Research by the BTO, just published in the journal Global Ecology and Biogeography, shows that the diversity of Britain’s birds has increased with a warming climate, but this was accompanied by a loss of habitat specialists. Using Breeding Bird Survey data from 1994 onwards, scientists at the BTO found that recent temperature increases had a positive effect on the number of bird species. However, there was also a negative effect of rising temperatures on ‘specialist’ species (species with specific habitat requirements). It appears that the winners from recent climate change were already common habitat generalists that have expanded their ranges, such as Great Spotted Woodpecker - up by 139%, likely at the expense of habitat specialists such as the Grey Partridge and Corn Bunting, which have declined by 50% and 29%, respectively, over the period of the study. Overall, this means that with climate change, bird communities across the UK are becoming more similar to each other as vulnerable range-restricted species are outcompeted by more resilient common species.

Newly published work by BTO scientists suggests that the increasing abundance of three widespread deer species, Reeves’ Muntjac, Roe Deer and Fallow Deer, have contributed to population declines of several breeding woodland birds in lowland England. The research applied novel analytical techniques to data collected through the Breeding Bird Survey, and indicated that populations of five bird species associated with dense understorey habitat have been depressed at sites where deer have increased. The two species of conservation concern, Common Nightingale and Willow Tit, are likely to have been worst affected. Modifications to the understory through deer browsing are thought to underpin these declines. Since high deer populations are now prevalent and likely to be affecting woodland biodiversity in large parts of lowland England, broad scale and co-ordinated deer management is required, and could usefully target areas that still support high populations of species sensitive to deer.

Many UK-breeding birds that migrate to Africa for the winter have undergone dramatic declines in recent decades. New BTO research shows that both winter habitat and the geographical regions visited had a significant effect on population trends between 1994 and 2008, as measured by data from the Breeding Bird Survey. Species overwintering in the tropical Humid Zone of west and central Africa (including the Congo basin where our tagged Cuckoos are currently located) declined more rapidly than migrants wintering in other regions. This might be because Humid Zone species have not advanced their return migration dates as much as species wintering further north, and are therefore not keeping pace with earlier springs in the UK. Humid Zone species affected included four on the Birds of Conservation Concern Red List : Turtle Dove, Tree Pipit, Spotted Flycatcher and Cuckoo. However, generalist species, including the House Martin from the Humid Zone, did well no matter where they wintered. This is because their ability to adapt to a wide range of habitats allows them to cope with changes to land use in their wintering grounds, for example deforestation and agricultural intensification. Conversely, habitat specialists, such as Nightingales and Garden Warblers that rely on understorey and scrub, are likely to be adversely affected by accelerating habitat changes in their wintering grounds, compounding similar effects on their breeding grounds. This study highlights the difficulties and complexities in defining the numerous and often interacting factors underpinning population changes. Further research is needed if we are to effectively protect our migrant birds.

Ecological indicators that measure the state of the environment are of growing political importance due to national and international conservation obligations. Birds are commonly used as biodiversity indicators because of their high trophic level, sensitivity to environmental change, public popularity and relative ease of survey. Indicators of general environmental health based on breeding bird population trends (from and BBS) have therefore become important drivers of conservation and land-use policy in the UK and further afield. New research by the BTO has investigated the selection of species within these indicators, which was originally chosen by expert opinion. BTO scientists used BBS data to generate an objective method of species selection based on a quantitative assessment of the habitat preference of each species. They also determined the effect of changing the minimum threshold for data quality and compared trends using data from specified habitats only with those using data from all habitats. The study found that indicators based on species selected using the new objective assessment produced similar results to the original indicators based on expert judgement and were relatively robust to the habitat-specificity of the trends. The most important consideration was scarce species, the omission of which may give over-optimistic results. The methodology adopted by this study is simple and robust, and could be applied to a range of species and habitats, leading to a better understanding of our environment, and improved evidence-based conservation policies.