds with three to four billfurrows have been 

 called adult, even if the genitals clearly seemed to show that 

 they had not been in function the preceding summer or before 

 (see Conclusions: Maturity). 



All birds were collected at Noordwijk-aan-Zee. 



Uria troille (L.). 



1920 December 1 cT ad., 1 9 ad., 3 cTcT juv., 1 9 juv., 1 9- 



1921 January 8 cTcT ad., 9 99 ad., 1 cT 1919, 3 99 1919, 2 99 



ad. vel 1919, 16 cTd" juv., 1 c^ 1919 vel juv., 4 



99 juv., 2 d^cf, 3 99- 

 February 1 cf ad., 1 cf juv. 

 March 5 cfcf ad., 4 99 ad., 5 cf cf 1919, 8 99 1919, 15 



cfcT juv., 10 99 juv. 

 April 1 9 ad., 2 cTcf 1919, 2 99 1919, 1 dd" juv., 4 



99 juv. 

 May 1 cT 1920 vel 1919. 



August 1 cf juv. (1921). 

 September 12 cf d" ad., 2 99 ad., 3 cf cT 1920, 5 cfcT ad. 



vel 1920, 1 cf (1920?), 2 c^cT juv., 2 cTö" juv. 



vel 1920, 1 9 JUV. 

 October 2 cTo" ad., 3 99 ad., 4 c^cT juv., 2 juv. (sex?). 



