Could mammals originate from birds?

No, It is Immpossible. Although all animals and all organisms, in principle, descended from a common ancestor, the path of development of different groups was very different. The presence of common ancestors explains some of the similarities in physique, the same internal organs, and so on. But scientists have found that mammals evolved from synapsids. They are very similar to mammals, especially later representatives of these animals. This is also proved by the structure of the skeleton of synapsids, some genetic studies and other archaeological data.