单项选择题
A．there was not enough food there in the winter B．there were too many birds C．there were too many glaciers D．there was too much daylight
A．exact reasons for migration are not known B．birds migrate because of changes in temperature C．the ancestral home of all birds was the tropics D．glaciers caused birds to migrate
A．To test the relationship between daylight and a disease of the glands common to birds. B．To test the relationship between daylight and migration. C．To test the relationship between migration and temperature. D．To test the relationship between daylight and changes in the season.