And don't think Birds are just animals driven by instinct.. They have a darn good memory.. I have a bath tub with goldfish in the garden, have it for over about 7 years and last year i had my first Heron visiting it and snatching my oldest and tamest fish from the tub. After that i saw the Heron fly over regularly, but the tub was covered with a net till the grape plant grew big enough to prevent the Heron to land. Anyway, in the winter the fish go indoors and around appril they go out again.
This year, the fish were out again for only 2 weeks and i saw the Heron again sitting on my neighbours roof eyeballing my tub. I scared it off with oppening the door.
6 years never seen a Heron and than once the small tub was spotted 2 years in a row a Heron visiting. If it wasn't the darn same Heron remebering it than he must have told his friends where to find my tub with fish in it.
I go for the first option, it was the very same Heron remebering my delicious goldfish tub.
There is only one thing you an do, that is prevent the bird from getting to it. pull a net over the pond which always isn't a decorative choice. Or put a net or lines over the biggest part of the garden and over the pond. For example take fishing line and tie the lines 50cm apart from eachother, one end to the other over the garden. High enough to walk under it. Is less obtrusive than a net over teh pond or net over the garden. But it still prevents the Heron to find a place to land. Other parts in the garden you can use these lines to grow vining plants over it. Giving it a decorative purpose as well..