Causes of change

Willow Tits have declined in woodland, probably because of habitat degradation. How this relates to demographic trends is unclear.

Further information on causes of change

Little evidence is available regarding changes in the demography of this species but CES trends suggest a decline in productivity since 1983 (see above). Lampila et al. (2006) found that adult survival was the main driver of Willow Tit populations in northern Finland, although a study in Norway recorded adult survival of around 50% (Hogstad & Slagsvold 2018) which is in line with the survival rates for Blue Tit and Great Tit published in this report. Both these studies were in boreal forests, so the processes may not be the same as for the British population. The British subspecies shows very different habitat preferences to the Fennoscandian one, preferring wet woodland rather than conifers, emphasising that Continental studies may not be very relevant to population change in the UK.

There are several hypotheses that have been put forward to explain the cause of population declines of Willow Tit. One is that deterioration in the quality of woodland as feeding habitat for this species through canopy closure and increased browsing by deer (Perrins 2003, Siriwardena 2004, Fuller et al. 2005, Newson et al. 2012) has been important. The area of wet woodland and scrub is also thought to have declined as a result of drainage and the occurrence of increasingly dry summers (Vanhinsbergh et al. 2003). A field study based on former CBC sites and other woods that were known to have held the species in the past provided good evidence that the sites still holding Willow Tit tended to be wetter, so drying out of woodlands may have been a factor (Lewis et al. 2007, 2009a, 2009b). Siriwardena (2004) analysed long-term CBC trends and found that, although population trends have been stable in their preferred, wet habitats, Willow Tit have declined in woodland, probably because of habitat degradation.

A second hypothesis is that nest predation pressure, from Jays, Great Spotted Woodpeckers and grey squirrel, for example, has increased, both because some of these predators have grown more abundant (Harris et al. 1995, this report) and because restrictions in nest-site availability are likely to have forced more birds into suboptimal, more vulnerable sites. In the study mentioned above, Siriwardena (2004) found increases in Green Woodpecker abundance on CBC plots at the same time as declines in Willow Tit abundance, but this is unlikely to reflect a causal link - this woodpecker being unrecorded as a nest predator. A negative relationship between Great Spotted Woodpecker and Willow Tit abundance on farmland plots is more likely to reflect a real population effect, but farmland is only a minor habitat for the species, so it is unlikely that such a relationship has biological significance for Willow Tits nationally. There were no significant associations with other avian potential nest predators. Supporting this result, Lewis et al. (2007, 2009a, 2009b) found that sites that were known to have held the species in the past and that were still holding Willow Tits did not differ in the density of potential nest predators. In a study which monitored 128 nests over 2014-18 in Greater Manchester, predation accounted for 60% of the 45% of nests that fai

Thirdly, increases in the local populations of behaviourally dominant, sympatric species such as Blue Tit, Great Tit, Marsh Tit and Nuthatch could have led to increased competition, especially for nest-holes. There is little direct evidence specifically concerning foraging interactions involving Willow Tit in the UK but it is possible that increases in other tit species have placed extra pressure on Willow Tit populations through competition for food or nest sites (Vanhinsbergh et al. 2003). In Lanarkshire, central Scotland, Great and Blue Tits were found commonly to take over the nest sites of Willow Tit (Maxwell 2002, 2003) but it is unclear how widespread this phenomenon is. In the Greater Manchester study mentioned above (Parry & Broughton 2019), eviction by Blue Tits caused 23% of first breeding attempts to fail, but only 3% of second attempts, so the ability to re-lay may limit population level effects of usurpation. Excluding evicted and predated nests, 98% of eggs in this study resulted in fledged chicks, suggesting that competition for food was not an issue at this site, at least during the nestling stage. In the analysis of long-term CBC trends carried out by Siriwardena (2004), no negative relationships were found between Willow Tit and its potential competitors. Again, this was supported by field data from Lewis et al. (2007, 2009a, 2009b), who found that sites that were known to have held the species in the past and that that were still holding Willow Tits did not differ in the density of avian competitors.

Overall, therefore, habitat deterioration is the strongest candidate as the cause of Willow Tit decline nationally. As well as increasing woodland drainage, degradation has been hypothesised to have occurred via a reduction in nest-site availability resulting from falls in the amount of dead wood and number of dead trees in woodland reducing nesting opportunities (Vanhinsbergh et al. 2003). This has yet to be tested formally, however, probably because historical data on quantities of dead wood are not available.

Information about conservation actions

The long-term decline of this species is most likely to have been caused by habitat degradation, and hence conservation actions and policies which aim to restore and create habitat are most likely to benefit this species.

Wet woodland also seems to be a particularly important habitat for Willow Tits, and one study found that soil water content was the only significant difference between sites which were still occupied and those which had been abandoned. This study concluded that "Habitat provision for Willow Tit should involve provision of young woodland patches with moist soils" (Lewis et al. 2009a). Another study identified deadwood of silver birch, elder and alder as the most frequent trees used for nest sites (Parry & Broughton 2019). Deterioration of the woodland understorey is also believed to be important (see Causes of Change section, above); actions to promote understorey habitat could include management to create a more open canopy and to control deer numbers.

It is also possible that increased interspecific competition for food and nest sites might be having some negative effects on local Willow Tit numbers, although the evidence does not suggest that this is likely to be the main cause of the general population decline. Nevertheless, it might be prudent to discourage the increased provision of food and nest boxes within and near Willow Tit strongholds as a precaution.