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.

https://en.wikipedia.org/wiki/Proportional_navigation https://en.wikipedia.org/wiki/Constant_bearing,_decreasing_r...