YA Book about two children who are turned into a manta/sting ray and a large bird

Question

I am trying to find a book that I believe I read as a child, but I think is older than me. Either that or it's a really vivid fever dream.

In it, two children finds themselves on an island, shipwreck or plane crash I think, and are imprisoned in a kind of apartment by a scientist. I distinctly remember them using toothpaste and shaving foam to destroy food they didn't like. It was delivered when a light over the door came on and they had to wait in another room.

At some point some science happens and one turns into some kind of sea creature, a manta ray or sting ray I think, and the other into a large bird. I think they have some telepathic link at some point, but it fades as they become more animal than human. The bird is trying to plot via sticks and berries to get them to escape. I don't remember how it ends, and there may have been a third child who is changed earlier than them.

Answer 1

Dr. Franklin's Island by Ann Halam

Semi, Miranda, and Arnie are part of a group of 50 British Young Conservationists on their way to a wildlife conservation station deep in the rain forests of Ecuador. After a terrifying mid-air disaster and subsequent crash, these three are the sole survivors, stranded together on a deserted tropical island. Or so they think. Semi, Miranda, and Arnie stumble into the hands of Dr. Franklin, a mad scientist who’s been waiting for them, eager to use them as specimens for his experiments in genetic engineering.

From this review:

Dr. Franklin is a scientist involved in the works of transgenics. He's coming up with a way to create animals mixed with humans. Think about it - a human, who can change into a bird. He kidnaps Semi and Miranda and makes them his first human guinea pigs, turning Miranda into the previously mentioned bird, and Semi into a manta ray.

Found with a search for novel mad scientist children "Manta Ray"