(I call mine Robin Cake because they literally go mad for it).
First make a crumble mix with plain flour and margarine ( or butter if you are rich).
Rub in half fat to plain flour with hands or in mixer – as much as you want, I generally make a Delia Smith recipe and leave a fair amount over when making one just for the two of us.
Add oats, any fresh or dried fruit, nuts of any kind, whole or ground, but not salted, cake or biscuit crumbs, breadcrumbs, grated cheese, cereal, anything really that birds like. Add any oil you may have lurking in a frying pan or similar. Suet goes down very well. Raid the fridge and your cupboards for suitable additions.
Add a little water to bind and then roll in oats (or flour if you have no oats) so it is reasonably firm and not sticky. Make a nice cake shape. No need to harden in the fridge (!) - just stick it out on the bird table, preferably one in view of a window.
Watch it disappear.