PLAN OF MAPPING MIGRATORY BIRDS. 328 



Avas at Wye on April 23rd, 1908 (the day before the 

 snow), when seven passing Willow- Wrens were observed 

 where only eighteen local ones had arrived. 



The method of arrival which we have observed in these 

 two districts agrees with what has been made clear in 

 the " B.O.C. Migration Reports," namely, that each 

 migratory " wave " drops a few individuals of a species 

 in a district. Thus the filling of these two districts 

 with any one species takes several weeks. In 1908, 

 for instance, the first male Chiff chaff belonging to the 

 Tunbridge Wells district was seen on March 30th, while 

 another of the Chiffchaffs of the district did not arrive 

 until about April 30th ; some Willow- Wrens at Wye 

 did not arrive until after May 4th ; two Tree-Pipits 

 arrived near Tunbridge Wells on April 11th, while some 

 did not come until early May ; and one Blackcap reached 

 the same district on April 10th, others not until the 

 beginning of May. 



So far as we have been able to judge, certain males 

 are habitually among the earliest, others among the 

 later, arrivals. 



The males inhabiting one small district (such as a wood, 

 or stream valley) often appear to arrive together. In the 

 case of such a district which contained several pairs of a 

 particular species in 1907, but none in 1908, we conclude 

 that the males were travelling together and were overtaken 

 by some calamity. At Tunbridge Wells, the only five 

 Willow- Wrens of a district known as Bishopsdown, and 

 three Chiffchaffs close together near Langton, were 

 absent in 1908, and at W^ye three Sedge- Warblers, which 

 had inhabited a part of the river for at least three seasons, 

 likewise did not appear in 1908. 



The females seem to be always a few days behind the 

 males, and we have observed the curious fact that those 

 males which arrive first are first joined by the females, 

 so that an early pair may be building before the male of 

 a late pair has arrived. This suggests to us that the same 

 female returns to the haunt of its mate year after year. 



