One good egg, one bad egg

I noticed a fluffed up, sickly looking pigeon the other day. It was tucked up in the pigeon "coop" at the skytrain station and sleeping during the day which I thought was unusual. Yesterday, I went to peek at it and, lo and behold, an egg sat beside it. The pigeon looked more perky, but it was still all fluffed up. "Look at the egg," I urged the pigeon mummy, "Go sit on it!" Mummy pigeon looked at the egg, but she made no effort to go sit on it. Instead, she got up to show me another surprise - another egg underneath her already! "Wow, " I thought, "Twins!" I thought that maybe she had abandoned one egg because she could only handle raising one baby bird as the weather got colder and food became less scarce. Upon more investigation on the www, however, I found that mothers only abandon eggs if they are unfertilized or have similar problems. So I guess it wasn't twins after all. It was one baby and one dud.