36 
EXCURSIONS OF THE BIRD. 
to forest. Birds to whom flight is a painful and difficult effort, 
accomplish a great part of the distance " on foot ; " the aquatic birds 
alternately swim and fly. If the wind face them, the journey is 
made with rapidity ; if it blow from behind them, their rate of speed 
slackens, and sometimes they rest for several days. 
The travelling expeditions or excursions of the birds resemble 
their migrations in this respect, that they take place at a certain 
e]50ch, with more or less regularity. Many of the Northern families 
are travelling birds, wandering all the year round within sufficiently 
extensive limits ; but it is only in very severe winters that they 
direct their course towards the south— penetrating even into Southern 
Europe. 
" Wild birds that change 
Their season in the night and wail their way 
From cloxul to cloud." — Tenntson. 
