Causes of change

The reasons for change are currently unclear. Although agricultural intensification is likely to be a primary driver, over-winter survival and changes in habitat on the breeding grounds may both be having an effect.

Further information on causes of change

Population fluctuations are most strongly related to variable levels of survival (Robinson et al. 2014), most likely on their wintering grounds (Baillie & Peach 1992). More particularly, annual population change has been shown to be correlated with rainfall in the western Sahel prior to the birds' spring passage through West Africa, but with neither cattle numbers nor nest-site availability in the UK (Robinson et al. 2003). Annual survival rates from RAS sites in the UK for 1998-2004 were correlated positively with mean monthly rainfall during the early austral summer in southern Africa (Robinson et al. 2008). A review of research into threats to adult survival of several hirundine species, including Swallow, identified weather conditions throughout the annual cycle as a key threat (Imlay & Leonard 2019). There is also evidence suggesting that weather conditions experienced by Swallows during winter may have carry over effects which could affect breeding productivity the following summer (Saino et al. 2012).

It is likely that, in eastern parts of the UK, the loss of livestock farming and grazed grassland, together with arable intensification, has caused the Swallow population to decline, while an increase in the area of pasture in the west and north has promoted a population increase which apparently has more than compensated for declines elsewhere (Evans & Robinson 2004). A link between regional changes in the availability of preferred feeding habitats and the regional patterns of UK population change again suggests that habitat change on the breeding grounds may explain population trend, at least partly (Henderson et al. 2007). Brood size increased up to the late 1980s, however current data show no difference in brood size compared to the late 1960s, while nest losses have increased and the number of fledglings per breeding attempt shows no trend. A Danish study over 22 years found that the number of nesting pairs and the rate at which birds made feeding visits to nests declined in parallel with declines in insect abundance (Moller 2019); however numbers of fledglings were not counted by this study. Further evidence is therefore needed to confirm whether agricultural intensification and changes in insect abundance may have been sufficient to directly affect nest productivity and hence cause population changes at local and wider scales.

Climatic warming is leading to an earlier start to the breeding season for European Swallows, and analysis of phenological data has found that the arrival date in the UK has advanced, between the 1960s and 2000s, by 15 days (Newson et al. 2016), with the laying date also advancing (see above). Facey et al. (2020) found that nestling mass was negatively correlated with temperature, although the relationship with other weather conditions was complex and fledgling body mass was less sensitive to weather, hence the lower nestling body mass does not necessarily have any subsequent effect on juvenile survival and hence on population trends. However, Turner (2009) found that there has been increased chick mortality in hot, dry summers and reduced post-fledging survival because of poor conditions for birds migrating through North Africa. A study in eastern Germany also highlighted reduced breeding success despite earlier breeding, and suggested that a mismatch between local and large-scale climatic changes may mean that, for this species, earlier breeding was not sufficient in that region to respond to climate change (Grimm et al. 2015).

Information about conservation actions

The decline of the Swallow in some parts of the UK may relate to agricultural intensification and changes of land use. There is evidence from several studies in Europe, including one in the UK, that the retention of cattle for meat or dairy farming may improve breeding performance and colony size and buffer Swallow declines ( Moller 2001 ; Evans et al. 2007; Gruebler et al. 2010; Ambrosini et al. 2012; Sicurella et al. 2014), with the presence of manure heaps also important ( Gruebler et al. 2010), and the presence of greater numbers of hayfields within 200 m of the colony increasing colony size at farms without livestock (Sicurella et al. 2014).

Artificial nests have also been suggested as a possible conservation option. In a study in Denmark, artificial nests had a low predation rate which was similar to natural nests, and showed higher breeding productivity. The author speculated that this could be due to the saving of energy and time costs from nest construction ( Teglhoj et al. 2018).