A bird that is "keen" could also be said to be . . .

A bird that is described as "keen" conveys a state of alertness, readiness, and eagerness, which can indeed parallel the meanings of being hungry, sharp-set, and ready to kill. When a bird is keen, it is not just physically prepared but also mentally focused on hunting or capturing prey.

If a bird is hungry, it is more likely to exhibit keen behavior as the desire to hunt intensifies. Being sharp-set implies that the bird is in prime condition, physically fit, and mentally alert—characteristics that are essential for successful hunting. Lastly, a keen bird could be directly ready to kill, as this term reflects the bird's hunting instinct and readiness to engage in the hunt.

Thus, the combined meanings of all these phrases contribute to the broader understanding of what it means for a bird to be keen, making "all of the above" the comprehensive answer.