The sawflies will make a tasty meal for the blue tits, to help encourage the birds you could hang a sunflower feeder nearby to draw them to find the caterpillars more quickly if necessary. I would leave them be as Alan has suggested.
What a great picture of the male azure damselfly Punkdoc, not seen so many of these about this year, although we have in previous years. Plenty of empty Emperor dragonfly cases lurking in and around our pond this year.
Thanks guys, yeah I'll leave them be. We do have a couple of feeders very close, actually, and bluetits, sparrows and starlings have all been very active on them today. They haven't clocked the sawflies though! Must have been there for at least a few days, given the size of them. On closer inspection I've found another two leaves that are covered in much smaller, darker caterpillars. I am assuming these might be younger of the same? At the end of the day we put this birch in precisely to try and attract wildlife, so it would seem a bit churlish to then kill it once it arrives. Having said that, it is a very young tree so if it would get significantly set back then I might consider it for the greater good (of a bigger tree in the future). I'll keep an eye on it.
I had sawfly strip a very young birch whip a couple of years ago, the leaves grew back quite happily within a few weeks and the tree seemed none the worse for it and is making good growth now, Id just keep it well watered if they really go to town on it, they are pretty tough trees in my experience
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Male azure damselfly on the pond
When you don't even know who's in the team
S.Yorkshire/Derbyshire border