How can birds still fly if they have to constantly eat to have enough energy to fly? Doesn’t eating make them so heavy that they need even more energy to keep flying?

Question

When I watch the birds at the feeding station in my garden, I notice that individual birds often eat a lot at once. Doesn’t all that food make them so heavy that they need more energy/food to fly than they can carry in flight?

In: Biology

Answer 1

You are thinking that constantly eating means they have to carry around more weight, but actually this is backwards. Constantly eating means they have to carry around less weight.

Consider two hypothetical birds. One eats ten percent of its body weight once per day, the other eats one percent of its body weight ten times a day. Same amount of food, but one is eating constantly and the other isn’t.

The bird that eats 10% of its weight once per day has to carry around a whole extra 10% right after eating. As it digests this food and burns it/eliminates wastes, it’s weight slowly drops down back to normal. But it spends time carrying around a lot of extra weight. The bird that eats 1% of it’s weight 10x a day, on the other hand, is never carrying around more than 1-2% of extra weight, and maybe not more than 1%. It’s burning and eliminating wastes as it goes, so the weight doesn’t build up.

By eating all the time, birds distribute their mass intake across the day and avoid getting heavy at any one point in time. Exceptions are birds like vultures, which somebody else mentioned, that have to take in big meals periodically because of their particular ecological situation.