C because it sums up the whole passage
When migrating, birds usually migrate when there's not enough food, shorting of daylight, and when it's getting colder in winter. Birds that migrate do so in the late summer through the fall and in the late winter through the spring. Birds and use a radical pair-based compass to "see" the Earth's magnetic field when migrating.
The answer to you question could be while staring at the sun, it hurts my eyes or it could when i stare at the sun, it hurts my eye.
