Causes of change

The reasons for this decline are unknown, but there is good evidence that they lie at least partly outside the breeding season and are thought to be linked to changing conditions on wintering grounds and migration.

Further information on causes of change

The reasons for this decline are unknown, but there is good evidence that they lie at least partly outside the breeding season (Goodenough et al. 2009). No trends are evident in the number of fledglings per breeding attempt. Although the failure rate at the egg stage has shown a decrease, failure rate at the chick stage has increased. Clutch size increased until the mid-2000s but has since decreased slightly.

A study in the Netherlands found that a large proportion of Pied Flycatchers arrive later at the breeding grounds and do not breed in their first adult year (Both et al. 2017). Assuming that the same is true in the UK, this may complicate interpretation of trends and modelling to investigate the causes of change, particularly if the proportion of non-breeding first-year birds varies regionally and over time. Fatal interspecific competition for nest boxes is higher when Pied Flycatcher arrival coincides with peak laying in Great Tit, although late arriving male Pied Flycatchers were most likely to killed and therefore the deaths did not have any effect on the breeding population (Samplonius et al. 2019).

There is good evidence that declines are related to conditions outside the breeding season. Mallord et al. (2016) found no evidence that changes in woodland structure affected populations in six study areas in the west of the UK. Goodenough et al. (2009) found that decreasing breeding performance is contributing to decline, but that non-breeding factors are more important. Winter NAO index is a strong predictor of breeding population, probably because the North Atlantic oscillation influences food abundance in Africa and at migratory stopover points. Long-term autumn bird monitoring data from Russia were related to monthly mean temperatures on the West African wintering grounds; the positive relationship suggests that increasing bird numbers are explained by increasing mean November temperatures. Precipitation and European autumn, spring and breeding-range temperatures did not show a strong relationship (Chernetsov & Huettmann 2005). Thingstad et al. (2006) found that weather conditions at the flycatcher's wintering areas in western Africa were suspected to be responsible for the decrease in Scandinavia, although the breeding success of the sink populations was significantly correlated to June temperatures.

In the Netherlands, climate change may have brought about decline in Pied Flycatchers by advancing the peak period of food availability for this species in deciduous forests - the birds being unable to compensate for the change in food supply by breeding earlier (Both 2002, Both et al. 2006). A subsequent paper found that timing of spring migration has responded flexibly to climate change as recovery dates during spring migration in North Africa advanced by ten days between 1980 and 2002, which was explained by improving Sahel rainfall and a phenotypic effect of birth date. However, there was no advance in arrival dates on the breeding grounds, most likely due to environmental constraints during migration (Both 2010). Futhermore, declines were found to be stronger in forests, as these were more seasonal habitats whereas less seasonal marshes showed less steep declines (Both et al. 2009). Another more recent study in the Netherlands confirmed that arrival dates had not changed, but found that the timing of breeding and moult had both advanced, with earlier breeding increasing the time available for fledgling development and the probability that they will survive and join the breeding population (Tomotani et al. 2018). Climate change was also given as a potential factor by a Swedish study, that suggested warmer springs favoured resident Blue Tits and Great Tits over Pied Flycatchers, which were not able to adjust to increasing spring temperatures (Wittwer et al. 2015). Another study, looking at 10 European nest box schemes (including one in the UK) found that, although both tits and Pied Flycatchers had advanced their laying dates, tits had advanced more strongly by, on average, approximately one day per decade between 1991 and 2015 (Samplonius et al. 2018). It should be noted, however, that data presented here show that Pied Flycatchers in the UK have advanced their laying date by ten days since 1967, matching the change shown by Great Tit and exceeding the change of Blue Tit by two days.

Information about conservation actions

The reasons for the decline are unknown but it is likely that they lie at least partly outside the breeding season and hence it is uncertain whether conservation actions taken in the UK will have significant effects on the population trend.

However, Goodenough et al. (2009) suggested that, although less important than non-breeding factors, decreasing breeding performance may also be contributing to the decline, Therefore, breeding season conservation actions do remain useful for this species. Goodenough et al. (2009) recommend both the optimal placement of nestboxes (avoiding south-west facing boxes) and the management of woodland habitat to provide host plants for lepidotera larvae in order to increase food supplies. Elsewhere, a study in Finland found that birds preferred large (>5 ha) or medium sized (>1 ha) deciduous woodlands with the larger woods occupied first (Huhta et al. 2008); hence suitable sized patches of mature wood within the range of this species should be conserved where possible.