Stoned Birds are the result of swallows being affected by the Petrification alongside Humanity.
The entire swallow population was trapped in stone along with the entire human population of the earth and it is unknown why. After awakening, Senku used a number of stoned swallows for his experiments. He tested a number of cures until, after a year, he set a swallow free from petrification with the stone formula.
↑ Dr. Stone Manga: Chapter 2, Page 20 ↑ Dr. Stone Manga: Chapter 1 ↑ Dr. Stone Manga: Chapter 48, Page 8