Birds of prey are birds of prey.
Saguru Hakuba had always made sure that his hawk was allowed to perambulate freely when not needed for a case. Though he may have been domesticated, Watson was still feral at heart, and Saguru understood this. Being under the ownership of a human did not overwrite millions of years of natural instinct. He was a hawk, and it was a widely accepted fact that hawks flew around and ate things. [But hey, there are conspiracy theorists out there for everything.]
But Saguru had recently started to question whether or not he allowed Watson too much freedom. Now, he wasn't sure exactly what qualified as "fat" in bird standards, but his hawk had gained a significant amount of weight in the past month. There had been about five Kid heists in that month alone, so Watson's assistance hadn't been needed. The hawk had had quite a bit of time to do some roaming and hunting, to say the least.
As the weeks went on, Watson continued to grow in size, until Saguru was sure that either his hawk was obese, or had some kind of medical problem. Finding a veterinarian in Japan that specialized in birds was no easy task, and the first two refused to treat a hawk for various personal reasons. The third vet stated that nothing was wrong, and a fourth confirmed this. Watson was simply overeating.
Saguru found this hard to believe. Tokyo was not known for its wildlife population. He usually had to feed Watson raw meat since the hawk couldn't find enough prey outdoors. Where was his hawk getting all this food?
His question was answered about a week later, after a Kid heist. Saguru stumbled into his room, dead-tired and furious from yet another fruitless chase, just in time to see his beloved Watson bring a clawful of plump, well-fed white doves into his cage.
Saguru casually closed his bedroom door and carefully pretended he had seen absolutely nothing.
Watson's diet plan could wait.