The profound stillness a typical response immediately following detection of overhead threat. Everybody sits quite and stops making noises to avoid detection by predator. If threat a bird eating hawk (i.e. Coopers), then it will be looking for movement and listening for chatter during hunt. When Coopers buzz my yard all birds hush, unless hawk is low and approaches a nest, then some bird species will mob it. The birds seem to know when a hawk is prone to give a prolonged chase and make every effort to avoid its starting. I have seen prolonged chases on various blackbird species (starlings, redwinged blackbirds, Brewer's blackbirds, common grackles) and hawk usually catches a meal which is very big nightmare for potential targets.