Glutton Goldfinches: What are they eating?

More than likely seeds. That's the staple diet of goldfinches.

Today I watched three goldfinches--two male, one female, nibbling at unrendered beef suet.

They may be feeding on fallen thistle seeds, one of their favorite foods, or the grass seeds, or any wildflower seeds. I assume the field you're referring to is left wild with the plants left to seed-out. As most birds are oppurtunistic I wouldn't doubt that they would pass up an easy meal of protein-rich insects.

Colber--

The grassy area is my all-organic moss, grass and broad-leaved weeds yard.

http://www.laspilitas.com/California_birds/Finches/Lesser_Goldfinch/Lesser_Goldfinch.html

This is about a different finch, but the notes at the bottom left may add some possibilities to you list of finch food...

This nursery site is one of my favorites, even though I don't live in that region anymore.

The bird page is a rich treasure..
Bird Page, Las Pilitas nursery website

Osso--

Great links. Thank you.

My oddest feeding bird was a wren taking teeny-tiny nibbles of a cake of rock salt.

I looked goldfinches up on google and saw mention of rocksalt in one of the listings..
just tried to grab a link for american goldfinches, which I think are the goldfinches in your area, and lost it in the process of getting it here. The list of items of interest to it was similar if not the same as the lesser goldfinches in California, but it mentioned fly and wasp larvae as well as aphids and caterpillars, if I remember right.

Osso--

I have the American Goldfinch, but they are similar.

Last year a number of spiders built webs outside my computer window and eventually produced a number of egg cases. One morning I watched a tufted titmouse perform some incredible acrobatics to harvest them.

The effort involved was considerable. Spider eggs must be very tasty.

Wonder if they're salty..

If insect "juice" is like animal blood, those egg cases are very salty. After all, Mama Spider eats nothing but liquids.