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That does look like a sarcophagid wing. It's difficult to say precisely why the flies were breeding there, but Helicophagella melanura has been recorded as causing myiasis in birds; and other species (especially Parasarcophaga) are known to feed on carrion. It would be worth searching for a male to make a positive identification of the species - even if none has emerged, it may be possible to dissect a pharate male from one of the puparia.