The woodcock is one of William Shakespeare's favorite birds. particularly in reference to the gin, or springe, with which it was captured; thus, it came to represent some trick or contrivance by which a person is caught or deceived. In the play Hamlet, for instance, Shakespeare's Ophelia answers her father's questions regarding Hamlet's attentions with great naivete, describing how the Prince
Hath given countenance to his speech/With almost all the holy vows of heaven. (I,iii)
Unconvinced of Hamlet's motives, Polonius interrupts her,
Ay, springes to catch woodcocks, I do know! (I,iii,122)
The remainder of his speech to his daughter is an extended metaphor as Polonius elaborates upon the comparison of Hamlet to the hunter who wishes to lure his prey, so that he may capture it. For, he tells his daughter to not believe Hamlet's "vows,"
...for they are brokers [ pimps], not of that dye which there investments show (I,iii,134-135)
These promises are not what they appear to be; they are mere
implorators [beggars] of unholy suits/Breathing like sanctified and pious bonds (this is a simile within the metaphor), the better to beguile. I,iii,134-138)
The woodcock's being snared is the metaphor for Ophelia's being tricked by Hamlet's snare of false vows and promises.
There are plenty of metaphors in this speech. Remember, what's going on here is that Polonius is telling Ophelia to be careful because he doesn't trust Hamlet's intentions towards her. Ophelia tells him not to worry -- Hamlet truly loves her. That's when he gives the speech you cite. The first two metaphors:
- The first line. He's comparing Ophelia to a woodcock (some kind of bird) and saying that Hamlet is setting a trap (spring) for her.
- Blood burning. This is given as a metaphor for lust. Of course a man's blood doesn't literally burn from lust, but that's the comparison Shakespeare is making.