While we have good evidence of climate change impacts on birds, there is still a lot to learn about the specific climatic drivers behind these observed changes. Data from long-running monitoring schemes can be used to determine how bird populations respond to temperature and rainfall changes. The evidence for a changing climate Evidence for the impact of climate change on species’ populations and ecological communities has been presented in an increasing number of scientific publications. For birds, whose populations have been well studied compared to other groups, there is a growing body of evidence which associates warming with the population declines seen in long-distance migrants and habitat specialists. Other work links warming with the establishment and range expansion of southerly-distributed species, and with homogenization in the structure of avian communities. While we have good evidence of climate change impacts, there is still a lot to learn about the specific climatic drivers behind these observed changes. Looking for effects Using data from the BTO Common Bird Census (CBC) and the BTO/JNCC/RSPB Breeding Bird Survey (BBS) for 59 species, BTO researchers – working with Dr Dan Chamberlain from the Università di Torino – tested the effects of monthly temperature and precipitation on the population trends of common and widespread bird species present within England. The work examined whether the responses to temperature and precipitation varied with respect to migration strategy, habitat specialization and thermal association. The analyses looked at both immediate impacts – for example, the effects of low winter temperatures on bird populations the following spring – and those operating over a longer time period. The latter may reveal lag effects for particular climatic events; hot dry summers, for example, may limit the productivity of invertebrates living within soil, the effects of which may not be felt by the birds that feed on them until the following year. Critical seasons Overall, the analyses revealed three main periods of sensitivity across the year: the breeding season (particularly June), the winter (December to February) and the previous summer (July and August). Negative effects of cold winter weather on the population growth of resident and short-distance migrants were evident, the effects being stronger for residents than for short-distance migrants. Cold winter weather can impact the survival rates of birds, from small passerines, like Treecreeper and Wren, to waders, such as Lapwing and Golden Plover. The positive effects of June breeding season temperatures are probably mediated through the increased abundance of invertebrate food, greater foraging time and reduced thermoregulatory requirements associated with warm weather. These were also most apparent in resident species. Interestingly, there was a strong negative effect of May temperature on the population trends of long-distance migrants, something that is consistent with the hypothesis that such species have failed to advance their arrival time to match the advance seen in the timing of peak abundance of their invertebrate prey – known as a phenological mismatch. Longer-term impacts The detrimental impacts of hot, dry summers (July/August) were evident but were found to operate with a lag, not influencing the bird populations until the following year. While this effect had been identified previously for some upland birds, to see its impacts more broadly reflected in this national scale analysis suggests summer drought may be a wider problem. The work also highlights the negative impacts of warming on habitat specialists and northerly distributed species associated with cold climates. Generalist species may be better equipped to cope with a changing climate and this may be one reason why we are seeing the development of less-diverse avian communities. While this work has provided some novel insights, it is important to remember that climate change is not the only challenge that our bird communities face. Habitat change, much of which is linked to human activities, may also be important and we need to carry out more work in order to understand how these different drivers influence our bird populations and avian communities.

New research by the BTO has used detailed distribution maps of 122 species of bird to measure the ways that climate change could be affecting our avian populations. Species distributions were found to have changed, but the range shifts detected could not be explained by any single climatic factor, indicating that the distribution changes for British birds are complex, multi-directional and species specific. New research by the BTO has used detailed distribution maps of 122 bird species, drawn from breeding bird atlases two decades apart, to measure the complex ways that climate change could be affecting our bird populations. Atlas data are collected in a standardised manner by thousands of volunteer bird surveyors, providing a unique barometer of the impacts of environmental change on this one component of British biodiversity. From the early 1980s to the early 2000s, temperatures in spring and summer increased, which should have pushed species to the northwest if this aspect of climate is key to their success, whilst higher temperatures in winter should have pushed them to the north and northeast. In contrast, if spring rainfall is critical to species, they should have been pushed to the west. Analyses looking at how bird distributions had actually changed over this period found that birds had indeed shifted to the north, on average by 13.5 km, which continued a trend seen in previous decades. However, more than a quarter of species had also extended their ranges to the northwest and northeast, while almost half had retreated from southerly directions. The ranges of a few species such as Greylag Goose and Great Tit had extended in all directions while others like Lesser Spotted Woodpecker and Corn Bunting had retreated from all directions. Overall the range shifts could not be explained by any single climatic factor, indicating that the distribution changes for British birds are complex, multi-directional and species specific. The impact the arrival of species new to particular areas will have on existing biodiversity is not yet clear. As some species are not shifting their ranges as fast as others, or moving in the same direction, the bird communities of the future could be very different from those seen today. There is still much to learn if we are to manage the impacts of anthropogenic climate change on our wildlife, and studies such as these will help policymakers to adopt conservation and land management strategies that effectively assist species survival.

New BTO research uses data from the Breeding Bird Survey (BBS) to quantify habitat-specific trends, providing novel insights into the processes that regulate UK bird populations, and allowing population responses to environmental change to be predicted with greater accuracy. The "buffer effect" and population regulation Previous studies of individual species have shown that the “buffer effect” regulates populations at large scales through the mechanism of density-dependent habitat selection (Gill et al. 2001). As population densities increase, higher quality habitats become saturated, forcing individuals to move into lower quality ones. Species that are increasing in abundance should therefore increase most rapidly in low-quality habitats. This study used data from BBS to examine whether population trends of 85 species varied in a manner consistent with the buffer effect at a national scale. Habitat trends and preferences After calculating population trends across a range of habitats, habitat-specific trends were found to be strongly positively related to that species’ national trend. Species that increased most in a particular habitat tended to be those that had increased overall across the country. In addition, there was an interaction between habitat-specific population trend and a species habitat preference, as estimated by Jacob’s index from the proportion of sightings of each species recorded in each habitat. This means that for declining species at a UK level, losses were greatest in their least preferred habitat, whereas for increasing species; population increases were greatest in least-preferred habitats.. Local population trends driven by large-scale processes The results were consistent with predictions from the buffer effect, and show how this process is an important driver of population trends for many species at large scales. This highlights an important message for conservationists; that the most important driver of population trends at an individual site may be a large-scale process rather than the result of local environmental changes. It is therefore important to consider the wider context when making management decisions in response to species population changes. However, this study showed that a great deal of variation in trends between species and habitats could not be explained by the buffer effect alone, and were probably caused by habitat-specific environmental changes. Understanding the mechanisms underpinning these environmental changes will therefore remain an important focus for BTO research, as shown by our current Farmland Bird Appeal.

New research examining the relationship between the availability of young woodland and two declining species - the Tree Pipit and the Lesser Redpoll - indicates that while important, young woodland availability is not the primary driver of population trends in these species. New research by the BTO, RSPB Science and Natural England has examined the relationship between the availability of young woodland and the breeding population trends of two species on the Birds of Conservation Concern Red List – Tree Pipit and Lesser Redpoll. The availability of young woodland in the UK is largely dictated by national programmes of woodland planting and felling, which peaked after the middle of the twentieth century. Young plantations are considered to provide important breeding habitat for both Lesser Redpoll and Tree Pipit, which underwent sustained and substantial population decline and breeding range contraction, starting in the 1970s. The study used data from the 1988-91 and 2007-11 Bird Atlases, the Breeding Bird Survey and statistics from the Forestry Commission to show that population declines in England were strongly associated with decreases in the availability of young coniferous woodland, and were more weakly associated with an increase in young broadleaved woodland. Abundance was also related to availability of young woodland at the 10-km square scale. However, overall the results suggested that young woodland availability is not the primary driver of population trends for these species. Other factors, such as pressures experienced outside the breeding season, must therefore also be important. More research is needed to better understand the requirements of these species so that they can be conserved and their population declines reversed.