Birds are metabolically much more active than mammals yet many have long lifespans. This prompted another finding that the number of cortical neurons can explain a large part of the variation in lifespan across species.
Flight gives birds greater protection from predation, which makes evolution towards biologies that pay a higher metabolic cost for greater longevity more advantageous. The same thing applies to larger animals, like cows, which tend to have longer lifespans once you normalize for heart/metabolic rate.