324 BRITISH BIRDS. 



The young seem to disappear earlier than their parents 

 in most species, the Red-backed Shrike being an ex- 

 ception. In those species in which the sexes are ahke, 

 and in which the males do not sing in the autumn, we 

 cannot tell which sex leaves first. In the case of the 

 Chiffchaff the males are the last to leave ; a pair of 

 Wliinchats left together in 1908 between September 

 30th and October 1st ; the last two Blackcaps seen in 

 1908 were females, but there is a possibility that these 

 were merely passing. 



It seems that certain individuals habitually stay 

 latest, just as certain ones habitually come earliest. We 

 have not been able to detect any correlation between 

 early arrival and late departure. 



Our mapping also provides a census of certain species 

 in the two districts, and shows the variation in numbers 

 from year to year. It will be seen in the subjoined table 

 that the numbers in 1908 were practically the same as 

 those in 1907, except in the Chiff chaff and Red-backed 

 Shrike, which show considerable losses. In almost all 

 cases the losses appear slightly greater than the gains, 

 but this is probably due to the fact, that it is easier to be 

 sure that a bird which certainly was here last year, is 

 not here this year, than to be sure that one which is here 

 this year was not overlooked last year. The minus 

 number in 1908 compared with the 1907 number gives 

 the proportional loss on migration of the adult males, 

 and hence their length of life ; but it would not be safe 

 to work this out on one season's difference. 



What becomes of young birds we cannot pretend to 

 say ; the individuals shown in the plus number in 1 908 

 are presumably young of the previous year. We have 

 occasionally found two males of a species arrived at the 

 same place and singing at one another. Eventually, as 

 with Robins in the autumn, one has disappeared, or else 

 has settled in an unoccupied place near by. 



The observations of only two years are insufficient to 

 form a basis for definite conclusions in all cases, but in 



