THE BIRDS OF NORTHERN THAILAND 19 



and May, falls off in June, and then begins to rise again to another 

 high point in September (when many waterfowl are nesting). 



The great majority of species have a definite nesting season of 

 fairly short duration, but some others have a very protracted one, 

 with certain individuals breeding in the hot weather and others 

 during the rains. 



Comparatively few birds' nests have ever been found within our 

 area; my information has been gained almost wholly from a study 

 of gonadial condition and the collection of young birds. 



MIGRATION 



Hosts of migratory birds from more northern latitudes find in 

 Thailand during even the cold weather the abundance of vegetable 

 and insect life required by their economy ; as a result, birds are more 

 numerous, both in species and individuals, during the winter months 

 than at any other season. 



The earliest arrivals among these visitors appear late in August 

 and early in September, and the latest stayers leave toward the end 

 of May, but the great migratory waves, bringing most of the winter- 

 ing species, appear in October and leave in March (before the indi- 

 viduals have assumed the nuptial plumage or have begun to sing). 

 The provenience of our visitors cannot yet be told with any cer- 

 tainty, but a study of subspecific differences indicates that one large 

 group is formed of species that breed in the mountains of north- 

 western Yunnan and southwestern Szechwan, while another more 

 probably comes from notheastern China, Korea, Manchuria, and 

 southeastern Siberia. 



A form that winters on the mountains is not likely to occur also 

 on the plains and the reverse is equally true. In certain cases, how- 

 ever, such as the tree pipit, birds racially indistinguishable occur 

 commonly in both environments, and the two populations have dif- 

 ferent dates of arrival and departure. In all such cases we find that 

 we are dealing with a form of extensive breeding range, and it is 

 logical to assume that we are concerned with wintering populations 

 that have come from distinct portions of that range by different 

 routes. 



In addition to normal north-south migration, we have indications 

 of other kinds of movements among our native birds. These are : 



1. The appearance of a few species, such as Merops s. philippiniw, 

 from farther south, apparently to breed in our provinces, since they 

 are seen there only in summer. 



2. The arrival of summer (rains) visitors to breed on marshes 

 that have been dry during the cold and hot seasons; this type of 

 migration is shown by many herons and rails. 



