I find it hard to believe that any sentient being could somehow have non-abstract thoughts. Down to the very basics of language and cognition, everything we know is abstract concepts. Nobody has an immediate and intuitive understanding of the underlying processes.
That aside, you have to draw the line somewhere. I have seen a video if a crow picking up a piece of metal and bend it into a hook in order to obtain food. The bird was able to recognize the piece of metal as not only that, but a potential tool. The bird figuring that out on its own is evidence of abstract thought, if anything.
I've seen another video of a pigeon, that in order to grab a fruit out of its reach within an enclosed area too small to gain flight in, pushed a box underneath the fruit and stepped onto it. That way, it was able to eat the fruit. Seeing the fruit and the box, it was able to conceive a solution.
There's another story of an experiment with chimpanzees which I'm not sure is true or not (the description of it usually comes in the form of a badly compressed jpg forwarded by an old person). In the experiment, they put a few chimps in a cage with a banana. If any of them tried to take it, they would all be punished. After having figured that out, they stopped trying. Then a new chimp was let into the cage, not knowing the consequences of trying to take the banana. When it tried, the other chimpanzees beat it up.
This was repeated while the original chimps were replaced one by one. At some point, none of the chimps had experienced the initial punishment, but they would all beat up whatever new chimp would try to take the banana.
I'm not sure what exactly it would prove, but it's interesting.