Suet is raw beef fat from around the kidneys and loins. Suet is one of the best foods to attract nuthatches, woodpeckers, wrens, titmice, creepers, kinglets, chickadees, thrashers, cardinals and even bluebirds and unfortunately the starlings and squirrels! To discourage the starlings, purchase a suet feeder cage that is covered on all sides but the bottom. Only those birds that can hang upside down will use this feeder.
If the temperature outside is around 70 degrees Fahrenheit and warmer, beef fat can turn rancid and melt. There are many commercial suet cakes that can be purchased and some of these are called "no melt", "berry" and "insect" cakes.