Re: Which fox species can hybridize with which (if any)?

As you point out, there is a lot of variation in individuals in the same species. If someone claims to have seen a hybrid, they can't be certain. What you'd really need is do something like a genetic analysis of the individual in question, which is unlikely. No one is really sure if different fox species can interbreed; research on it hasn't been done, as far as I know. It may be possible for them to interbreed in captivity while never doing it in the wild. Lions and tigers are a great example of this phenomenon: although they do interbreed in captivity, they never meet one another in the wild, as they live in very different environments, and so it's not an issue in nature.

As a final comment, the status of swift and kit foxes is still uncertain. Although there have been reports of breeding between the two species in the wild, genetic and morphometric (measuring body ratios and dimensions) studies have been inconclusive as to whether to combine the two into a single species or not.