Guest blog by Professor Chris Redfern, Newcastle University, and Paul Morrison, RSPB Northumberland Coast Site Manager, discussing their new paper on oceanographic factors which are defining the migration ecology of one of Europe’s rarest breeding seabirds.
Roseate terns have a special significance for seabird enthusiasts, being the rarest breeding seabird in Europe, with colonies in the Azores and in northwest Europe at Rockabill, Lady’s Island Lake (both in Ireland), Coquet Island (UK) and Brittany (France). Historically, North Atlantic populations of roseate terns suffered significant declines, but this trend has been reversed in recent decades through intensive management of the extant colonies.
Conservation at breeding sites in the UK and Ireland has recently been supported by funding from a European Union LIFE project (LIFE14 NAT/UK/000394) with the long-term goal of improving the conservation status of the species across Europe.
A successful foraging trip by a geolocator-tagged roseate tern Sterna dougallii on Coquet Island in 2018 © Chris Redfern
A need for further study
Declines at breeding colonies may also result from changing conditions and reduced survival on migration or at wintering sites. Safeguarding roseate terns thus necessitates an understanding of their movements and ecology throughout their annual cycle. Persecution at wintering sites, such as Ghana where the trapping of sandwich, common and roseate terns on beaches was revealed from ringing recoveries over 30 years ago, could still be an issue reducing over-winter survival if roseate terns mainly winter at such sites.
Against this background, Daniel Piec, the LIFE Project Manager, Steve Newton and author PM, site managers for Rockabill (Eire) in the Irish Sea and Coquet Island (UK) in the North Sea, respectively, were keen to use leg-mounted geolocators to identify migration routes and wintering areas of roseate terns from these colonies to inform conservation priorities.
Although the previous use of geolocators has led to some concerns about their suitability for roseate terns, newer and slightly smaller devices with different attachment methods have been used successfully on Arctic terns.
Tagging gets underway
Therefore, we embarked on a geolocator study on the migration ecology of roseate terns, initially on those nesting on Rockabill, and then the following year using them on Coquet Island, a smaller, more compact colony where it was easier for David Kinchin-Smith, the resident Assistant Warden, to keep tagged birds under closer observation.
Figure 1. Roseate Tern fitted with a geolocator mounted on a plastic leg ring on Rockabill Island in the Irish Sea at the start of the study in 2017 (a, b). Roseate tern after being fitted with a geolocator on Coquet Island in 2018 (c) and a close up of the geolocator on a different bird after its return to Coquet Island the next year with its geolocator intact and still functioning (d).
While geolocators lack the locational resolution of GPS devices, they are relatively inexpensive and analytical approaches for interpreting light-level data are now much more refined so that, with other measurements, such as temperature, they can provide good locational resolution at an appropriate spatial scale for migration-ecology studies.
From 20 birds tagged with geolocators at each site, but in different years, 31 were recovered and yielded 30 datasets for analysis, 16 from Coquet Island and 14 from Rockabill with no apparent adverse impacts on birds’ physical condition and reproductive output. These data have given a fascinating insight into the migration ecology of roseate terns. Importantly, the data reveal, in broad terms, the importance of coastal upwellings for all stages of roseate tern migration and wintering.
Authors Paul Morrison (left) and Chris Redfern (right) masked up and socially-distanced on Coquet Island in 2020
Coastal upwellings
Coastal upwellings are usually driven by ocean currents flowing parallel to the coast, often driven by prevailing winds, and where the occurrence of upwelling or downwelling is dependent on position relative to the equator and the direction of flow in relation to the earth’s rotation (Coriolis effect). Upwellings bring cooler water and nutrients to the surface, allowing phytoplankton growth which supports higher trophic levels such as forage fish and the predators which feed on them.
Most of the tagged birds wintered exclusively in a relatively localised area off the coast of Ghana, where the Central West African Upwelling (CWAU) of the Guinea Current Large Marine Ecosystem (GCLME) in combination with coastal topography may generate good foraging conditions. However, one third of the birds wintered further west wholly or partly in the Sierra Leone-Guinea Plateau (SLGP) subsystem of the GCLME.
Figure 2. Wintering areas of Rockabill and Coquet Island roseate terns. The mean longitude and latitude of stationary periods (stationary periods of > 50 days) for birds from Rockabill (central dot) and Coquet Island (central cross) with circle size indicating relative duration and vertical and horizontal bars representing longitude and latitude standard deviation, respectively; different colours are used for each bird within group. The yellow shaded area is the Senegal Upwelling Zone (SUZ) of the CCLME with the northern boundary of the SUZ indicated by a thick horizontal red line. The red hatched area represents the GCLME with vertical red lines indicating division into SLGP and CWAU subsystems. Lines connect different stationary period locations used by the same bird within the wintering period.
Migration implications
Outward and return migration was within the Canary Current Large Marine Ecosystem (CCLME) along the coast of West Africa, and for most birds was interrupted by stationary periods in this cold-water upwelling system. Differences in the distribution of stationary periods between the outward and return migration may relate to seasonal differences in upwelling intensity between upwelling subsystems at the northern and southern ends of the CCLME.
Surprisingly, the return migration was slower than the outward journey. In many species the reverse is the case, and we might think that birds would be in a hurry to get back to breed. However, in this respect, roseate terns are similar to the common terns from Europe that winter in the CCLME, and to the Icelandic whimbrel that winter along the coast of southern West Africa. The prevailing north-easterly trade winds may be an important factor reducing the speed of return migration and increasing the importance of suitable foraging areas for refuelling on the return journey.
Figure 3. Phenology summary of roseate tern migration. Lengths of each phase (Outward, Wintering, Return) of the timeline from the time of departure from the British Isles to arrival back at the colony the following year are to scale. The width (top to bottom) of the central rectangle for the wintering phase represents the number of birds (30) and the widths (top to bottom) of the coloured bars represent the proportion of birds that adopted each of the three wintering-area strategies. On the Outward journey two-thirds of the birds had short stationary periods or stopovers along the way, indicated by vertical yellow dashed bars, and some of these were in the CCLME. A similar proportion of birds had stationary periods on the return journey (vertical yellow solid bars) but these were in the CCLME and of longer duration than on the Outward phase.
The coastal upwelling systems of the Guinea and Canary Currents are broad-scale oceanographic features underpinning biological productivity. Other factors within them, such as seasonal effects, eddy currents arising from coastal topography, thermal fronts and interactions between ocean currents and deep cold-water domes, may focus biological resources and sustain wintering and migratory birds at different times in the annual cycle.
An important take-home message from this study then, is the importance of learning how physical oceanography drives biological productivity and moderates forage-fish availability. This is essential if we want to understand factors that generate the foraging environments needed by seabirds such as roseate terns.
In this context, it is sobering to realise the extent to which the anthropogenic pressures of climate change and overfishing at national and international scales threaten the viability of seabird foraging resources.
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