On 26th March in Salcey Forest, Northamptonshire, I came across a Eurasian Treecreeper Certhia familiaris building a nest. The site was about 3 m up in an old Oak Quercus robur and behind loose bark, as is usual for this species. I watched for some time as it came in with short grass stems or small slivers of bark. These visits were about four minutes apart and on one occasion when it was absent, a Long-tailed Tit Aegithalos caudatus landed next to the nest site, poked its head behind the bark and pulled out some wool from the nest. It was joined by a second Long-tailed Tit, which started removing wool from the opposite side of the nest. The two birds spent some time tugging out the wool and then left. The Treecreeper subsequently returned with more stems and continued to build.