Philomela, Sir Phillip Sidney

 Upon further research I find that what I thought was true. Sidney did base this poem off of a myth or a folk tale, to be more exact he based this poem off of a Greek myth about Philomela, princess of Athens. Her brother-in-law Tereus son of Aries agrees to go to Athens and escort her from Athens to his hometown, Thrace for a visit. He lusts for her on the way and when they get to Thrace he ends up taking her to a cabin in the woods and rapes her. So she won't tell he cuts out her tongue but this proves useless because she weaves a tapestry telling her older sister, Tereus' wife Procne what happened. To get revenge on him Procne ends up killing their son and feeding him to an unknowing Tereus. He finds out and tries to kill both Procne and Philomela but the gods end up turning them three into bird; Procne into a nightengale (hence the "the nightengale as soon as April bringeth" in the poem), Philomela into a swallow, a bird with no song, and Tereus into a bird similar to a hawk.
 This adds alot of background to this poem and helps me understand it a little more because at first I didn't know what Sidney was talking about. I now know that the speaking in the poem is Procne talking to Philomena. In the first stanza it is as if she is in mourning for what happened, "what grief her breast oppreseth".  In the reoccurring stanza, stanza 2, it looks like she is saying that Philomena should atleast be glad that Tereus wants her because Procne is older and describes herself almost as baren "Thine earth springs, mine fadeth". This seems a little vain and selfish but this could have been the opinion Sidney formed himself, because the myth was told in many variations over time.