What sort of fat is it? The fact that it's on the ground indicates either that the birds aren't eating it, and are picking at the seeds, or that it melts and drips onto the ground. I'm sure you know that only hard fats like beef fat/suet and lard should be used. No vegetable fats at all.
Fat 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 prone to smearing, not good for birds' feathers. It is a breeding ground for bacteria, so potentially bad for birds' health. Salt levels depend on what meat is used and if any salt is added during cooking.
Lard and beef suet on their own are fine as they re-solidify after warming and as they are pure fat, it is not as suitable for bacteria to breed on.
Polyunsaturated margarines or vegetable oils
These are unsuitable for birds. Unlike humans, birds need high levels of saturated fat, such as raw suet and lard. They need the high energy content to keep warm in the worst of the winter weather, since their body reserves are quickly used up, particularly on cold winter nights. The soft fats can easily be smeared onto the feathers, destroying the waterproofing and insulating qualities...." (extract from RSPB website)