This is for any "garden variety" detectives out there. Who/what left this curl on one of the trees in my garden?

Other clues;
The only people who come into the garden are me and my husband (and neither of us is in the habit of scratching the bark off the trees).
This is a tall tree with hardly any branching off except at the very top and it's very flexible.
At the top of the tree a pair of wood pigeons are nesting. Right next to them hangs a home made bird feeder that is popular with sparrows.
This is a macro, so the curl is actually quite small.

Will publish the identity of the culprit tonight. Meanwhile ... does anyone care to have a guess?

PIP ADDED and ...

Congratulations, Karl and Stephan! You pretty much guessed it!

We do not have woodpeckers or other birds that feed on bark parasites and no squirrels, so Suspect N. 1 is .... Mina! I have added a pip to introduce her to you. She is not ours but likes to visit and is possibly the fiercest hunter of her kind I have met, daring to mess even with creatures that are nearly as big as she is. She grew up with dogs so she has a number of dog like behaviours that are quite funny, such as laying on her back and showing her belly when she wants to play. A pleasant visitor but ... a terror for the poor birds!