$\begin{array}{1 1} (a)\; \text{The jaws of a bird and their horny covering} \\ (b)\; \text{The ridge on the breastbone of a flying bird to which the flying muscles are attached} \\ (c)\; \text{The hollow central shaft of a feather from which the barbs grow} \\ (d)\; \text{Hooks that hold the barbs of a feather together in flying birds} \end{array}$
