In Henrik Ibsen's play A Doll's House, Nora, the protagonist of the play, is referred to as skylark and squirrel by her husband, Torvald.
One could easily make the connection between Nora and the squirrel, while the connection to the skylark may be a little more difficult.
Torvald refers to Nora as a squirrel based upon the fact that she likes to hoard secret little stashes of things which Torvald demands that she does not have (like the macaroons which she must hide). By referring to her as a squirrel, Torvald is letting Nora know that she cannot "get anything by him." He is simply letting her know that he does know about her stashes. This, actually, is quite ironic given that she was able to fraudulently obtain a loan without her husband's knowledge.
As for why Torvald refers to Nora as a skylark, one can only make assumptions. Given that the name of the bird alone contains the word "sky," Torvald may be referring to the fact that Nora is a little to flighty for his taste and that she needs to be more grounded. By calling her skylark, he is letting Nora know that her behavior is not appropriate. Torvald treats Nora like a child. Given that he refers to her with silly pet names enhances his oppression over her (by being "silly" while letting her know that he is in control).