system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it that animals such as birds are "scared" of humans, while smaller insects like flies are not?
gpt
Birds have a lot more "brainpower", which is metabolically expensive but gives them greater behavioral flexibility than insects.  In general, birds are more capable of learning about humans (or even recognizing them as distinct entities), while insects rely on high reproductive rates and a series of inflexible but very fast and effective escape maneuvers.

EDIT: flies in particular might be better off being attracted to humans, since we aren't particularly good at killing them and tend to be associated with good food sources.