(The). The eggs of this bird are spotted with red. The tradition is that the bird fluttered about the Cross, and got stained with the blood in its plumage, and by way of punishment its eggs were doomed ever after to bear marks of blood. 'Tis a very lame story, but helps to show how in former times every possible thing was made to bear some allusion to the Redeemer. Because the bird was “cursed,” boys who abstain from plundering the eggs of small birds, were taught that it is as right and proper to destroy the eggs of the bunting as to persecute a Jew. (See Christian Traditions.)
Hammer is a corruption of the German ammer, a bunting.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Yellowhammer from Fact Monster: