The BTO/RSPB/JNCC Breeding Bird Survey is the primary source of information about changes in national populations of over 100 bird species. Here we describe analyses of this dataset which has identified important spatial variation in population indicators, highlighting previously unreported declines in habitat specialists in southern and eastern Britain. Using biodiversity indicators In the UK, wild bird population indicators are produced annually on behalf of the Government to monitor the health of the environment. These multi-species indices are calculated by averaging the population indices of birds that typically breed in the habitats of interest. The farmland and woodland bird indicators are those that show the largest declines with losses of 54% and 20% respectively compared to the 1970s baseline. However, these figures mask significant regional and local variation. The BTO has used existing land-cover data to develop a new approach to model breeding densities of birds across the UK from BBS surveys for each species in the indicator for two time-periods (1994-96 and 2007-09). These maps were then combined for each indicator set to show fine-scale spatial variation in temporal trends of both multi-species indicators. Declines greatest in the south-east Farmland birds were found to be doing better in western areas where they have been mostly stable compared to eastern areas where they declined extensively since the mid-1990s. Declines were greatest in south-east England and were driven by losses of more than 50% in farmland specialists such as Linnet, Skylark, Starling and Yellowhammer which are Red Listed species in the UK. In contrast to the east-west pattern seen in farmland birds, the woodland bird indicator showed a north-south gradient, with populations in the south faring worse than those in the north. Again, the largest declines occurred in south-east England, particularly for Willow Warbler, Garden Warbler and Bullfinch, where losses of specialists averaged more than 25%, whilst populations generally increased significantly in Scotland. For both farmland and woodland bird communities, indicator trends for specialist species were much more negative than those for generalist species which tend to occupy a wider range of habitats, and showed positive population trends. What are the reasons for these spatial patterns? The fact that both farmland and woodland indicator trends are most negative in the south-east of England shows that bird populations in this region are under significant and previously unrecorded pressure. Compared to other regions, south-east England has experienced the greatest degree of long-term agricultural intensification, fragmentation of natural habitats and urban expansion. While these factors are very likely to have continuing negative effects, rising temperatures and risk of drought may also play a role and further work is required to properly attribute these spatial patterns to specific drivers. The UK Government is committed to national and international policies to reduce threats to biodiversity and the results of this research are important to help policy makers identify the areas where conservation actions should be prioritised.

BTO research has used information collected by bird ringers to investigate large-scale differences and flexibility in the timing of feather moult across 15 passerine species that breed in the UK. Different moult strategies were found between migrant and resident species, alongside within-species regional variation in moulting schedules. Most passerines replace their 10 primary flight feathers in sequence after breeding, and ringers are encouraged to score the progression of this moult in the birds they catch. This study found significant differences in when species begin to moult, with Blue Tits, which commence this process earliest in the season, replacing feathers more than two months before Bullfinches, the latest moulting species. Data from the Nest Record Scheme showed that these differences can be completely explained by the time at which species finish breeding, and consequently, species that produce only a single-brood each year start moult earlier in the season than multi-brooded species. There was also significant variation in how long species take to complete their post-breeding moult, with long-distance migrants replacing their feathers at a faster rate than the resident species. Migrants achieved this by moulting a greater number of feathers simultaneously than residents. For example, migratory Whitethroats moutled faster than any other species in the study, replacing their primary feathers in about two months by moulting an average of three and a half feathers per wing at one time. Conversely, resident House Sparrows, the slowest moulting species, took nearly four months to complete their moult by only moulting two feathers simultaneously. It is likely that these differences in moult strategies are governed by the constraints each species faces at the end of the breeding season. Residents are unlikely to travel very far from breeding grounds during the winter and can consequently take their time over moult, but a rapid post-breeding moult, although energetically costly, may allow migratory species to undertake their perilous southerly journeys as early and quickly as possible. Within species, there were clear regional differences in the commencement of moult that seemed to influence moult duration. This was particularly apparent in the multi-brooded species, in which moult started later but progressed faster in southern Britain (where the duration of the breeding season was longer) than in the north. This finding demonstrates flexibility within birds’ moult schedules so that a later end to breeding can be compensated for by a faster post-breeding moult. Such flexibility might allow species to change their moult schedules in response to prevailing breeding season conditions. However, this may be constrained by the requirement to complete post-breeding moult prior to migration or the onset of winter, which may be especially limiting for later nesting species.

Aerial surveys that capture high quality photos are increasingly being used to monitor bird populations, but these images are not always good enough to identify birds to species-level. A new study led by the BTO investigates how best to resolve this issue. In certain habitats, aerial surveys can monitor bird populations by taking high quality photos or videos. This is an example of how new technology is revolutionizing monitoring. Such passive survey techniques can often cover a larger area faster than human surveyors can, and may cause less disturbance to the animals concerned. However, these advantages may be offset by the loss of accuracy in identifying species or individuals, as the human ability to take in aspects of size, colour, behavior, habitat and group dynamics (all of which contribute to identification) has not yet been surpassed by technology. A new study led by the BTO has analysed aerial photos of birds at sea taken to estimate species’ population sizes at a site where an offshore wind farm has been proposed. Traditionally birds at such sites have been counted by observers on boats, but digital aerial snapshots are quicker, and therefore avoid the issue of double counting that can arise from boat surveys. However, these aerial photos do not always create an image that is good enough to identify birds to species-level. Only 23% of photographed birds were identified to species-level with any confidence in this study, with some individuals classed only by family, e.g. “auk”. This low proportion identified presented a major hurdle to estimating population sizes. To overcome this problem, the study incorporated data from boat surveys carried out at similar times and places to the aerial surveys. On boat surveys, observers routinely identify the species of up to 95% of the birds they record. Sophisticated statistical models were used to compare the proportions of different species observed on boats with that from planes, allowing “intelligent guesses” to be made on the species identification of birds in the aerial surveys. This is the first time population estimates have been calculated for individual species with uncertain identification. This approach could be applied in a range of situations that do not identify species with certainty, resulting in large-scale, quick, efficient and non-invasive monitoring that has obvious conservation benefits in today’s rapidly changing world.