system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Does the action of flying in birds tire them out the same way that running tires us out?

 Is there a function in humans analogous to flying in birds?
gpt
It does. However, unlike a human running, a bird doesn't have to constantly flap its wings to fly.

Although the usual way people think of birds flying is constantly flapping their wings, that's really not the case. Over very short distances, especially if trying to get to a higher spot, birds will flap their wings furiously, creating the image that they must do that all the time. However, when it comes to going a long distance, nearly all birds "coast" a great deal of the time. The pattern varies from species to species, but it generally involves a few wingbeats, then the bird tucks its wings in against its body and flies without flapping, much like a thrown football. For a human, it would be like running four or five paces, then sailing over the next several meters. Birds can get away with this because their physiology is all geared toward making them lighter - hollow bones, lower density tissues - making them more football-like than human-like.

Really long-distance flying birds use a process called soaring. These birds have larger wingspans, enabling them to glide long distances. In order to save energy, soaring birds catch a ride on thermal updrafts that naturally occur during the day. These updrafts, which are really nothing more than rising warm air, carry the birds higher into the sky so they can gain altitude and keep gliding even longer without having to flap their wings once.

The exception is hummingbirds. Hummingbirds only stay in the air because their wings are beating constantly, and beating fast. Hummingbirds flap their wings an average of 10-15 times per second, allowing them to hover and be extremely agile. Don't they get tired? You bet they do, which is why unless they're feeding or moving to a new feeding ground, they're usually perched. All birds tend to conserve energy, but hummingbirds have more of a need than the average.