METEOROLOGICAL CONDITIONS 75 



westerly winds, which are mostly accompanied by rain, and also 

 during fogs, no matter in what direction the wind may be at the 

 time. 



The extent to which these conditions of weather prevail at the 

 time of a particular period of migration positively determines the 

 extent to which the birds appear; and should they prevail 

 throughout the whole of the autumn or spring months, we may 

 not reckon upon seeing either Snipe, Thrushes, or any other 

 common or uncommon species of birds on the island. Whether, 

 however, the migrations come within the sphere of our observations 

 or not, there is no doubt that they proceed regularly under all con- 

 ditions, during a period of time peculiar to each particular species. 

 This is proved by the fact that if this particular migration time of 

 a species has once expired, not a single one of its members will 

 thereafter be seen, even though the most favourable weather for 

 their appearance may set in immediately. 



It is true that retardations in the migration of one species or 

 another do sometimes seem to occur. I say ' seem to' intentionally, 

 for the assumption of their being retardations is based on a wrong 

 interpretation of the phenomena. Thus, for instance, if in spring 

 the first half of the migration time of a species happens to be 

 already past, without any member of the species having as yet been 

 observed, and if thereupon the weather assumes a character favour- 

 able to the occurrence of this species, the latter will at once be 

 found to make its appearance. From a mere superficial observa- 

 tion of a retarded appearance of this kind, one might be very easily 

 led to suspect that the migration of the species in question was 

 only just then commencing, and had been delayed until then by 

 unfavourable conditions of weather. Such a conclusion would 

 however be incorrect, for in a case like this the separate individuals 

 of the species in question do not consist of fine old males, as ought 

 to be the case if one were really dealing with the commencement of 

 the spring migration of the species, but instead thereof such male 

 birds as are seen are scattered, younger individuals, while the 

 majority of the birds are females. This however only shows that 

 the old males, who form the advance guard of the spring migration, 

 have long since arrived undisturbed, and therefore unseen, at their 

 breeding stations, and that these apparently later arrivals, instead 

 of being, as erroneously supposed, the vanguard of the migratory 

 host, are in reality the second division of the main body of 

 migrants of that particular species. 



What has been said above is amply proved almost every year 

 by one or the other of such species as display more or less marked 

 difference in the colour of the plumage according to age and sex, 



