Here's a nice photo my brother Simon took of a yellowhammer.
I loooked up where the bird got its name and there are different theories. All agree about the yellow bit. But what is a "hammer"
I found two suggestions and there may be more possibilities.
Old English amore, a kind of bird (probably the yellowhammer, so that would make it a yellow yellowhammer, or maybe just a bird of the bunting group)
Old English hama = feathers
The first seems to be a common Germanic word denoting a bunting or some similar bird. It's been suggested there is a connect with emmer, a kind of wheat, and indeed grains are part of a yellowhammer's diet. But the OED has a long discussion and I don't think we shall ever know the word origin; it's probably a conflation of more than one source word.