Three different kinds of source lie behind the roc and they are usually run together. The Thousand and One Nights supplies the story most readers know, in which Sindbad ties himself to the leg of a bird whose egg he has mistaken for a dome. That is fiction and presents itself as such. Arabic and Persian geographical writing supplies a second layer, in which sailors report an enormous bird in the western Indian Ocean, and those reports are offered as information.
Marco Polo supplies the third. He describes a gigantic bird on an island he calls Madagascar, strong enough to lift an elephant, and says a feather was sent to the Great Khan. Polo did not go there and is reporting what he was told, which is his usual method for the region.
Madagascar did have an enormous bird. Aepyornis, the elephant bird, stood taller than a person and laid eggs of several litres, and it survived into the period when sailors were reaching the island. Its eggs were used as containers and shells still turn up. The connection between that bird and the roc is an inference, not a report; nobody in the sources says the two are the same. It is a good inference and it should be presented as one.
The spelling is unsettled in English, which is a symptom of the same tangle. Roc, rukh and ruc all appear, and the Persian simurgh and the Indian garuda are pulled in alongside them by writers looking for one great bird behind all the reports. There is no reason to think there was one.