Another surprise. I found an old pellet on the roof, trapped in a hydrangea branch that had grown over the tiles. I assumed it had been regurgitated by a tawny owl sitting on a branch of the oak tree above. The pellet was hard and about 40mm by 10mm.
A tawny owl pellet is mostly made of fur and will break apart easily to reveal the bones inside - usually the bones and skulls of three or four small mammals. This one was tough so I soaked it in water. It was still difficult to tease apart because it was made of decomposed feathers rather than fur. And the largest bones it contained were the legs and feet of a bird.
So now I think the pellet is not from a tawny owl at all. The bones were all broken - an owl swallows its prey whole. There were quite a few other bone fragments but no skull or other parts that I could recognise. My best guess is that the pellet is from a sparrowhawk. I am a bit puzzled by what look like a few strands of grass, best seen in the top photos. They were much less obvious in the grey mush once the pellet had been teased apart in water. I can understand how a bit of grass might be swallowed during eating but I would have expected it to be digested easily. Here are the pellet contents, minus the feathers. You can see the claws, two lines of bone fragments, skin strips, stones and a few small bits of insect wing case as well.
The several small torn fragments of skin look like bird skin.
The most intriguing finding was these four small stones, about 5-10mm in diameter.
Birds can't chew and they mash up their food in a muscular stomach, the gizzard. Many birds swallow small stones to help this process, stones known as gizzard stones or gastroliths.
Plant-eating dinosaurs used gastroliths as well. Their stones were much larger and stayed inside for years, becoming smooth and rounded like pebbles on a beach. I found this photo of Jurassic gastroliths on Wikipedia.
Every day a school day!
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