Since a bird's body temperature can increase as much as 10 degrees Fahrenheit during long, strenuous flights, birds are in danger of overheating during the prolonged flights typical of seasonal migrations. This risk is greater during daylight hours than at night, because air temperatures are generally lower at night. It is, therefore, likely that the tendency of many bird species to migrate at night is due to the need to avoid overheating.

Which of the following, if true, most strengthens the argument given?

A. Birds that migrate in warmer months are more likely to migrate at night than are those that migrate in colder months.
Correct, as in warmer months more birds migrate in night thereby showing the importance of day temperatures in overheating that is not the case in colder months when the temperature is low in day time also i.e. day time(colder) < day time(hot months).
B. Birds that migrate during the daytime are in danger of being blown off course by atmospheric turbulence, which is strongest during daylight hours.
Atmospheric turbulence O.O.S(out of scope)
C. Birds that begin to overheat during migration can lower their body temperature significantly by briefly stopping to rest.
contrary it weakens the conclusion
D. Nighttime migration enables birds to avoid certain predators by flying in the dark and remaining hidden during the daylight hours.
Weakens , tendency in conclusion is due to overheating here it mentions an alternative cause for the stated effect.
E. Nighttime migration enables birds to forage by daylight, when they can most readily find food.

D and E weaken the argument by providing an alternative explanation.
C is irrelevant.
B is irrelevant.
A is correct in that it gives a fact that further strengthens the argument.
+1 A