While it is possible to make great images of birds and wildlife with short lenses, be sure to use common sense. If a creature flees once or twice upon your close approach, it is best to look for another subject. Getting close to a deer or a duck will rarely, if ever, get you in trouble. Getting too close to a hawk's nest, walking up to a moose that's with a calf, or attempting to photograph a venomous snake with a 100mm lens, however, might leave you injured or worse. There is no substitute for knowing your subjects.