54 THE RIDDLE OF MIGRATION 



ever the climate, there is no exception to the varia- 

 tions in day-length. The further north one goes, 

 the shorter become the days in winter and the longer 

 in summer. 



The mean annual temperature at Edmonton, 

 Alberta, is 36.8°F. The winter minimum is about 

 — 52°F. (though it may drop below — 60°F. in the 

 river valley) and the summer maximum (shade) 

 just over 100°F. Rapid changes of temperature 

 are characteristic through most of the year. The 

 summer nights are nearly always cool, often cold. 

 Annual average precipitation is in the neighborhood 

 of 12 inches: snowfall by itself, 45 inches. The last 

 sounds a good deal, but owing to the dryness of the 

 climate and periodic chinooks, it is but rarely that 

 a foot of snow is to be seen on the ground at any 

 time. Average monthly sunshine, 191.2 hours. On 

 June 21 the sun rises at 4.07 a.m. and sets at 9.04 

 p.m., i.e. the day is some 19 hours long, actually 

 somewhat more since both dawn and twilight are ex- 

 tremely protracted. On December 21 the sun rises 

 at 8.40 a.m. and sets at 4.24 p.m., giving a day- 

 length of less than 9 hours. 



On the face of it, it is obvious that many species 

 of birds could not under any circumstances survive 

 a northern winter. With many of them the food 

 question alone would settle that. None of the 



