The feathers of a bird are, for their weight, among the strongest structures in the world. The bones of this Magnificent Frigatebird weigh less than its feathers! To further reduce weight while maintaining strength, many bird bones are fused. In addition, the pectoral and pelvic girdles and ribs are joined to make a rigid box that supports those long wings, just as the wings support the bird.

More info and transcript at BirdNote.org. 

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