Causes of change

The causes of the decline are currently unclear. A number of causes have been suggested but remain speculative and robust evidence to identify the main driver(s) of population change is lacking.

Further information on causes of change

Analysis of phenological data has found that the arrival date in the UK has advanced, between the 1960s and 2000s, by 16 days (Newson et al. 2016), although it is not currently known if and how this may affect breeding productivity for this species. Winter weather and food resources may be more limiting than conditions during the breeding season: annual survival rates from RAS sites in the UK for 1994-2004 were positively correlated with maximum monthly rainfall in West Africa. Some declines in survival rate are apparent over this period but they do not correspond well to the period of population decline (Robinson et al. 2008).

A number of breeding season factors have also been suggested as potential causes for the decline and for the contrasting trends between the north-west and south-east of the UK, but these are all speculative and robust evidence to identify one or more of these possible causes as a driver of population change is lacking. Previous research found that clutch sizes and the number of second broods varied by latitude and longitude (Moller 1984,Bryant 1975 ), and that breeding success is predominantly affected by foraging conditions (Bryant 1975, 1979,Poulin et al. 2010). Hence it is feasible that factors such as climate change and land use changes could have caused increases in breeding productivity in some parts of the UK and decreases elsewhere, causing the observed geographical variation in trends. However in the absence of demographic data it is not possible to confirm this. Competition with House Sparrow for nest sites has also been found to affect populations at local colonies (Tryjanowski & Kuczynski 1999). However, there is no evidence as yet to show that the overall proportion of usurped nests has changed over time and hence that House Sparrows could be having a population level effect.

Information about conservation actions

The main drivers of the decline for House Martin are not yet clear; hence, it is uncertain which conservation actions are most likely to directly benefit the species. However, it is known that House Martins usually prefer to use existing nests remaining from the previous year and will be more likely to raise two broods if they existing nests (e.g. Piersma 2013). Therefore, wherever possible, homeowners should leave existing nests in place over winter to enable them to be reoccupied the following spring.

Whilst research has not yet confirmed what factors are likely to be driving the decline and has therefore not tested possible solutions, there are some other actions which individuals and organisations can take as a precaution to address some of the potential factors whilst research is ongoing. Artificial nests will sometimes be used by House Martins and can therefore be provided alongside or to replace natural nests, particularly in cases where soffits and/or wall surfaces are replaced with PVC materials as anecdotal observation suggests such nests are more likely to collapse than those built on wood or brick surfaces. Another action which could be considered by site managers is the creation of 'House Martin towers' to provide potential nesting sites for House Martins on protected areas away from buildings. Such towers have been built for both House Martins and Swifts in several European countries.

Warmer, drier conditions as a result of climate change has also been suggested as a possible cause of the decline, and some homeowners have created muddy puddles during times of drought but the efficacy of this has not been tested.

Previous research suggests that breeding success may be predominantly affected by foraging conditions (Bryant 1975 , Bryant 1979; Poulin et al. 2010). Therefore, wider scale policies which aim to ensure habitat in the wider countryside supports invertebrates may also be important, e.g. reducing herbicide and pesticide use and creating natural habitat features.