I have only sketchy knowledge of the theme, but for two reasons would discount pallid.
1) The pallid harrier is a hawk. Hawks are generally distinguished from falcons by methods of hunting and killing.
2) If you just said 'pallid' would anyone understand that you were discussing a bird ? Don't you need also to include 'harrier' ? But 'gentle' is a one word definition of a bird, which fits the right bird...