FG - I think it is to do with the levels of salt & nitrates used as part of the bacon curing process - high level of salt are toxic to birds. Some sites say to avoid bacon altogether but here is what the RSPB say - quote:
"You can put out fat from unsalted cuts of meat in large pieces for the birds. Birds, such as tits, can remove morsels from them. Make sure they are well anchored to prevent large birds flying away with the whole piece! This kind of food can attract magpies and gulls, and also neighbourhood cats. If this is likely to be a problem, it's best avoided.
There is a lot of debate about the suitability of bacon rind, since much of it is salted during the curing process. As long as you can be sure the bacon is not salty, you can put it on your bird table. Since bacon can be too tough for many birds to tackle, chopping it finely will allow a wider variety of birds to eat it"
I thought it was also interesting that the RSPB do not recommend using any fat that comes off cooked meat - quote
"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."
The RSPB site says you can feed cheese - but I find that confusing as cheese has high salt levels!
I think I'll stick to fat balls, seeds & nuts - although they apparently like baked potatoes as well!! (With or without sour cream & chives Lesley )
Just hope I haven't poisoned too many birds over the years in my ignorance!
PS I like Alan's point in his original posting about getting some TV coverage about the harm that can be done by the netting on fat balls - I found a dead blue tit with a broken leg caught in one several years ago - absolutely heartbreaking - always remove the net now or make my own.