A fishy tale

I have fed the birds in the back garden for 30 years with seed feeders and bowls of water.

All was okay but the ornamental bird baths were not big enough for the larger and more numerous starlings, so I put a plastic bowl on the grass, 18ins x 2ins deep.

When I went out a few days later to top up the feeders I found the bowl had about 70 small dead fish. They are about two inches long, dark grey on top and cream underneath.

The birds are not using the bowl or eating the fish. Could they be some type of slug? It’s a puzzle. Mr R Peacock