Evie, it might really be a bluebird nest – you need to give it a few days to see. My pair of blues this spring built a nest which I questioned might be a HOSP nest because it was not as neat as usual and with more coarse grass, but it had a nice cup. Watch closely to see if the HOSP are around or if the bluebirds are building. I know it is hard to watch all the time – but early morning is usually when I find most building goes on – first 2-3 hours after daylight. I still believe it was a snake which got your babies, or the nest would have been disturbed. Keep us posted.