Fat-based bird feed is a type of bird feed that is made from SUET, which is a hard fat found around the kidneys and loins of cattle and sheep. SUET is often used to make bird feed because it is high in energy and provides birds with essential nutrients. It is also an excellent source of fat for birds during the winter months when food is scarce. Fat-based bird feed is a great way to attract a variety of birds to your backyard.