Catkins. I grew up knowing them as Lamb’s Tails and a bit of a personal harbinger – in the midst of our grey and heavy-sky winter – that spring and brighter days will follow.
Are these silver birch or hazel? I am no expert – but l know at least that these pendulous flowers are Nature’s way of letting the wind assist in pollination.
I’m no expert but I think these are Silver Birch. But other trees have them as well:
https://www.woodlandtrust.org.uk/blog/2018/12/which-trees-have-catkins-and-how-to-tell-them-apart/
A lovely harbinger of Spring!