The fused pelvic vertebrae, or synsacrum, of a flying bird absorbs compression shock whenever the bird lands at high speed.

References

"Several features of the bird skeleton are specially designed for life in the air. The pelvic vertebrae are fused into a solid mass of light bone, the synsacrum, which provides support for the independent movement of wings and legs, and absorbs the compression shock that occurs every time a bird lands on its feet at speed." (Foy and Oxford Scientific Films 1982:39)

Book
The grand design: Form and colour in animalsJanuary 1, 1970
Sally Foy

Organism
BirdsAves