Thank you for your question.

If you can put the nest where it was, or on a lower branch in that tree (or even stick the nest into a tupperware container bottom and attach that to the tree), then she is likely to find this remaining hatchling. If you put on the ground, she is likely to find him anyway, but it would be a vulnerable location for the wee one and for the parents when they return to feed (and the predators would likely take advantage of this).

Overall, she will come back if you put her nest out for her. If you monitor from afar over the next few days and the little one is showing signs of not being fed and weakness, then you can consider turning him over to a rehabilitation centre. To find one near you, you can check at Wildlife International (LINK), US Wildlife Rehabilitation (LINK), Wildlife Rehabilitation Information Directory (LINK) Wildlife Rehabbers (LINK), or Wildlife Sanctuaries (LINK).

Alternatively, if you are struggling in that situation, then you can turn the bird over to your nearest vet practice. They will be able to evaluate him, treat him as required, and turn him over to the rehab organizations (usually they will have a relationship with the organization and can get birds in, even if the public cannot).

So, do put the nest back and give this family a shot. Sometimes nature is cruel and nests dislodge, but hopefully mum will find her wee one and continue to raise him to be a robin.

I hope this information is helpful.
Please do let me know if you have any further questions.
If you have no further questions, feedback is always appreciated.

All the best,

Dr. B.

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