Willow / Marsh Tits and nest hole excavation

Ian - If the tits were excavating a nest hole they're almost certain to be Willow Tits.  Marsh Tits don't excavate nest holes from scratch, although they will often enlarge existing holes. This behaviour difference is given as a useful distinguishing feature between the two species in the excellent 2009 Bristish Birds paper on separating Marsh and Willows:  http://www.britishbirds.co.uk/wp-content/uploads/2010/10/Willow-Marsh-Tits.pdf​