I've read a few posts debating which is best -vegetable fat balls vs animal fat balls. This has prompted me to have a quick look at the scientific literature to try to work out which is actually best for the birds.
Around half of the birds that typically feed off hanging bird feeders in the UK are predominantly plant eaters, mainly consuming seeds, berries etc, with some also opportunistically consumings insects, this includes species like finches. The others such as tits are flexible omnivorous, eating insects, seeds, berries in varying proportions depending on the season and what is available, so mostly insects in the summer months and seeds and berries when the insects are not around. Reading the scientific literature, the predominant source of fats in the majority of wild birds natural diets therefore appears to be from seeds. The fat content of most insects is relatively low overall (typically under 5%, some up to 10%, but depends on species). From the papers I found, insect fat is predominantly unsaturated fat - up to 70% polyunsaturated fat, with palmitic acid also featuring, whilst other saturated fats are relatively low. The high unsaturated fat, low saturated fat content of balls made from vegetable fats and seeds is therefore much closer to the birds natural dietary fats. Over millions of years, birds have evolved significant levels of specialism to adapt to their natural diets - this includes significant physiological changes that make the different species of birds what they are today - you can therefore reasonably assume that their digestive tracts and metabolic processes have also become optimised to eat their natural diet. For example, in migratory birds, De-Novo Lipogenesis, the body's pathway to convert carbohydrates to fat for storage, is pronounced.
I have read an argument on the RSPB site that saturated fat is more slowly digested by the birds or somehow has more energy than unsaturated fats, so therefore lard or animal suet based balls are best to keep our wee friends warm, because they will burn the polyunsaturated fats too quickly. The idea that saturated fats have more calories than polyunsaturated fats is simply untrue. The theory about saturated fats taking longer to digest and therefore keeping the birds warmer for longer, fails to take in to account the ability of the birds to rapidly store unused digested fat in their body tissues, just as we humans can, so that it is readily available for burning when they need it. It also fails to take account of the high speed of food passage in small birds - saturated fat or polyunsaturated fat - it will be digested and stored quickly. I couldn't find any evidence to support this theory that saturated fats will keep the birds warmer. It is strange that the RSPB site states that seeds, including sunflower seeds, are good for birds, but seed oils, including sunflower seed oils, are bad for them. I am somewhat baffled by some of the very unscientific statements made on the RSPB website.
The other argument put on the RSPB site is that vegetable oils could get on the birds feathers and therefore damage their waterproofing. The use of solid vegetable fats and avoidance of using processed margarines would seem sensible when making bird feeders, just in case a bird brushes its wings against the feeder.
Having a science PhD, I tend to trust the natural selection processes of evolution to match a bird and its diet. Vegetable fat balls are far closer to the birds natural diet and for me, that means that they are what we should be feeding to birds. My suspicion is that the reason lard based fat balls are the norm, is a way of dumping all of that unwanted lard, meaning cheap fat balls in the shops. We are making solid vegetable and seed fat balls.
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