Willow tits select stumps and standing dead stems of willow, birch or elder that are rotten enough to work with a small bill, and excavate a chamber, carrying the chips away from the site rather than leaving a telltale pile beneath. Most pairs cut a new hole each season rather than reusing an old one.
The requirement is specific. The wood must be soft enough to excavate but sound enough not to collapse, which means a narrow window of decay, and the birds need damp scrubby woodland where such stems are common. Removing dead wood, draining wet woodland and allowing scrub to grow into closed high forest all take that away.
Britain has lost a very large share of its willow tits since the 1970s, one of the steepest declines of any British breeding bird, and the species is red-listed there despite a global assessment of Least Concern. Deer browsing of understorey and competition from blue and great tits for the freshly cut holes are both implicated, but the ranking of causes is not agreed.