Mama Birds

I think my robin has abandoned her nest. My husband said I was morbid when I said she was probably dead, but really, why else would she work hard to build a nest, lay eggs, sit on them for a week and then not return? Anyway, I'm going to include pictures of her from last week anyway. I also saw a mama bluebird on her nest yesterday, and she wasn't budging. Then there was this other nest. I know what it is, do you? We stumbled across it and scared mama off for a few minutes, but when we discovered she was on a nest we quickly left the area so she could return.