Research by the University of Birmingham and the BTO used data from the Waterways Breeding Bird Survey and the National River Flow Archive to show how the occurrence of river birds is influenced by the magnitude and timing as well as variation (i.e. extreme events) in river flow. Flow affects aquatic communities of invertebrates that birds consume, through changes in velocity, depth, temperature, turbidity and nutrient availability. Such influences on the lower trophic levels have implications for the whole food web, with consequences for river birds in terms of survival and breeding success. Data from 17 river birds were analysed to show that species associated with upland environments (e.g. Grey Wagtail) had a higher probability of occurrence with high flows, while those that forage within aquatic macrophytes (e.g. Coot) preferred stability in low flows, perhaps because an undisturbed substrate promotes greater plant diversity and growth. Species that predominantly forage at river margins (e.g. Lapwing) benefitted from variability in low flows, possibly because this increases the availability of foraging habitat. Dipper, which typically feeds in river channels, also favoured variability in low flows, which may indicate a propensity for this species to forage opportunistically outside breeding territories when marginal habitats are exposed. Flow timing, especially in spring, was important for diving species (e.g. Great Crested Grebe) and bank-nesting species (e.g. Kingfisher). Bank-nesting species may be vulnerable to flow variability during the breeding season, as nest sites on marginal habitats are prone to flooding, but benefit from annual high flows that create suitable breeding habitat of exposed sediment banks. River systems are vulnerable to climate change. Current models predict that river flows in the UK will have altered considerably by 2050, with decreases in summer flows and increases in winter flows. This research helps suggest how bird communities might respond to a climate change-induced shift in river flows and highlights which species will be at risk if the predicted increase in the intensity of floods and droughts comes about.

Recent research (including by the BTO) on declines in Afro-Palaearctic migrants has primarily focussed on conditions in these species’ wintering grounds. However, population changes could also be influenced by factors operating during breeding and migration, as a new study by the BTO and UEA shows. Scientists analysed data from the Breeding Bird Survey for 46 species of passerine and near-passerine, including residents, short-distance migrants (wintering in continental Europe) and long-distance migrants (wintering in the Arid and Humid Zones of Africa). Overall, they found that species breeding in Scotland are generally doing better than those in England, with several species either declining in England but increasing in Scotland, or increasing in England at a slower rate than in Scotland. These differences were especially stark in long-distance migrants, and in particular those that overwinter in the African Humid Zone, with species such as House Martin and Garden Warbler strongly increasing in number in Scotland only. Taken together, these results illustrate how population trends can be affected by interactions between breeding season processes, wintering conditions, and the costs of making long migratory journeys. While many migrants may be facing increasingly tough conditions outside the UK, it is likely that these costs are being offset by better breeding conditions in Scotland than in England, which could be related to differences in land-use between the two countries (e.g. lower agricultural intensification in Scotland than in England). Further exploration of such geographical variation is essential to properly understand the demographic processes underpinning population trends of these migratory species, many of which are on the Birds of Conservation Concern Red List.