Larger native woods with less conifer plantation support greater populations of the marsh tit Poecile palustris, a declining forest specialist

Many forest specialist birds are in widespread decline across Europe. In Britain, marsh tits are an indicator species of mature native woodlands, but have suffered an 81% population decline since the 1960s. We assessed whether habitat degradation during the mid 20th century, through the widespread conversion of native deciduous woodland to conifer plantations, could have impacted marsh tit populations. We surveyed the recent number of occupied marsh tit territories in 74 discrete woodland patches (‘woods') of 1–296 ha in England, comprising purely native deciduous woodland or with varying coverages of conifer plantations (0–89%). We found that the number of marsh tit territories increased with the woods' size, but this increase was significantly greater for deciduous woods, and lower for woods with a greater proportion of conifer plantation. The area of woodland in the local landscape, reflecting a wood's isolation, had no significant effect on marsh tit abundance in a focal wood. The results indicated that the historical conversion of native deciduous woodland to conifer plantation likely degraded a substantial proportion of formerly high‐quality habitat for marsh tits, affecting up to 37.3% of potentially suitable woods and possibly one‐fifth of the former marsh tit population directly, likely contributing to the species' national decline. Many of the larger coniferized woodlands are in public/state ownership, which could facilitate habitat restoration for the conservation of woodland specialists, like marsh tits, via centralized policies, with additional incentives targeted at woodlands in private ownership. We cautiously estimated that restoration of native woodland could re‐establish a median of 24 610 marsh tit territories in Britain, equivalent to an additional 86% of the current national population.