Bird wings are actually optimized for different Reynolds numbers (https://en.wikipedia.org/wiki/Reynolds_number https://en.wikipedia.org/wiki/Reynolds_number).

Bird wings are actually optimized for different Reynolds numbers (https://en.wikipedia.org/wiki/Reynolds_number https://en.wikipedia.org/wiki/Reynolds_number). More flapping for smaller birds/insects, more gliding for larger (they fly like airplanes almost).