Who wrote 1950's book about teaching wild birds to count?

Question

She taught Bluetits mostly. They would tap out the numbers she spoke.

Answer 1

Probably this article:

KOEHLER, O. 1950. The ability of birds to "count." Bull. Anim. Behav. 9: 41-45.

And a good article on the subject:
Birds’ Judgments of Number and Quantity

http://www.pigeon.psy.tufts.edu/avc/emmerton/default.htm#ethological