Tool use by animals: research history
Best tip I can give you is buying a textbook or popular science book about ethology. That should give you an overview about research done in the past decades. Konrad Lorenz was a pioneer of ethology, so a book should summarize his work, e.g. this one

For more current stuff look up some journals in this field:


Animal Behaviour
Journal of Comparative Psychology


Animals having a small brain compared to mammals but nevertheless showing very intelligent and reflective behaviour like this rook are currently interesting research objects. This kind of research is not really tricky or expensive as you see, it just has to be done by a student :) But behaviorism as the leading paradigm in the middle of the 20th century has probably made a lot of more fruitful research approaches impossible.