You can go so far in applying human traits to animals or birds, but here is a great opening line to a piece from the New Scientist. "Before leaving the nest, some birds decide to go on a crash diet and do press-ups to make sure that their wings will be able to carry them." So reasons a zoologist in Scotland. Common swifts feed on insects, and in bad weather there may be a gap in their food source for a day or two. So, to proctect themselves, the nestlings put on extra fat - presumably by overeating beyond hunger.
But this could be a problem when the time comes to fly; so the birds lose that weight again before they leave the nest. Some theorists believe that the parents stop supplying the food, so that the young will lose their excess weight. Then again, could that young diet of their own will? In the University of Stirling, Thais Martins thinks that young swifts and perhaps other species do just this. She reaches this conclusion after two years study of common swifts in different weather conditions. Birds from twelve days old are weighed and measured. She also - hard to believe this - mixed up broods to make sure that a chick's weight was not just due to having good parents.
She found that the birds always leave the nest when the ratio of their body weight to their wing area, known as the wing-load lowers to a certain point. Also surprising is that the chicks took the same time to shed the extra weight, regardless of how fat they were when they started to slim. How do you know when a young swift is sulky? Well, she tells us that, when preparing to leave the nest, they sit sulkily on the edge of the nest entrance and won't take food. When they can balance on their wings for ten seconds or more "in a kind of avian press-up," they know they can leave.
It's good to think that when all sorts of machines of destruction are being produced and refined, humanity can put its mind also to some of the insignificant, but nevertheless intricate problems in the wonderful world of nature.