Caspian Tern from the 1990s

After my Caspian Tern experience yesterday, I thought I'd check the last Caspian Tern I sketched. Here are some drawings from the 1990s. These were for a book I wrote a long time ago.  On the second figure, lower left corner, there's a Caspian Tern.  And there it is, the down-tilted bill posture.  I had drawn the posture accurately at the time, but somehow it never triggered in my mind that this might be a useful feature for identification on the fly of faraway birds.