I participated in an interactive falconry demonstration, using a Harris hawk. These creatures can maneuver with incredible precision in extremely tight spaces. It's not surprising to me to see their behavior described like this, with terms from modern control theory.
They can fly through spaces barely wider than their body (pulling their wings in as tightly as necessary, then immediately unfolding them to continue flying), over a wide range of pitch/roll, while following a curved trajectory, while pursuing prey.
To describe the article as an application of modern control theory is an exaggeration; these rules (proportional navigation and proportional pursuit) are extremely simple and ancient. It's difficult to imagine a time when humans didn't intuitively understand the law described in the paper as proportional pursuit, and the idea behind proportional navigation predates guided missiles by hundreds if not thousands of years.
It's a relief that the heavy mathematical prerequisites of optimal control theory aren't needed to understand what the birds do.
Watch this, for example: https://m.youtube.com/watch?v=HYGz32iv1vw
They can fly through spaces barely wider than their body (pulling their wings in as tightly as necessary, then immediately unfolding them to continue flying), over a wide range of pitch/roll, while following a curved trajectory, while pursuing prey.
Can you imagine writing software to do that?