Since Pavlow is so famous you see often the dogs depicted as cute and someone ringing a bell. Not as fixated pepsin machines. At least in the past that was my mental image. Might be a me problem.

FWIW I used to hang out with behavioral psychology grad students, who were in the Skinnerian tradition of operant conditioning research. They mostly worked with pigeons, and to transport the birds they used juice pitchers with a few air holes cut into the bottom. I asked them once how they got the birds into the pitchers and they laughed and showed me: they would just open the bird's cage and hold the pitcher up and the birds would dive head-first into the pitcher, sometimes knocking themselves out in the process.

As part of the research protocol, the birds were kept on a diet that included about 80% of their normal caloric intake; the rest of their food was provided by the reinforcements of the experiments themselves (this was done to maximize the reinforcement effect of the rewards). So those birds were way the fuck into those experiments. To add to that, these students were all behavioral pharmacologists, so in addition to getting food reinforcement the birds were also getting drugs like cocaine and heroin.

BTW a lot of people confuse the operant conditioning research with the people who put animals into cages and shock them. This is definitely not what BF Skinner was all about. In fact he wrote books on the subject of how punishment is a bad thing for all animals (including humans and pigeons).