When the BTO began ground-breaking Cuckoo tracking project in 2011, we had very little idea where these birds spent the winter or how they got there. Our latest research not only reveals this information, but also shows that Cuckoos’ use of autumn migration routes helps explain population declines. 56 Autumn migration routes 2011-14. Infographic by Nigel Hawtin.Like many migrant birds, the Cuckoo is in long-term decline as a breeding bird in Britain, and has been on the Red List of Birds of Conservation Concern since 2009. The breeding behaviour of the Cuckoo has been very well-studied, but until recently, knowledge of this species’ migratory and wintering behaviour was very limited. In 2011, a BTO team, led by Chris Hewson, set out to change this, using satellite-tags to track the movements of male Cuckoos breeding in East Anglia. This study has since been extended to track males from several other sites across Britain. This work has not only revealed details of Cuckoos’ migration timings and wintering grounds, but has also showed “our” breeding birds take one of two routes south to Africa after breeding. Crucially, differential rates of mortality have been found on these routes, which strongly correlate with breeding population trends. The study included information from 42 male Cuckoos tagged at nine different breeding locations in England, Scotland and Wales, and tracked over 56 autumn migrations between 2011 and 2014. Birds either migrated southwest via Spain and Morocco (the ‘west route’) or southeast via Italy or the Balkans (the ‘east route’) before converging in the Congo basin of central Africa. Birds taking the west route left eight days later on average than those taking the east route. Interestingly, birds on the west route were more likely to die before completing the Sahara crossing, even though their route was 12% shorter to this point, demonstrating route-specific costs of migration for the first time. There was no difference in survival during the rest of their southward migrations, or during their return northward journeys to the breeding grounds. The differences in survival were correlated with breeding population changes assessed via two independent datasets – from bird Atlases in 1988-1991 and 2007-2011, and from the annual BTO/JNCC/RSPB Breeding Bird Survey. The strong relationships found provide the first direct evidence that conditions encountered during migration can have an impact on breeding populations. Despite the obvious ecological barrier presented by the Sahara, most of the mortality on the west route occurred in Europe, suggesting the conditions at stopover sites are responsible. In recent years, Spain has suffered droughts and wildfires, which might have affected the Cuckoos, although other factors like large-scale habitat change might also have played a role. Declines in Cuckoos’ main food source on the breeding grounds (the caterpillars of large moths) were also particularly severe in areas in which birds migrating on the west route were tagged, which might have compounded difficulties encountered further south, especially given their later departure dates from the UK but similar arrival time south of the Sahara, which suggests they may undertake more fattening here than birds that migrate via Italy. The fact that the western route persists despite the apparent disadvantages suggests that the conditions leading to increased mortality have arisen recently. This study not only demonstrates how conditions during migration can influence the population dynamics of long-distance migrants through effects on survival, but also underlines the need for a full annual-cycle approach to understanding migratory birds and their conservation needs. The results show unexpected variation in migration routes and mortality and this illustrates that we need to collect information on the variability in migratory behaviour and performance if we are to understand migratory animal declines in general. Future work of this sort will be vital for the conservation of Afro-Palaearctic migrants. It will be needed to identify key areas where stopover site quality has declined, and could be used to predict future responses of species to climate change, for instance.

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.