It is also unclear to scientists just how birds know exactly, and collectively it seems, when the time has come to leave. How, exactly, that pivotal moment is reached: changes in day length, temperature, hormones… What has been observed can best be described as a sort of restlessness, an innate pull out and up that, if birds were human and scientists prone to using such terms, almost looks like longing.

It has been observed that birds feel a sort of pain before taking off, almost like fear, and that nothing alleviates that feeling except the rapid motion of wings.