Migratory decisions for Icelandic Oystercatchers

New Oystercatcher research in Iceland aims to explain how migration patterns change over time and how individual behaviour may drive population change

Which Oystercatchers will cross the Atlantic and which will stay in Iceland? (Tómas Gunarsson)

Most waders leave Iceland in the autumn, with vast numbers heading for the British Isles, coastal Europe and even Africa.  Amongst the exceptions are hundreds of Oystercatchers which ‘tough it out’ in wintry conditions, thereby saving themselves two long trans-Atlantic flights and potentially ensuring that they are able to make a prompt start to the next breeding season. Why do some Oystercatchers migrate when others don’t?  Is it the same birds each year?  Do resident birds have a competitive advantage when it comes to choosing a territory and raising chicks, and do these chicks follow the same migratory strategy as their parents?  In an era of changing weather patterns, can birds change their migratory preferences and how does this happen?  These are some of the questions that are being tackled by Verónica Méndez, as she monitors the movements of colour-ringed individuals in a joint University of East Anglia (UK) / University of Iceland / University of Aveiro (Portugal) project.

Migration in a changing world

Throughout the world, the distribution, abundance and behaviour of species is changing, partially as a response to climatic change. Given that most bird protection relies on a legislative framework that is site-based, changes to bird distributions are going to make life challenging for conservationists.  Will today’s Special Protection Areas be fit for purpose in 20 years’ time?  As they look to the future, conservationists want to be able to set up a network of sites that can flexibly support migratory species, and this entails understanding why and how migratory patterns are changing and might change in the future.  Icelandic Oystercatchers may well provide an ideal model species through which scientists can learn how migratory decisions can influence individual fitness, population demography and species distribution.

“I’m sure that I started a nest scrape somewhere here”. Spring conditions in 2015 were distinctly wintry: Verónica Méndez

There are likely to be different pressures facing resident and migratory Oystercatchers.  Two birds that breed on adjacent territories might have completely different annual cycles.  The bird that spends the winter in Iceland will not have to migrate but its probability of survival may be reduced by harsher winter conditions, possibly with a more limited set of feeding options and certainly with shorter periods of daylight.  In mid-winter, there is 6hr 40 min between first light and dusk in Reykjavik but 9 hours in Bangor, North Wales.  Come the spring, however, the same bird will be well placed to take advantage if spring comes early, whilst his migratory neighbour will face a hazardous sea-crossing, perhaps being delayed by a fortnight or more if strong northerly winds set in.

Individuals matter

An adult Oystercatcher providing food for a growing chick: Tómas Gunnarsson

Although patterns of change in migratory strategies can be detected by looking at timings of first arrival and counting flocks, these changes can only be explained if we understand what individuals are doing.  It will be interesting to see, for instance, if Icelandic-winterers pair up together and what their offspring subsequently do in their first autumn.  Oystercatchers look after their chicks for longer than most waders so perhaps youngsters will be more likely to adopt the strategy of their parents than other species?

To answer these and many other questions, Verónica and colleagues from the Universities of East Anglia, Iceland and Aveiro have been colour-ringing Oystercatcher adults and chicks on the breeding grounds between the north-west and southern lowlands of Iceland.  Both adults and chicks (when big enough) are ringed with two colour rings on the left tarsus and a flag above another colour ring on the right tarsus (metal ring goes on either tibia and is not part of the scheme).  See update below to learn about a second scheme.

Wintering flocks of Oystercatchers will be checked in Iceland but most of the colour-ringed birds are likely to have flown south.  The team therefore need birdwatchers around the British Isles and along the Atlantic coast of Europe to look out for these marked Oystercatchers and to report them to icelandwader@gmail.com

Catching adult Oystercatchers

The study season for an Oystercatcher fieldworker starts in April, when the first nests start to appear.  Once a full clutch of between 1 and 4 eggs has been produced, both parent birds take turns to incubate for between 3 and 4 weeks.  During this period it is possible to catch the adults by temporarily replacing the eggs with plastic eggs and setting a spring trap or walk-in trap over the nest.  Sometimes the second adult can be caught while the first bird is being ringed, as he or she settles down on the unattended eggs.

Juvenile wearing new colour-rings: Verónica Méndez

Catching chicks is harder.  Although they can be metal-ringed at an early age, their legs are too short at this stage for additional colour-rings.  By the time that the youngsters are big enough they can run very quickly – or even fly – providing excellent entertainment for the ‘spotter’ in the research team, as the ‘chaser’ is often outwitted and out-run.

What has been learnt so far?

The first adult Oystercatchers were colour-marked in the summer of 2012 and more were ringed in 2013 and 2014.  In the winter of 2014-15, eight migratory individuals were seen in Ireland, Northern Ireland, Wales, Scotland and Shetland and five resident birds were shown to have stayed in Iceland. Additionally, five birds wearing BTO rings that they acquired in Ireland and Wales have been recruited to the colour-ring project, increasing the sample of known migrants.  By the end of 2015, over 200 adults had been colour-ringed, as well as about 100 new chicks, so there should be plenty of birds to look for this winter.

Breeding adult Oystercatcher: Tómas Gunnarsson

A century or more of metal-ringing has shown that Icelandic Oystercatchers migrate mostly to the British Isles and West coast of France, but some have been recovered in Germany, The Netherlands, Spain and Portugal, so please check out any Oystercatcher flocks you see in any of these countries.  Although most observations of colour-ringed Oystercatchers will be made during the winter months, young birds may well be spotted in the summer, as Oystercatchers do not start to breed until they are two or more years old.  If some juveniles winter in Iceland perhaps they might be able to breed at a younger age than their migratory cousins?  That would be yet another way for a change in strategy to not only impact on the fitness of the individual but also on the population demography and the overall migratory strategy of the species.

Oystercatchers are also being ringed with two letter engraved rings, along with two colour-rings: Photo Tómas Gunnarsson

An update…

Oystercatchers caught in later catches have been ringed with white, grey or green engraved rings, alongside two colour-rings.  If you come across a bird with a yellow engraved ring then it is probably from a project in Dublin Bay.

When researchers from the two projects got together at the 2015 International Wader Study Group Conference in Iceland they realised that five more Dublin Bay birds could be added to the list of Icelandic breeders that are definite migrants.

Please check flocks of Oystercatchers and report any sightings to icelandwader@gmail.com

Photo: Tómas Gunnarsson

---

 

WaderTales blogs are written by Graham Appleton, to celebrate waders and wader research.  Many of the articles are based on previously published papers, with the aim of making wader science available to a broader audience.

@grahamfappleton