As you predicted;
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The chap holding him is from the Barn Owl Trust, they asked if they could come and check the owlets, weigh them and ring them to help monitor populations and gather data to help in their conservation efforts. The owlets were then replaced in the box.
We have three owlets, all in very good health and well fed, so we are very pleased because we manage our field to maximise the number of voles, mice and shrews available. The adult owls are hunting from 8 PM every evening which provides us with more entertainment than any television could!