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The Marsh Tit Poecile palustris is a small, hole-nesting woodland passerine whose national population in the UK has declined by more than 50% in the last 25 years. To investigate possible causes for the species long-term decline, we examined habitat selection by Marsh Tits at three scales. For individual foraging birds, winter time budgets and foraging behaviour, recorded using instantaneous sampling, differed little between Marsh and Blue Tits Cyanistes caeruleus, but Marsh Tits spent more time in the understorey and more time lower down in both the woodland canopy and the understorey. At the scale of breeding territories, the characteristics (numbers by size class, vegetation density, species richness) of trees and shrubs were compared using 100 × 10-m sample transects of ten territories in each of four woods. The characteristics of the trees differed significantly between woods whilst those of the shrubs did not, suggesting that the characteristics of shrubs were more important in territory selection by Marsh Tits than were those of trees. Furthermore, in one of the four woods (Wytham Woods, Oxfordshire), Marsh Tits were largely absent from areas with dense tree canopy, but poor shrub cover. On a national scale, using data from 157 of the woodlands surveyed by the RSPB/BTO Repeat Woodland Bird Survey, Marsh Tit abundance in 2003/04 was found to be positively related to vegetation cover at heights corresponding to the shrub layer, especially at 2–4 m. These relationships were not apparent in data for the same woods for the 1980s, but shrub cover had increased substantially by 2003/04 and Marsh Tit abundance had increased in woods with the most cover in 2003/04. Thus, factors damaging the shrub layer, such as overgrazing by deer, shading out by canopy closure and managed clearance of shrub cover, may reduce the suitability of woodland for Marsh Tits. Habitat use by a closely related species, Willow Tit Poecile montanus, is also discussed.