Just take it or ask depends on some obvious factors.
For pictures like this how else you are going to take it? Do you seriously think it is possible to get it more less OK like in OP without asking? With long tele lens for birding?
Read "Dialogues" book with Henri Cartier-Bresson interviews to receive complete guidance from real expert on this topic and from humanist PoV perspective.
Also educate yourself on local regulations for public place photography to find out what is allowed to be taken without asking and how it is allowed to be used.
I made person fully aware what I'm taking it and he cooperated.
and talked to this:
but this one was quick encounter, strait, eye to viewfinder.
and in situations like this you are taking it as candid.