74 THE MIGRATIONS OF BIRDS 



with particular promptness on their appointed days. 

 Severe indeed is the weather that delays them for 

 any length of time. 



Departure for the south in autumn is prompt, but 

 has greater range of variation, particularly in middle 

 latitudes, as prolonged mild weather may induce 

 birds to remain beyond their custom. Many late 

 fall records are based on individuals that have been 

 injured, have not moulted properly, or for other simi- 

 lar reason have delayed their departure. Many of 

 these are unable to migrate, and may linger along 

 until winter, or may possibly endure through the 

 cold weather. Ducks with broken wings, or other 

 injuries from the hunting season, often winter where 

 they can find open water, and instances of winter 

 occurrence of unusual birds are frequently recorded. 

 Catbirds, gnatcatchers, and tanagers have all been 

 found in the north in midwinter, and in December, 

 when the ground was covered with snow, the Cape 

 May warbler has come to the window-sills of the 

 National Museum. The red-headed woodpecker is a 

 bird that may or may not retire southward, appar- 

 ently more or less through whim. When it remains 

 to winter in the north, it does so when acorns are 

 abundant; on other years, when mast is equally 

 available, it retires southward. 



As a general rule, among small birds the seed- 

 eaters compose the early migrants in spring as they 



