Long-tailed tit. Photo by Steve RoundWe humans are constantly bombarded with messages that Fat Is Bad. It's less lard, more olive oil these days. And rightly so, much of the time. Now that many of us live rather sedentary lives, sitting around in heated offices and houses, fat isn't as important for us as it once was.
For birds, it's completely different. Fat is their saviour!
If you were to take a peek under your garden robin's feathers, you might be able to see how much fat it's carrying around. When birds are able to, they carry a bit of fat to use as 'fuel' during their day-to-day lives. It looks like a smear of butter under their pink, almost transclucent, skin.
Fat is what keeps birds going through the day and, more importantly, the night. A bird has to eat enough to make sure it has the energy it needs - not just for flying, running and singing, but also to keep itself alive overnight.

Keeping warm takes a considerable amount of energy, so heavier (fatter) birds are more likely to survive a cold snap. Birds weigh less in the mornings than in the evenings before they go to roost, because of the fat they 'burn' overnight.

Fat reserves can also see a bird through periods of bad weather when feeding is difficult or impossible, and migrating birds feed up before setting off.

Fat = energy

Even if a bird doesn't eat fatballs, it can convert its food into fat, whether it's scoffing apples, worms, insects, seeds or fish. So do healthy birds carry around as much fat as possible...?
A bird that eats a lot and builds up fat reserves is actually taking a risk. It'll be better equipped to keep itself warm, but eating more means more food-finding has to be done. That in itself takes energy, and more foraging can put a bird in harm's way, when otherwise it might be hiding safely in a tree or bush.

For this reason, birds in a good environment with plenty of food (like your garden) actually carry less fat than birds in poor habitat, because they don't need to.
A plump bird can also be hampered by its extra load when it comes to predators. Birds are always on the lookout for rivals and predators, every day of their lives. Those that don't stay alert get eaten... 

Weighing a few extra tenths of a gram may mean that a bird isn't quite as quick off the mark when a sparrowhawk comes calling, compared to its skinnier colleagues. So there's a careful balance to be had.
The good news is that wild birds don't get obese. They live high-energy lives, but by putting out food we give them a little more leeway.
How you can help

There are lots of different ways to help your garden birds. As well as putting out foods like seeds, peanuts and fat, you can also leave out fruit like apples - always a hit with thrushes and starlings. Don't forget that water is important, too, even in winter. And for birds like goldcrests that don't really come to feeders, plants like ivy can be really important - it's very attractive to insects, which make ideal goldcrest food.

You can get lots of helpful garden advice from our Homes for Wildlife project. Sign up, it's free!

What do your garden birds like eating?