found this picThat caught my eye too.

This one looks shorter than the ones on the Spanish website, but might be easier to work with on little birds. Have no idea where you'd buy one, just found the pic in a search for polipotomo,
But I'm betting some talented tinkerer could make one. Appears to be a fine metal straw with one end slightly flattened and two rings attached to the other end. Then the wire loop is fastened to the plunger. I don't know how, perhaps the plunger has one or two small holes so you can tie the wire off on it. Plunger has a finger loop.
I think you would use it like the Chinese tool but instead of having to manipulate the wire with one hand and the tool, you would just slip the loop over the testes and pull up on the plunger to make the loop smaller.
Might check medical supply houses too; muddling through the Spanish site I *think* these are used to remove 'polyps' from intestines.