10 
The following species are given as representative of what birds are to 
be expected in each zone: 
Upper Austral Canadian Arctic 
Cardinal Hudsonian Chickadee Ptarmigan 
Orchard Oriole Red-breasted Nuthatch Snowy Owl 
Carolina Wren Olive-backed Thrush Snow Bunting 
Grasshopper Sparrow Three-toed Woodpecker Gyrfalcon 
Blue-grey Gnatcatcher Hermit Thrush Longspur 
Dickcissel White-throated Sparrow 
Canada Jay 
Transition Grey-cheeked Thrush 
Bobolink Slate-coloured Junco 
Wood Thrush 
Yellow-throated Vireo Hudsonian 
Baltimore Oriole Rough-legged Hawk 
Towhee Fox Sparrow 
Cuckoo Northern Shrike 
Field Sparrow White-crowned Sparrow 
Bluebird Pine Grosbeak 
Catbird American Pipit 
MIGRATION. 
The migration of birds, their periodical and seasonal appearance and 
disappearance, is one of the most obvious phenomena of nature. The 
fact that many birds disappear in winter is common knowledge and has 
attracted attention for ages. Though once regarded as a mystery, and still 
far from being throughly understood in many of its details, we are beginning 
to wonder less but admire more as accurate knowledge gives place to vague 
speculation. To-day, where most of our northern species spend the winter 
is known and many of the routes by which they come and go have been 
mapped out. We know that on the whole they are governed by ordinary 
and well known, though perhaps highly developed, senses and common 
every day influences, and not by the mysterious powers and instincts once 
ascribed to them. 
The fundamental cause of migration is obviously the waxing and the 
waning of the food supply. Birds leave the northern land of their birth 
because there is no other way by which to avoid starvation. Many species 
can withstand extreme cold but none can go long without food and though 
some bird food still remains in Canada throughout the winter, its amount 
is small and only sufficient for a imited population and even that supply 
rapidly decreases, or to the north is buried under deep snow. ‘The cause 
of the southward migration in the autumn then is obvious, but why should 
a bird leave the soft climate and plentiful food supply in the south to brave 
dangerous travel and finally find itself in a land where retiring winter stil! 
lingers and the danger of starvation is imminent. Many ingenious explan- 
ations have been advanced to account for this, longing or homesickness 
for the land of birth, hereditary memories of an ancient home enduring 
through geological ages, the seeking of special food for nestlings, and 
insufficiency of nesting sites in the southern areas, have all been given as 
possible reasons. However, it is unnecessary to advance a complicated or 
far-fetched explanation when a simple and direct one exists. If we 
remember that in the nesting season the bird population is increased many 
1Most of the species of this zone also occur in the Upper Austral, but reach their northern limit here. The occur- 
rence of these with the absence of the species of bordering zones are the most marked characteristics of the Transition 
zone. 
