The Eternal Triangle

I went across town for a session of Ashtanga yoga - the first time I'd done any since B.S. (before shingles) - so I definitely broke a sweat. Phew.

When I got back to Leith I noticed this trio dabbling around in the water of Leith. With mallards the ratio is almost always two males: one female. Why the imbalance? Do they practise polyandry or do they expose lots of the female ducklings out on the hillside?

Then I started to wonder where the expression "the eternal triangle" originated - but I am too tired to look it up. (I blame the Ashtanga). Help me out, blippers!