Since a bird's body temperature can increase as much as 10 degrees Fah

I think it's A.
The conclusion states the needs of many bird spieces to avoid overheating (by migrating at night). Only A demonstrates a migration pattern that reinforces the tendency to avoid heat.
A. Birds that migrate in warmer months are more likely to migrate at night than are those that migrate in colder months.

B,D and E are irrelvant.
C mentions overheat but "lower temperature ... by stopping to rest" is irrelevant.

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?

Since as per conclusion overheating is the reason behind birds migrating at night as temperature is low, something about the temperature in the below choices shall be helpful to support the conclusion.

A. Birds that migrate in warmer months are more likely to migrate at night than are those that migrate in colder months. - CORRECT. A comparison of bird migratory behavior w.r.t. warmer months and colder months in the desired directions strengthens the argument here.

B. Birds that migrate during the daytime are in danger of being blown off course by atmospheric turbulence, which is strongest during daylight hours. - WRONG. Irrelevant.

C. Birds that begin to overheat during migration can lower their body temperature significantly by briefly stopping to rest. - WRONG. Opposite to what is sought to strengthen the argument.

D. Nighttime migration enables birds to avoid certain predators by flying in the dark and remaining hidden during the daylight hours. - WRONG. Irrelevant.

E. Nighttime migration enables birds to forage by daylight, when they can most readily find food. - WRONG. Irrelevant.

IMO Answer A.
_________________

The best answer is option A.

Premise 1: 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.
Premise 2: This risk is greater during daylight hours than at night, because air temperatures are generally lower at night.
Conclusion: It is, therefore, likely that the tendency of many bird species to migrate at night is due to the need to avoid overheating.

The argument assumes that when birds fly at night, it is not because of any other reason other than to avoid overheating, and that birds have no other means to regulate their rising temperatures during the daytime, hence their likelihood to fly at night rather than during the day.

B: Birds that migrate during the daytime are in danger of being blown off course by atmospheric turbulence, which is strongest during daylight hours. This actually weakens the argument. If atmospheric turbulence is strongest during daylight hours can blow a bird off course, then birds would rather fly at night in order to have a more likelihood of reaching their intended destinations. This option presents an alternative reason as to why birds are likely to fly at night, and hence weakens the argument. Eliminate B.

C: Birds that begin to overheat during migration can lower their body temperature significantly by briefly stopping to rest. This a clear weakener since for the conclusion made in the argument to hold, then birds must be incapable of lowering their body temperatures during flights. So if it is indeed true that birds can lower their temperatures in-flight during the day by briefly stopping to rest, then the argument is weakened. Eliminate C.

D: Nighttime migration enables birds to avoid certain predators by flying in the dark and remaining hidden during the daylight hours. Another weakener. The argument relies on the assumption that birds must not have any other reason to fly in the night other than to avoid flying in warmer temperatures and reduce the likelihood of overheating their bodies. So, if it is true that birds actually fly at night in order to avoid some of their predators during the daytime, the argument is weakened. Eliminate D.

E: Nighttime migration enables birds to forage by daylight, when they can most readily find food. This also provides an alternate reason why birds may fly at night and hence a weakener. It punches holes into the conclusion of the argument. Per the assumption of the author, birds must not have any other reason to fly at night except to avoid the warm daytime temperatures. Eliminate E.

A: Birds that migrate in warmer months are more likely to migrate at night than are those that migrate in colder months. This is perfect. This option is saying that since birds only fly at night to avoid the warm daytime temperatures, if the temperatures are now colder in the day during colder months, then birds that migrate at such times have no reason to migrate at night. Hence they will more likely migrate during daytime than at nighttime. This strengthens the argument above. Hence A is the right answer.

Its A for me. As we have to strenghten the conclusion "that the tendency of many bird species to migrate at night is due to the need to avoid overheating.
" And only option A strenghtens it.

Posted from my mobile device

IMO B:
Birds that migrate during the daytime are in danger of being blown off course by atmospheric turbulence, which is strongest during daylight hours.


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.

B. Birds that migrate during the daytime are in danger of being blown off course by atmospheric turbulence, which is strongest during daylight hours.

C. Birds that begin to overheat during migration can lower their body temperature significantly by briefly stopping to rest.

D. Nighttime migration enables birds to avoid certain predators by flying in the dark and remaining hidden during the daylight hours.

E. Nighttime migration enables birds to forage by daylight, when they can most readily find food.

ARGUMENT
[prem] Bird's temperature increases significantly during long flights, and they are in danger of overheating;
[prem] Risk of overheating is greater during the day than night, because air temps are lower at night;
[con] So, the tendency of many to migrate during the night is due to the need to avoid overheating.

Ans (A) this strengthens the argument.