NATURE'S CALENDAR gj 



comparatively mild weather of our South- j^^ - 



ern States, but press on, picking up more 



and more recruits, until they find them- 

 selves well within the tropics. Of the 

 peculiarities of this southward movement 

 in autumn I shall have more to say when 

 I reach that part of the calendar ; let us 

 now consider what happens in spring. 



As the end of the dry season approaches 

 in the tropics, the birds that have been 

 wintering there grow restless and gather 

 into companies, some to go to New Zea- 

 land or South Africa or the Argentine 

 Republic, but far more (and, of course, 

 different ones) to Northern America, Eu- 

 rope, and Asia, because there is more 

 land in that direction. Just then the 

 tropical residents are preparing to nestle 

 (migrants never breed there, with a very 

 few exceptions), the rainy season is bring- 

 ing out flowers, fruit, and insects, so that 

 bird food is growing abundant, yet away 

 go these restless wanderers on their long 

 journey poleward, moved by reasons we 

 cannot yet perceive. 



Some always start much earlier than 

 others— perhaps a month --each species 

 or group keeping to its habit in this re- 

 spect. They form little companies, usu- 

 ally from two or three similar species, 

 and these grow and gather others as they 

 proceed. They follow particular routes. 

 Our own birds, for example, enter the 

 United States by three main roads. One 

 set follows the Pacific coast ; another. 



