Question : birds in migration

for the birds that develop infection and die in a migration versus the other ones, is it the personal immunity these birds have developed thus far, or is it other factors that determines the health and safety of the birds migrating 1000s of kms.

thanks

Answer : birds in migration

Survival of the fittest is all about Bell curves and probability.

Immunity to particular diseases is only one factor.

Micro climate, food distribution, and genetics all contribute to health and robustness.

If a bird is lucky enough to avoid the mosquitoes with West Nile virus, his immunity may not be tested.