Sophocles, Electra 145-149 (web address 1), with comment (on the feminine participle which is the present headword) from the scholia there.
Through her references to Itys and to "that bird mad with grief, the messenger of Zeus," Electra likens herself to Philomela. In the Attic version of the myth, Philomela was raped by her brother-in-law Tereus, who cut out her tongue to prevent her confession of the crime to her sister Procne (pi 2475); instead, Philomela embroidered the deeds of Tereus and thus revealed them to Procne. Procne then killed her son Itys and fed him to Tereus. The gods changed the fleeing sisters into birds. In the most widespread version of the myth, Philomela became a nightingale. Modern commentators often understand the nightingale, here the "messenger of Zeus," as a "harbinger of spring." Kammerbeek, however, reads the nightingale as a type of "inconsolable grief" because its love is faithful, and points out that Penelope compared herself to a nightingale (Homer, Odyssey 19.521-23).