UPDATE 21st April. I have now had 3 Brown House Moths (Hofmannophila pseudospretella) emerge from these larvae.
When I cleaned out our House Sparrow nest boxes last autumn I decided to keep the contents in a plastic bag overwinter to see if any moths emerged this year. The bag was hung in a garden shed and today we noticed several larvae crawling up the sides of the bag obviously looking for suitable pupation sites. There are several species that inhabit bird nests but, looking on the internet, there seems to be a dire shortage of larval images of the likely candidates so I shall have to wait and see if adults emerge, allowing identification. Patrick Clement