Shapeshifters, or at least the type of shapeshifter Silva (and Nightjar and that marten looking one in my scraps) can transform into any species they want, but they can't choose the subspecies (IE, they can choose to shapeshift into a jackdaw rather than a raven, but they can't choose the subspecies, they're permanently stuck with what they get, so if a shapeshift shifts to, for example, a blackbird, and the subspecies they get is turdus merula azorensis, they'll never ever be able to shift to t. m. maximus.) What subspecies they shift to is mostly based on their personality, like their main form (they have no control of what that looks like either, but that's another topic entirely). The same rule goes for domesticated species, if they shift to a dog, they have no control over the breed, colors, ect. Generally, the colours will be similar to their main form. The breed is another topic entirely, usually personality based, though.
Obviously, this prevents them from disguising as other people.