Have you seen the parents yet? If so, do you know what they are? If you describe what the parents and the nest look like, we can probably figure it out.
Last year, I found a colony of Barn Swallows nesting under a bridge overpass. They picked a perfect spot for it. They were over water, so much less chance of getting disturbed by people. There were no ledges, so they were safe from raccoons and cats. Even a snake would have had a tough time getting there. The bridge of course was concrete, and they were tucked up underneath out of the wind and rain. As you might imagine, they successfully raised a huge crop of young. When the young birds fledged, they weren't wary of people yet, and would sit about 5' away, just looking me over. I hiked back there recently, and they are here again for another nesting season. I intend to keep an eye on them this summer.