There are some birds that lay their eggs in another birds nest. The Cuckoo, Cowbird, Whyda, Honeyguide and Black-headed Duck come to mind. "These birds, called brood parasites, lay their eggs in another bird's nest and let the other bird parents feed and raise their chicks. The "egg abandoner" is then free to mate again and lay more eggs in another nest. The cuckoo is the best known brood parasite and an expert in the art of cruel deception. Its strategy involves stealth, surprise and speed. The mother removes one egg laid by the host mother, lays her own and flies off with the host egg in her bill. The whole process takes barely ten seconds. Cuckoos parasitize the nests of a large variety of bird species and carefully mimic the colour and pattern of their own eggs to match that of their hosts. Each female cuckoo specializes on one particular host species. How the cuckoo manages to lay eggs to imitate each host's eggs so accurately is one of nature's main mysteries."

I had a catbird using a robin's nest in my perennial g'house (roll up sides, and open to the air) up until last week. The robin had its eggs taken twice and finally gave up the nest for other digs. The poor catbird laid four eggs, and had been sitting for at least four or five days and we had a run of mid-ninety degree weather. She sat on those eggs with her beak open and panting. The structure had to be at least ten degrees above the ambient temperature. I checked the eggs daily when she left the nest and she deserted it. The last two days I checked, the eggs were cold. I have chickens who sit eggs and occasionally they'll destroy some in their nest and remove the remains. It leads me to believe they must be able to sense when an egg is not viable any longer. Perhaps the eggs from the catbird just simply fried in the heat. I had a dove try to sit a robin's nest in another location earlier in the year. She aborted it too, as it was in a busy location and I don't think doves are as comfortable with humans scurrying by as robins can be. You don't give your zone. It's a factor in how late a bird may nest. I have found blue jays babies on the ground dead after falling from their nest after storms in s.e. Ohio as late as October.

In addition, you don't know what stage of incubation another nest would be in. So, if the incubation periods are messed up, the other eggs (or both sets) still may or may not hatch. I would just leave it and let nature take its course. Robins are robust and able to readily renest after a failed nest attempt.

I have no idea; very strange indeed. Robins are usually loyal and caring parents. I would stay away; they would feed the baby while building others "nests" I think. Robins sometimes change their minds and build a few nests before egg-laying; not usually while raising young though.