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You don't need pneumatic bone (hollow bone) to have air sacs.

air sacs evolved before hollow bones, pneumatic bones is just a useful extra thing you can do with air sacs once you have them, (you can make bone stiffer). pneumatic bone is not necessary for functional air sacs, most air sacs are not even in bone but in the body cavity.

see the six big air sacs in this image here, they are in the body cavity not bone.

you can see it in air sacs casts, in B the light green portion (LVD and CV) is the only portion actually inside bone.

Air sacs evolved outside bone, but get moved into bone fairly quickly evolutionarily because it has several benefits (stronger lighter bones for a given size for a start) but it is not needed for the air sacs to work.

You may want to check this out (http://people.eku.edu/ritchisong/birdrespiration.html)

You don't need pneumatic bone (hollow bone) to have air sacs.

air sacs evolved before hollow bones, pneumatic bones is just a useful extra thing you can do with air sacs once you have them, (you can make bone stiffer). pneumatic bone is not necessary for functional air sacs, most air sacs are not even in bone but in the body cavity.

see the six big air sacs in this image here, they are in the body cavity not bone.

you can see it in air sacs casts, in B the light green portion (LVD and CV) is the only portion actually inside bone.

Air sacs evolved outside bone, but get moved into bone fairly quickly evolutionarily because it has several benefits (stiffer lighter bones for a given size for a start) but it is not needed for the air sacs to work.

You may want to check this out (http://people.eku.edu/ritchisong/birdrespiration.html)

You don't need pneumatic bone (hollow bone) to have air sacs.

air sacs evolved before hollow bones, pneumatic bones is just a useful extra thing you can do with air sacs once you have them, (you can make bone stiffer). pneumatic bone is not necessary for functional air sacs, most air sacs are not even in bone but in the body cavity.

see the three big air sacs in this image here, they are in the body cavity not bone.

you can see it in air sacs casts, the light green portion (LVD and CV) is the only portion actually inside bone.

Air sacs evolved outside bone, but get moved into bone fairly quickly evolutionarily because it has several benefits (stronger lighter bones for a given size for a start) but it is not needed for the air sacs to work.

You may want to check this out (http://people.eku.edu/ritchisong/birdrespiration.html)

You don't need pneumatic bone (hollow bone) to have air sacs.

air sacs evolved before hollow bones, pneumatic bones is just a useful extra thing you can do with air sacs once you have them, (you can make bone stiffer). pneumatic bone is not necessary for functional air sacs, most air sacs are not even in bone but in the body cavity.

see the six big air sacs in this image here, they are in the body cavity not bone.

you can see it in air sacs casts, in B the light green portion (LVD and CV) is the only portion actually inside bone.

Air sacs evolved outside bone, but get moved into bone fairly quickly evolutionarily because it has several benefits (stronger lighter bones for a given size for a start) but it is not needed for the air sacs to work.

You may want to check this out (http://people.eku.edu/ritchisong/birdrespiration.html)

Frame challenge you don't need pneumatic bone (hollow bone) to have air sacs.

air sacs evolved before hollow bones, pneumatic bones is just a useful extra thing you can do with air sacs once you have them, (you can make bone stiffer). pneumatic bone is not necessary for functional air sacs, most air sacs are not even in bone but in the body cavity.

see the three big air sacs in this image here, they are in the body cavity not bone.

you can see it in air sacs casts, the light green portion (LVD and CV) is the only portion actually inside bone.

You don't need pneumatic bone (hollow bone) to have air sacs.

air sacs evolved before hollow bones, pneumatic bones is just a useful extra thing you can do with air sacs once you have them, (you can make bone stiffer). pneumatic bone is not necessary for functional air sacs, most air sacs are not even in bone but in the body cavity.

see the three big air sacs in this image here, they are in the body cavity not bone.

you can see it in air sacs casts, the light green portion (LVD and CV) is the only portion actually inside bone.

Air sacs evolved outside bone, but get moved into bone fairly quickly evolutionarily because it has several benefits (stronger lighter bones for a given size for a start) but it is not needed for the air sacs to work.

You may want to check this out (http://people.eku.edu/ritchisong/birdrespiration.html)