1: Cooking fat
A few years ago it was a common site to see the roasting tin from Sunday Dinner put out for the birds, Well, of course stats tell us that we eat less Sunday roasts but also for the record it's best to avoid putt out fat from cooking out for the birds at from cooking is bad for birds.
The problem with cooked fat from roasting tins and dishes is that the meat juices have blended with the fat and when allowed to set, this consistency makes it really messy and easily smeared onto feathers which can affect their insulating properties. Same goes for any soft fats like vegetable oils and polyunsaturated margarines etc.
(PS Lard and beef suet on their own are fine for your birds)
2: Salty Foods
Avoid all salted foods as they dehydrate birds. This includes Salted crisps and peanuts, processed cheese and leftover frozen meals as they are all likely to contain levels of salt dangerous to birds.
Birds cant digest milk so it's important never to give milk to any bird. Birds can however digest fermented dairy products like mild grated cheese (and particularly yummy for robins and wrens).
Whilst fresh coconut in the shell is fine for birds (as long as you have washed out any coconut milk from the middle desiccated coconut is a definite 'No No' as it can swell inside a bird and cause death.
5: Mouldy and stale food
Many moulds are harmless, but some that can cause respiratory infections in birds, and so it is best to be cautious and avoid mouldy food entirely to be on the safe side. If you end up with mouldy food on the birdtable adjust the quantity of food as otherwise you will end up providing a breeding ground for salmonella which is bad news for birds and of course you will want to avoid attracting vermin to your garden and birdtables.
For our full range of goodies for your garden birds, wiggle on over to https://www.wigglywigglers.co.uk/