The amount of times I have read this paper … I should be able to quote it chapter and verse by now.
“The naturalist who studies animals in their natural surroundings must resort to other methods. His main source of inspiration is comparison. Through comparison he notices both similarities between species and differences between them. Either of these can be due to one of two sources. Similarity can be due to affinity, to common descent; or it can be due to convergent evolution. It is the convergences which call his attention to functional problems. “
Tinbergen, N. (1963). On aims and methods of ethology. Zeitschrift für Tierpsychologie, 20(4), 410–433.