Out of Africa: declines in migrants linked to UK breeding grounds
Recent research 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 and found that species breeding in Scotland are generally doing better than those in England. Several species were 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, with species such as House Martin and Garden Warbler strongly increasing in Scotland only.
These results illustrate how population trends can be affected by interactions between breeding and wintering conditions, and the costs of making long migratory journeys. While many migrants may be facing increasingly tough circumstances outside the UK, it is likely that these costs are being offset by better breeding conditions in Scotland than in England. Further exploration of such geographical variation is essential to properly understand the demographic processes underpinning population trends of these species, many of which are on the Birds of Conservation Concern Red List.
Cormorant control: it's not clear cut
The UK Cormorant population has increased in size and range in recent decades, with more birds breeding and wintering inland. This has led to conflicts with some fisheries, so licences have been issued to kill up to 2,000 birds annually since the mid-2000s. New research by the BTO has examined whether this control has been associated with changes in Cormorant numbers on WeBS sites, particularly on Special Protection Areas (SPAs) designated to protect species under the European Birds Directive.
The study found no evidence that killing Cormorants one winter affected numbers at local sites the following winter. Cormorant population growth was associated with higher intensity control, although this does not show whether control has influenced the national population trend, as Cormorants may simply disperse as a result of disturbance. Further work is needed to monitor Cormorants outside WeBS sites and to research their population dynamics and behaviour. The key questions of whether Cormorant control has the desired effect of reducing predation at fisheries, and how cost effective it is compared to other measures, remain to be answered.
Winter feeding: birds need a balanced diet too
The effects of winter feeding on wild bird populations are not yet well understood. Winter food could enhance birds’ ability to invest in future reproduction, although this is likely to depend on the type of nutrients received. New work involving the BTO has shown exactly that, by examining the consequences of different winter food supplements for egg production in Blue Tits.
Provisioning with fat alone resulted in smaller relative yolk mass, and reduced yolk carotenoid concentrations in early breeders. This suggests that females consuming a fat-rich diet in winter were less able to acquire important resources needed to form egg yolk. However, the addition of vitamin E mitigated these negative effects, because as an antioxidant, vitamin E protects against oxidative damage arising from increased metabolism after eating fatty foods. These findings indicate that birds require a balanced diet to aid their reproduction, highlighting the importance of the nutritional value of provisioned foods.
Conserving Ugandan birds: land sharing or sparing?
Increases in human population and per capita consumption are placing growing pressures on ecosystems as agricultural demands escalate. This has led a debate about how best to farm for both high yields and biodiversity. Two contrasting models have been proposed: “land sharing”, where low-yield farming enables biodiversity to be maintained within the agricultural landscape, and “land sparing”, where high-yielding agriculture is practiced, requiring a smaller area of land to attain the same yields and therefore leaving greater areas of natural habitat untouched.
A new study led by the BTO has examined these two models in southern Uganda. The population densities of 256 species of bird were measured, along with crop yields and farmers’ income. Most birds were found to fare better under land sparing, especially those species with smaller geographical ranges, which are more likely to be of conservation concern. However, the authors suggest that high-yield farming can only be effective in delivering land sparing if combined with strong measures to protect natural habitats, other ecosystem services and human livelihoods.
Avian pox: a new infectious disease in British tits
New collaborative research has used information collected by BTO Garden BirdWatch volunteers to document the emergence and spread of a severe form of avian pox virus in British tits, especially Great Tits. The disease, which typically leads to unsightly growths on a bird’s head, was first reported in Sussex in 2006, and has spread north and westwards since. Although birds can recover from the virus, the lesions it causes may impair their vision and ability to feed, as well as leaving them susceptible to secondary infections and predation. The disease is thought to be spread by biting insects, leading to a peak in incidence in late summer after warm wet weather when insect population densities are high.
This study illustrates the power of collaborative research and value of citizen science in characterising and understanding wildlife diseases. It also underlines the importance of continued vigilance and reporting of illnesses in wild animals, as where avian pox and other emerging diseases (such as finch trichomonosis) lead, others may follow.