Research by BTO and Natural England, modelling the impact of a suite of climatic variables on the abundance of breeding bird species in England, suggests that climate change has had a detectable impact on a sizeable proportion of England’s avifauna over the last 50 years. The results of the study provide evidence that can inform future assessments of species’ vulnerability. Climate change is widely regarded as a major threat to the functioning of natural systems and to the maintenance of species’ populations. Birds are one of the taxa where climate change impacts have been most studied, the research demonstrating a range of documented responses, including advancement in the timing of migration and breeding, poleward shifts in distribution, and changes in community structure. While such work highlights some of the effects of climate change, the attribution of specific changes in species’ populations to a changing climate remains challenging. The research reported in this paper examines the potential impacts of climate change on individual species for as wide a component of England’s breeding avifauna as is currently possible. The study used data from the BTO’s Common Bird Census and the BTO/JNCC/RSPB Breeding/Bird Survey to generate annual indices of population change for 68 species, from 1966 to 2015. These data were then examined in relation to changes over time – and differences between regions – in a series of climatic variables derived from the National Oceanic and Atmospheric Administration / Earth System Research Laboratory, Physical Sciences Division series. This enabled Pearce-Higgins and Crick to consider climatic variables across the different geographical regions of relevance to individual species. For example, climatic conditions in England, Iberia and the Sahel region of Africa needed to be considered when examining population changes in those migrant species breeding in England but wintering in Africa’s arid-zone and migrating via Spain and Portugal. The results of the statistical analyses suggest that climate change has had a measurable effect on the population fluctuations of about a third of the breeding species considered, resulting in notable positive impacts on the abundance of 13 species and notable negative impacts on three species. The majority of the effects found related to climatic conditions within England, which were particularly apparent for resident species during summer and winter. These findings support previous BTO work, highlighting the role of winter temperature as a key driver of resident bird populations, including Goldcrest, Long-tailed Tit and Dunnock. With the exception of Common Whitethroat, Reed Warbler and Spotted Flycatcher, the study failed to find strong evidence for significant effects of climatic conditions away from the UK impacting on migratory species, something that may reflect the difficulty of assessing climate changes impacts on migratory birds, brought about in part by our incomplete knowledge of the migration routes and wintering areas for many of these species. This emphasizes that research seeking evidence for climatic impacts on species must also include a mechanistic understanding of the potential links between climatic variables and the population of interest. Gaining such understanding is an important component of current BTO work on migrant bird populations, with the knowledge gained on migration routes, wintering areas and differences between regional breeding populations key to future work.

New BTO research has used citizen science data to assess the effects of housing developments on Britain’s bird populations, predicting that almost half of the bird species currently found on sites earmarked to become the government’s flapship ‘garden villages’ could decline once development starts. Like in many countries, Britain faces the challenge of housing a growing population, but urban expansion to address this can lead to locally high rates of biodiversity loss, along with fragmentation of natural and semi-natural habitats. In Britain, most new housing is built on farmland or previously developed land, not on pristine natural habitat, meaning lessons from elsewhere may not be useful when assessing the effect of housing developments on wildlife. The BTO study used data collected by volunteers during the Bird Atlas 2007–11 project to assess how bird populations vary along a gradient from farmland to city centre. For 146 breeding and wintering bird species in England and Wales, bird abundance was related to the number of people living in each area surveyed. As expected, species such as Skylark were completely intolerant of any form of development, while others such as Feral Pigeon thrived in heavily developed landscapes. Species like Song Thrush were intermediate, becoming more common in the suburbs and other places with moderate human populations than in either open countryside or urban centres. Although these relationships may seem obvious, knowing at precisely what levels of development each species' abundance changes is crucial for estimating how birds will be affected in future. For example, the study examined the possible effects of the UK government’s 14 proposed garden villages on local bird populations. The results suggested that under current plans, the abundance of approximately 47% of species breeding at the garden village sites is likely to reduce in response to the proposed development, with 18% expected to undergo no statistically significant abundance change and 35% expected to increase. However, various measures were found to improve the situation for birds, such as including patches of trees and wetland areas in the garden village designs. Previous work involving the BTO has demonstrated that contact with birds improves people’s well-being, so understanding the effect of housing development on bird numbers has important implications for the future human residents of these villages as well as for the birds themselves. The results of this research can now be used to develop tools to help developers and planners optimise the design of new developments, both to benefit wildlife and the people that live there.

Warmer springs have seen many of our birds breeding earlier, but scientists still do not know how they actually decide when to start building nests and laying eggs. Do they respond directly to temperature or do they respond to other aspects of their environment that change in spring like days getting longer, trees coming into leaf or more food becoming available? A team of biologists from the University of Edinburgh in collaboration with the RSPB, BTO and the Woodland Trust, set out to answer this question for Blue Tits, collecting information from 40 sites in Scotland over five years. The team found that temperature does play a role, but that night-time temperature is particularly important. This could show that cold weather constrains when Blue Tits are able to start the very energetically costly processes of building nests and laying eggs. On top of this they found that birds lay earlier when birch trees come into leaf earlier and this is some of the first evidence that birds are using trees as a cue for timing their breeding. That birch trees are used more than other tree species in the landscape may be due to their comparably early leafing. As a follow up step, the authors wanted to check whether their findings could be transferred to a much larger geographic scale and longer time-period. To do this they used observations from two citizen science projects that focus on phenology (the timing of biological events) in the UK, the Woodland Trust’s Nature’s Calendar and the BTO’s Nest Record Scheme. Encouragingly they found very similar effects of temperature and birch leafing in these larger data sets, giving confidence in the findings. Working out what information a bird uses to time breeding is key to us accurately predicting how their timing may change under future conditions and what effect this will have on them.

England’s House Sparrow population fell by 70% between 1977 and 2016, and this once ubiquitous species is now absent from many urban areas. New research involving the BTO has found evidence that malarial parasites may be linked to this species’ decline. During a three-year study led by the ZSL Institute of Zoology, in collaboration with the RSPB and later BTO, almost 400 individual House Sparrows were colour-ringed to allow their survival to be tracked through the winter. Blood and faecal samples were also collected from each individual, which were then screened for parasites and bacteria. The birds trapped and tested all originated from private gardens across 11 colonies scattered across London, where the House Sparrow population has fallen by 70% since 1995. Seven of these colonies had a negative population trend whilst four had a stable or increasing population. The results showed that the parasite Plasmodium relictum, which causes avian malaria, was found in 74% of House Sparrows, the highest prevalence recorded in populations of wild birds in Northern Europe. It was also found that the intensity of the infection (the number of parasites per individual bird) of avian malaria was correlated with negative population trends, especially in juvenile sparrows. Survival analyses on individual House Sparrows showed that birds with higher infection of malaria, whether juveniles or adults, had a lower probability of surviving the winter than did individuals with less severe infections. It was therefore concluded that avian malaria was linked to population decline through reduced overwinter survival of juveniles. It is unclear why avian malaria, which was likely native to the UK before the onset of the population decline of House Sparrows, may be affecting this species so significantly in London. House Sparrows are sedentary and BTO ringing data suggest that dispersal distances of this species have decreased from 1970 to 2010. It is possible that loss of habitat within cities might have led to isolated populations which are not very genetically diverse. This can result in a less-effective immune system and higher mortality, which, combined with low recruitment of new birds into the colonies, can lead to population decline. Whilst the underlying causes of House Sparrow declines remain unknown, and are likely due to a combination of factors, this study suggests that avian malaria may be implicated. Improved disease surveillance and continuing monitoring of the species at national level are essential for effective conservation management.