What are these????

Whilst sweeping leaves this morning, I found a huge pile of these maggots of some kind…. there were literally thousands of them. They were about 2 cm long, plain brown in colour. I scooped a load up and put them on the bird table, all mixed up with the soil they were mixed up with. Within about half an hour, they had all congregated together and moved their way across the bird table and were ‘jumping’…. well falling, off onto the ground, where they congregated in a pile again… very weird!

Does anyone know what these are the larvae to?? I am intrigued…..