Hollow Bones Disadvantage?

[Bruno]

Birds, specifically, have a life-threatening problem if eg wing bones or some ribs are broken, because their respiratory system actually extends into the hollows in those bones. A compound fracture of a wing upper-arm doesn't just cripple a swan, it can cause a respiratory failure equivalent to a collapsed lung in a mammal.

[Verjigorm]

Originally Posted by Bruno

Birds, specifically, have a life-threatening problem if eg wing bones or some ribs are broken, because their respiratory system actually extends into the hollows in those bones. A compound fracture of a wing upper-arm doesn't just cripple a swan, it can cause a respiratory failure equivalent to a collapsed lung in a mammal.

Wait? Whuuuut? Shows dat work?

[Bruno]

Originally Posted by Verjigorm

Wait? Whuuuut? Shows dat work?

So, mammals and reptiles have two air bags for lungs, and they work like bellows. Mammals have a diaphram muscle to help pump the bellows, reptiles don't, and of course some species have some quirky features, but generally the whole respiration is very familiar to the human system.

There are two problems with bellows lungs which can cause problems with flight.
One, when you breathe out you don't get rid of all the stale air, so you only ever end up with "half fresh" air in the lungs. So for higher performance you need bigger lungs which is more weight which is bad for flying. Flight is a very high performance activity. Some deep diving mammals can nearly collapse their lungs so their efficiency is much better, but that approach exacerbates the second problem.
Two, all this pumping requires a flexible ribcage and constantly changing the volume and profile of your torso. Birds have a very rigid reinforced spine and chest for stable anchoring of the flight muscles and wings. All the flexing of bellows lungs is really bad for flight stability.

So, birds don't have bellows lungs. Instead, they have a kind of flow-through system, like the air supply for a car's engine. It's called circulatory lungs. Instead of two bags of air that do dual duty as the air pumpers/storage and the air processors, birds have eight or nine air sacks that do nothing but hold the air, and then two flow-through lungs. The air sacks are split between a set in front of the lungs (which extend right into the humerus and in some species into various other bones) and a set behind the lungs (I have no idea if these extend into the bones in some species).
Breathing in inflates the rear set. Breathing out pushes the air through the lungs into the front set. Contracting the front set pushes the stale air out and the bird breathes out. They do something too complicated for me to understand to balance out all the air sacks and keep the respiratory system relatively constant volume.

There's a hypothesis that at least some of the lizard-hipped dinosaurs had circulatory lungs (they're closest related to modern birds) and a slightly wilder hypothesis that the big bird-hipped dinosaurs also had some form of circulatory lungs, to just keep all that tissue oxygenated without spending their entire time struggling to breathe.

Bats don't have circulatory lungs, but they mostly get away with it by only flying at low altitudes. Birds can fly at much higher altitudes where the air is thinner because of the great efficiency of the circulatory lungs.

I have no idea how bats cope with the chest flexion.

EDIT: Apparently crocodiles and monitor lizards have something like the bird respiration, although the air sacks don't go into the bones. Pterosaurs, on the other hand, didn't just aerate the bones, but the air sacks went right into the wing membranes in some configuration that I don't understand, and even into the femurs in some types.

[whswhs]

Originally Posted by Bruno

I have no idea how bats cope with the chest flexion.

I believe bat wings are anchored to the hindlimbs and thus the pelvis as well as the forelimbs. That might make a difference.