How do air sacs in the bones help pelicans, frigate birds, and boobies?

In larger and more aerial pelecaniforms, such as pelicans, frigate birds, and boobies, the skeleton is extensively pneumatized. This means that nearly all the bones contain air sacs connected to the respiratory system, resulting in major limb bones that are essentially thin-walled, hollow tubes reinforced with internal struts.1

This skeletal pneumatization contributes to the overall strength and lightness of the avian skeleton, which is essential for flight. Additionally, for species that plunge into the water from a height, such as boobies and brown pelicans, these birds possess a system of air sacs under the skin connected to the lungs. This structure forms a spongy mattress that serves to protect the bird upon impact with the water. In some pelican species that do not plunge, these air sacs may also provide advantageous buoyancy.21