( 103 ) 

 MAPPING MIGRATORY BIRDS. 



BY 

 H. G. ALEXANDER. 



My brother and I recorded our plan of mapping 

 migratory species in their nesting areas in Volume II. 

 (pp. 322-6) after only two years (1907 and 1908) of 

 observation. In 1909 I was able to map the same part 

 of the Tunbridge Wells district very thoroughly, but 

 in 1910, and 1911, I have not been there enough to do 

 such a large district with any thoroughness, and as I 

 have been absent during the chief part of the song-period 

 of a few species, it is difficult to give any figures that 

 would really show the relative numbers from year to 

 year. But as far as I can make out, the conclusion 

 which we reached in 1909, alter comparing that year 

 with 1908, needs no modification : namely, that the 

 numbers of a particular species hardly vary at all from 

 year to year. As we pointed out in our previous article, 

 the males inhabiting one small district seem to travel 

 together, and sometimes none of these turn up one year ; 

 such a case as this is probably responsible for the frequent 

 assertions, that a particular species is very scarce in one 

 year ; whilst on the other hand, if several new birds arrive 

 close together and settle in a part which is not already 

 occupied b}^ the species, that may lead to the statement 

 that a species is unusually common. But my own 

 observations in a larger area, with every individual 

 carefully noted from year to year, have forced me to the 

 conclusion that the aggregate change from year to year 

 is very slight. 



This year (1911), however, there has been one notable 

 exception, which I think should be recorded. This is 

 in the case of the Chiff chaff (Phylloscoims rufus), and 

 considering the wintry condition of the weather at the 

 end of March and beginning of April, it is easy to account 

 for it. As all the Chiff chaffs arrive in April, and as most 

 continue to sing until the middle of July, I have been 

 able to make about as sure of their presence this year 



