Causes of change

There is good evidence that changes in the habitat quality of woodlands, particularly a loss of understorey, have been responsible for the decline in Marsh Tits. Analysis of the BTO's ring-recovery archive provides evidence that there has been a significant negative trend in annual survival rates during the period of decline, although this is based on a small sample size.

Further information on causes of change

Analysis of the BTO's ring-recovery archive provides evidence that there has been a significant negative trend in annual survival rates during the period of decline, although this is based on a small sample size. The absence of any reduction in breeding performance as the population has declined supports a reduction in annual survival as the demographic mechanism (Siriwardena 2006). Nest failure rates have fallen during the period of decline, but no trend is evident in the number of fledglings per breeding attempt.

One hypothesis relating to the causes of decline is that changes in woodland understorey have reduced habitat quality, due to increased browsing by deer (Perrins 2003, Fuller et al. 2005). Carpenter (2008) and Carpenter et al. (2010) conducted a detailed study providing good evidence that Marsh Tits were more likely to locate their territories in sections of woodland with more understorey cover. Carpenter found that birds in territories with more understorey raised more and heavier young than did birds in territories with less understorey, although this was based on only one year of data. The same study reported that understorey and low canopy sections were also important during winter while Hinsley et al. (2007) provide further evidence that this was important, showing that that Marsh Tits were selecting the understorey and habitat lower down in the woodland canopy. Another field study conducted by Broughton et al. (2006), however, did not find any difference in the amount of shrub layer in Marsh Tit territories compared to pseudo-territories, although this was from just one site and the authors noted that the understorey there was unusually healthy and complete, perhaps explaining this result.

A reduction in habitat quality through fragmentation is another possible factor that has contributed to declines, although there has been little fragmentation of woodland in a gross sense in recent years. Nevertheless, Hinsley et al. (1995) found that Marsh Tits need a minimum wood size of 0.5 ha and it's possible that habitat deterioration has reduced effective habitat patch size.

Another hypothesis concerning causes of decline relates to competition and nest predation. Marsh Tit is subdominant to both Great Tit and Blue Tit but Siriwardena (2006) found no evidence for population effects of the Marsh Tit being outcompeted for natural nest cavities. Similarly, the same study found no evidence that avian nest predation is a major factor in the long-term decline as Marsh Tit abundance was not significantly related to abundance in the previous year of any of the nest predators considered (Siriwardena 2006). Amar et al. (2006) found no association between population change and grey squirrel abundance and adding to this, Smart et al. (2007) conducted an initial analysis and showed that Marsh Tit declines were also unlikely to be caused by predation by grey squirrel, as presence and abundance of Marsh Tit was positively related to squirrel density.

Information about conservation actions

Marsh Tits have large territories and hence require large, mature woodlands with extensive coverage by understorey vegetation (Broughton et al. 2006, 2012; Hinsley et al. 2007). At a local level, therefore, actions to encourage woodland maturation and to improve the quantity and quality of understorey and shrub habitat in woodland may benefit Marsh Tits. These might include a reduction in active management clearance of the shrub layer, and control of deer populations which may be affecting the understorey through grazing. A minimum wood size of 0.5 hectares is believed to be required for this species (Hinsley et al. 1995), and habitat deterioration may have reduced effective habitat size.

On a wider scale, given the requirement for large territories, landscape scale management is likely to be important to maintain or restore connectivity between suitable woodlands and hence ensure that dispersing juveniles are able to connect with surrounding populations in fragmented landscapes.