Marsh Tit Poecile palustris population decline in Britain: the role of foraging behaviour and interspecific competition

The Marsh Tit, a strict deciduous woodland bird, has undergone a severe long-term decline in Britain of over 50% in the last 25 years. Possible reasons for this decline were investigated at two woodland sites in southern Britain. Winter foraging behaviour of the marsh and Blue Tit was compared, using instantaneous time sampling. The subdominance of the Marsh Tit was demonstrated by its higher within flock vigilance; and the overlap of specific foraging behaviours demonstrated that the two species foraged in similar ways, competing for food resources. However, niche separation was observed; the Marsh Tit spent more time in the understorey, and showed different tree and shrub species usage, thereby reducing this interspecific competition. Declining quality of the understorey in British woodland, e.g. due to deer browsing, is placing pressure on the Marsh Tits niche; and furthermore the Blue Tit population is increasing. For a species which remains on its territory year round, these combined pressures may be important in driving its decline, especially when food resources are minimal in late winter.