This is the 21st annual report of the BTO/JNCC/RSPB Breeding Bird Survey (BBS), containing the population trends of widespread UK bird species during the period 1994-2015.

New BTO research shows a recent imbalance in Willow Warbler sex ratios, with 60% of adult birds being male. Such a skewed ratio has implications for the conservation of this migrant species. Male-biased sex ratios have been documented in a number of bird species, in particular those whose breeding populations are small or in decline. Various reasons have been put forward for why the sex ratio in a population should move away from one-to-one, with sex-related differences in mortality or dispersal behaviour two of the most likely. Understanding which of these factors are important, particularly in the context of why it is that small and/or declining populations show more strongly skewed sex ratios, has important consequences for conservation. One way to establish the importance of potential sex-related differences in mortality, recruitment (the number of young that survive and join the breeding population) and dispersal for small populations is to look at a single species. This should be a species for which we have good information on the local variation in abundance across a wider spatial scale. Using information from the Constant Effort Site Scheme (CES) and the Breeding Bird Survey (BBS), a new BTO study has looked at Willow Warbler – a summer migrant, which winters in Africa and for which there are strong regional differences in both abundance and population trend here in Britain. CES data, which come from standardized mist-netting across a network of study sites, were used to examine sex ratio and the survival rates of male and female birds. BBS data, which come from survey transects monitored in a standardized manner by volunteers, were used to predict the relative abundance of Willow Warblers at each CES site. Sex ratios were found to vary markedly among sites, with more male-biased sites occurring towards the south and east of Britain. Sex ratios, which were closest to equality at those CES sites with the highest levels of Willow Warbler abundance, became significantly more male-biased as relative abundance declined. Interestingly, across Britain, sex ratios have also become significantly more male-biased since 1994, when BBS began; in 1994, similar proportions of males and females were estimated to occur at CES sites, but more recently males comprised roughly 60% of the adult population at these sites. Male survival rates were typically higher than those of the females but, while the results suggest that higher levels of female mortality could be occurring in low-density, skewed populations, this difference is unlikely to explain the widespread occurrence of skewed sex ratios. This suggests that sex-related differences in dispersal and recruitment are likely to also be involved. It is possible that females may preferentially recruit into sites with a high abundance of males, perhaps attracted by high levels of song or by the presence of large amounts of suitable habitat. The results of this work, a collaboration between BTO and the University of East Anglia and part of a wider study, suggest that conservation efforts for this species should focus on maintaining and enhancing those sites capable of supporting those populations that are larger and which have more equal sex ratios. It could be that the fragmentation of suitable habitat is leading to small populations with skewed sex ratios, something that might have profound consequences for this particular Afro-Palaearctic migrant.

Little Owls are in decline in the UK, but are hard to monitor, making it difficult to establish this species' conservation and management needs. Newly-published research by the BTO demonstrates how playback could be an effective tool for helping to detect and monitor this species. A non-native species, the Little Owl was introduced to the UK and first bred here in the late-1800s. Although it spread rapidly across lowland agricultural habitat, the population has declined fast in recent years, in a similar trajectory to that found in parts of its native range elsewhere in Europe. However, because this species is largely nocturnal and found at a low population density, it is hard to construct reliable population estimates, and the most recent number of 5,700 pairs is derived from work published in 2008. As a declining predatory species that largely occupies agricultural areas and has a varied diet, a better understanding of the Little Owl population and the drivers of its change is valuable irrespective of this species’ native status, so new and improved techniques monitoring Little Owl numbers are desirable. New BTO research, led by Gary Clewley, has tested the use of playback to monitor Little Owl numbers. Here, the response of local birds to recordings of Little Owl calls is assessed. This method was tested in a well-monitored Little Owl population in the Lincolnshire Fens, on whose habitat 12 squares of 2 km by 2 km were surveyed on four occasions between sunset and midnight in March and April 2015. The results showed that playback was a very effective means of detecting Little Owls. Little Owls were found in 100% of squares where individuals were known to be breeding after only two visits. This method could be scaled up for use in wider ecological research or indeed a national survey of Little Owls, the need for which BTO is currently considering. Either way, the application of playback in Little Owl monitoring could prove valuable in decisions about how to manage and conserve this species.