I was surprised to catch a Blackbird in my garden last week that was rather heavy. I had originally caught this bird in November 2005, when it weighed a pretty normal 113g. Last week, I re-caught it and it weighed 137g, an increase of 24g. A lot of this extra weight was stored as fat (visible if a ringer gently blows on the breast feathers to reveal the skin underneath) and you might like to put 25g (or one ounce) of fat on your kitchen scales to see what this amount of fat would look like!
There has been some recent information from ringing providing evidence that some species of bird, particularly Blackbirds, put on weight in harsh winter weather. You might imagine that this would make perfect sense - if food is in short supply, load on as much as you can in case you can't find some later on. However, it isn't quite as simple as that as a heavy bird takes up more energy to move and is thus less efficient at using its food whilst worse, it is less manoeverable and thus more at risk of being caught by predators. Whilst much more research is needed for humans to understand the processes involved in weight gain in birds in harsh weather, it appears as if the birds know what to do - and this seems to be to feed up!
Incidentally....I wonder where this bird had been for the past 3 years as I hadn't caught it since 2005 despite some intensive efforts, but the faithfulness of birds to urban gardens is a topic for another post!