Suet
Suet is made of the hard fats around the kidneys of cattle and sheep prepared with the right ingredients, is irresistible to birds (squirrels too but that's another story). Suet is an important source of fat and protein for birds and this energy packed food is necessary throughout the year, whether it be to survive the winter months or to help birds through nesting season. Almost all birds will eat suet and the birds will love all the great flavours.
Full Fat Suet
No-Melt Suet Doughs
By using No-Melt Suet Doughs in the summer, you can feed suet through-out the whole year! Suet Doughs have been re-melted several times before packaging, making them slightly harder than normal suet and able to withstand higher temperatures (mid 30 C) before melting.
Bark Butter 
Another type of suet that birds love is Jim's Birdacious Bark Butter, found only at Wild Birds Unlimited stores.
It is available in Original or Hot Pepper as Spreadable, Bits and Cakes.
For extra protein, why not try the Bits and Bugs, a mixture of regular Bark Butter Bits and meal worms.