The RSPB has urged gardeners to never put out cooking fat for wild birds.
Wildlife experts say cooked roasting tin fat is extremely dangerous for birds – many of which struggle to survive at this time of year.
The fats remain soft when cool, and can smear onto birds’ feathers, ruining their water-proofing and insulating qualities.
Fat from the Sunday roast can also contain meat juices.
The concoction goes rancid quickly and is a perfect breeding ground for salmonella and food poisoning bacteria.
Also, many people sprinkle salt on their roasts to crisp the skin. High levels of salt are toxic to birds.
RSPB wildlife advisor Richard James said putting roasting tin fat outside could result in the death of wild birds.
Richard said: “People pour fat onto bird tables or mix it with seeds because they think it will give birds energy and nutrients, just as fat balls do.
“But this is a completely different kind of fat and could have catastrophic effects. Only pure fats such as lard and suet should be used to make home-made fat balls.”