BTO-led research highlights the importance of the quality of protected areas in their effectiveness.

Satellite-tracked male Cuckoos show higher fidelity to sites used during the breeding season than to sites used at other times of the year.

Many long-distance migratory bird species are experiencing severe population declines. These declines are being caused by human-driven changes, such as habitat loss and climate change, but the relative severity of these varies in space and time. Understanding this variation is key to understanding where species might be most exposed to population-limiting effects. This study used 10 years’ worth of satellite data from birds tagged as part of the BTO Cuckoo Tracking Project alongside data on human impacts on the environment (including the location of infrastructure, habitat change and climate change) collected via remote sensing to address these information gaps.

The average spring arrival dates of many summer migrants have advanced over recent decades, but the Cuckoo is one of several species in which this has not happened. The long-term decline in UK Cuckoo populations, together with the link between a changing climate and changing spring arrival dates more generally, raises the question as to why there has been such a limited shift in Cuckoo arrival times. This study sets out to address this question. The authors use 11 years of satellite tracking data from 87 male Cuckoos, tagged at 11 sites across the UK, to examine variation in migratory timing throughout the annual cycle and its potential consequences. A number of key questions were addressed through this work. How much variation and repeatability is there in migratory timing across the annual cycle, and how likely is it that the Cuckoo’s ability to respond to environmental change is being constrained? How is spring arrival at the breeding grounds impacted by the timing of previous migration stages (such as departure from the wintering grounds and spring stopover sites)? Does the timing of migration impact mortality, and does this vary by breeding habitat and/or the post-breeding migration route used? The analyses revealed that the degree of variation in migratory timing differed significantly across the annual cycle. It was largest for movements from the central African wintering grounds, then decreased sharply to its minimum at departure from the West African stopover. It then remained low through to arrival at the breeding grounds, before increasing again during southbound migration through Europe and across the Sahara. The major determinant of the timing of spring arrival in the UK was departure from West Africa. Individuals that left West Africa earliest arrived back on the breeding grounds earliest. The high level of synchrony observed in Cuckoos at departure from West Africa suggests a relatively narrow optimal window of departure from this stopover, and limited flexibility to adjust the timing of this event. The results strongly suggest that the arrival of UK Cuckoos at their breeding grounds is limited by the environmental conditions constraining the timing of their departure from the spring stopover site in West Africa. Conditions within Africa improve from south to north in the northern hemisphere spring, with the arrival of rains associated with the Intertropical Convergence Zone. The rains deliver a flush of vegetation and a burst of invertebrate activity, the latter providing food for many migrating birds. The timing of increases in resource availability across the dry-to-wet season transition, which limit fattening rates in migrating birds, likely delay Cuckoos reaching the appropriate body condition to initiate a northward crossing of the Sahara. By looking at how mortality risk varied with timing across the annual cycle, the team was able to gain some important insight into how these limitations impact individuals and, therefore, potentially populations. In periods following the migration stages that were most important in determining breeding grounds spring arrival timing, early birds were more likely to die than later ones. This suggests that in order to try to gain the benefits of arriving early at the breeding grounds, birds suffer increased mortality risk; in other words, they are trading off exposing themselves to survival risks against the benefits of early arrival. Such risks may become more necessary as birds try to keep up with climate change on the breeding grounds. The fact that the authors found that migration timing didn’t reset during the midwinter period, and that part of post-breeding migration is included in the periods when birds appear to be exposing themselves to these risks, suggests that in the case of UK Cuckoos, all ecological levers are being pulled in an attempt to advance spring arrival. The resulting increase in mortality rates may form part of the demographic mechanisms linking failure to advance spring arrival to the population declines that we have seen. This work adds to our understanding of why UK Cuckoos have only shown a limited advance to the timing of their spring arrival. This new knowledge allows for better prediction of future responses to a changing climate, and reveals how conservation measures might mitigate the consequences of timing stresses at different points of the Cuckoo’s annual cycle.

Although we have lost over three-quarters of the UK Cuckoo population, we are unsure of the causes of this decline. Alongside this, the strong divide in northern and southern populations of Cuckoo and their local trends are cause to question why the local differences arise. This study, led by PhD student Chloe Denerley and using BTO's BBS data, used local data from Devon in comparison against data on Cuckoos and their hosts, habitats, and prey items. In Devon, Cuckoos were more likely to be found in areas with semi-natural habitat and more Meadow Pipits (but fewer Dunnocks) and with a higher number of moth species, which are predated by Cuckoos in late summer. Across the UK, Cuckoos have become more associated with upland heath with Meadow Pipits, and with wetland habitats with Reed Warblers, and the distribution of Cuckoos shifting from south to north within the UK. The abundance of moth species preyed upon by Cuckoos has declined four times faster than that of other moths. Overall, the results suggest that Cuckoos have contracted out of the farmed countryside and into heathlands and the uplands. Coordinated monitoring of moth, host and Cuckoo numbers across such projects could tell us much about the long-term prospects for reversing Cuckoo population decline in the UK

Cuckoos have been documented using a range of host species but most of their eggs are laid in the nests of just a small number of species. Why do they select these species and to what extent might other species be suitable hosts, as yet unused to any great degree? The Cuckoo is a generalist avian brood parasite, known to have utilized at least 125 different bird species as a host within Europe. Despite this, individual female Cuckoos are thought to be host-specific, preferentially laying their eggs in one – or a few – host nests. This has led to individual female Cuckoos being classified into host races, termed ‘gentes’, some 20 of which have been described within Europe. Given the large number of songbird species breeding in Europe, and the relatively few female Cuckoo gentes, it raises the question of what makes a good host. By examining data from three different sources, including nest record data from the BTO, Bård Stokke and colleagues sought to establish which characteristics determined the suitability of a songbird species as a potential Cuckoo host. Some of the characteristics they investigated included nest placement, chick diet, habitat, abundance and the timing of the breeding period. The results of analysing two independent data sets, one from the UK and one from Germany, produced similar results. The analyses revealed that species breeding in forest and rocky areas were used less than species breeding in other habitats; that those feeding their nestlings with plant material were used less often than those feeding their nestlings with invertebrates (though not significantly so in the UK) and that hosts tended to be of intermediate body size and have larger populations. The analyses also showed that, perhaps unsurprisingly, species nesting in cavities are used less frequently than those nesting elsewhere. Nest height, nest depth and overlap in the breeding period did not appear to affect parasitism in any of the data sets examined. Through the work the researchers were able to calculate a ‘host suitability index’ for each of the European species in the study. Interestingly, there was no evidence of a clear separation between suitable and unsuitable host species’ instead, there appeared to be a continuum from low to high suitability. However, all of the species with a recognized corresponding Cuckoo gens are ranked towards the high suitability end of the continuum. The host suitability index could indicate species that might become the focus of Cuckoo nest parasitism in future if, for example, the host population increases because of climate- or habitat- change.

This stunning new book brings together the latest research findings, delivered through an accessible and engaging narrative by the BTO's Mike Toms, with the wonderful artwork generated through the BTO/SWLA Flight Lines project. If you have an interest in our summer migrants, then you'll welcome this fantastic opportunity to discover their stories through art and the written word. By pairing artists, storytellers and photojournalists with the researchers and volunteers studying our summer migrants, we are able to tell the stories of our migrant birds, and the work being done to secure a future for them. Includes artwork by SWLA member artists Carry Akroyd, Kim Atkinson, Federico Gemma, Richard Johnson, Szabolcs Kokay, Harriet Mead, Bruce Pearson, Greg Poole, Dafila Scott, Jane Smith, John Threlfall, Esther Tyson, Matt Underwood, Michael Warren, Darren Woodhead and others.

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.