The robin probably had an instinctual urge to cover the eggs. If they have good place to build, robins will build several nests, with Mama making the final choice. But I've seen Mama sit on nests she previously abandoned. (?)
Susan

I agree with OB- it's pure instinct I believe that got the robin to "protect" the eggs while the mom was out. During this time of year, robins are usually defensive. I've watched them chase away other birds and even chipmunks that got too close to them. Once, when blue jays got too close to a cat bird's family, the cat birds cried out and the robins stopped feeding and rushed to help out. They are awesome birds.

Joan- forgot to say that you're a terrific bird "Mom" :-)

Well, thank you surya. I never knew being a "birder" could be so stressful lol. Upon further observation of the robin, I think she was trying to find a spot for a nest and liked the spot the waxwings have. The mother waxwing was sitting on the nest when the robin approached but took off when the robin intruded. I don't know if then she decided to sit on the nest to protect the eggs or was just checking out the real estate. She came back a couple more times and scared off the mama waxwing, but never seemed aggresive. I really think she just wants the waxwing's spot and can't figure out what to do with the nest that's already there lol.

LOL- so that's what she's up to - she's thinking that the waxwings "took" her spot and she wants it back!! Hopefully, she'll find something close by that she likes. Robins can be so picky.

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."