I was sure it was spread by rats-- I didn't see any reason to doubt what everyone seemed to agree on. Unfortunately, poking round online has reminded me of an article I read within the past few years that I haven't been able to find again, which included the point that pigeon coops (in England?) weren't designed to prevent rat depredation, so rats were either absent or not prevalent at the time of the plague there.

This article goes with the gerbil theory, but says that rats probably made a small contribution to spreading plague.

Here's an argument that it was people, not rodents, which spread black plague in England, and suggests that we don't know whether it was bubonic plague or some other disease.