Have you ever seen a colour-ringed Sanderling and perhaps wondered why it spends the non-breeding* season on a British or Irish beach rather than on one in Portugal, Ghana or even further south? Why fly from Greenland to Namibia, a distance of over 20,000 km, when spending the winter months in the UK or Ireland requires a flight of as little as 3,700 km? Perhaps the chance of survival is greater in other countries or perhaps birds that travel further have a larger lifetime breeding output? A paper by Jeroen Reneerkens and colleagues provides some of the answers.
*The term non-breeding season (rather than winter season) is used in this blog because Sanderling travelling as far as Namibia experience a southern summer at the same time as UK birds are experiencing a northern winter.
Pros and cons of travelling further?
At the end of the summer, juvenile Sanderling from Greenland start heading south. The first migration might take an individual to Scotland or Namibia, in southern Africa – or anywhere in-between. The circumstances that lead to these initial settlement patterns are unknown but an individual will repeat its first migratory journey every year, with some birds travelling just 7,400 km annually and others travelling over 44,000 km. It has been argued that, for a range of migration strategies to persist, different wintering sites will have balancing pros and cons. This suggests that costs of longer migrations might be matched by benefits gained at the non-breeding destinations. Is this really true?
Using data provided by colour-ring sightings, Jeroen Reneerkens and colleagues assessed three factors that might affect the fitness of individual birds that spend the non-breeding period in different areas.
- Annual adult survival: If a bird from one non-breeding location is more likely to survive than a bird that spends the non-breeding season somewhere else, it should live longer and potentially have more breeding attempts within a lifetime.
- Age when a bird makes its first migration northwards: A young individual that flies north in its first spring will potentially have one more breeding opportunity than a bird that remains in the non-breeding area for its first summer.
- Timing of migration: There is a short breeding window in the High Arctic so a bird that migrates north earlier in the spring may have higher reproductive success, because it will have a higher chance to re-nest if a first clutch is lost. (There is more about this in Time to nest again?)
Studying marked birds
Sanderling were captured and ringed in breeding sites in northeast Greenland, at staging areas during migration (SW Iceland, N Scotland and the Dutch Wadden Sea) and in the non-breeding season (Scotland, England, Portugal, Mauritania, Ghana and Namibia). The different analyses in these studies used data from 5,863 Sanderling, of which 5,220 were individually colour-ringed.
Survival rates. By visiting key sites and collating additional reports of ringed birds from hundreds of birdwatchers, the research team were able to estimate the annual survival rates of Sanderling that spent the non-breeding season in England, France, Portugal, Mauritania, Ghana and Namibia. As can be seen in the table, the apparent survival rate in West Africa (Mauritania and Ghana) was much lower than that in Europe or Namibia. A bird with an annual survival rate of 0.75 is 67% more likely to die in any given year than a bird with a survival rate of 0.85. Confidence limits and methodological notes are provided in the paper. There is a WaderTales blog about the importance of measuring survival rates.
Age of first northward migration. The proportion of colour-ringed juveniles that migrated north in the first spring varied significantly, with virtually all Portuguese and English juveniles migrating north but only 35.8 % of those from Ghana and 9.6 % of those from Mauritania. (There were insufficient data to work out figures for Scotland, France and Namibia).
Timing of northward migration. Observations in Iceland provided information on the timing of migration of Sanderling from a range of non-breeding locations. This is the last possible stop-over site on northward migration, before birds migrate to their Arctic breeding sites in Greenland or Ellesmere Island in Canada. Birds from Ghana were observed in Iceland between 5 and 9 days later and those from Mauritania between 10 to 13 days later than the birds from Europe or Namibia. That is a considerable difference, given the short breeding period.
The authors asked the question “is there equal fitness throughout the non-breeding range?”, as inferred from the three measured discussed above. The answer seems to be “no”. Sanderling from non-breeding areas in West Africa had lower annual adult survival, delayed first northward migration and later passage through Iceland than birds wintering either further north or south.
Using geolocators**, the team was able to show that birds from Namibia bypassed potential staging sites in West Africa on the way north, flying north across the African continent to Europe, with some birds stopping briefly in the central part of the Mediterranean before spending a longer stop-over in NW Europe, thereby overtaking the Sanderling from West Africa. Namibian individuals used both Mauritania and Ghana as staging areas during southward migration.
** The use of geolocators is discussed in this blog.
The West African sites seem to be relatively poor places in which to spend the non-breeding months of the year. Food availability in spring is likely to be the chief problem. Theunis Piersma and colleagues have shown that the quality and biomass of prey available to shorebirds is lowest close to the equator, resulting in low fuelling rates and low body masses at departure for northward migration in Knot (Piersma et al. 2005).
Sanderling occupy a variety of different and widely-dispersed non-breeding sites between the northern tip of Scotland and the southern tip of the African continent. Here, they experience very different conditions which affect potential, life-time breeding outputs. Sites which appear to be poorer continue to be used, even though there are better options elsewhere, simply because individual birds have no knowledge of other potential areas where they could spend the non-breeding months.
A roll of the dice?
Once a juvenile Sanderling has settled upon a particular migration strategy and a spot in which to spend the non-breeding season, he or she will continue on the same annual cycle for the rest of his/her life. One of the big unknowns for waders/shorebirds – and for other groups of migrant birds for that matter – is just how these settlement patterns develop. (See Generational Change to read how young birds can create new patterns of migration).
Checking their data, Jeroen and his colleagues could see no pattern in the juvenile/adult proportions, sex ratios or sizes of the birds in different non-breeding areas that would help to explain differences in fitness. Birds from the same breeding areas of Greenland end up in non-breeding locations along the whole north-south range. It is almost as if the dice are rolled and a juvenile ends up where chance events take it.
At the level of the individual:
- A Sanderling that spends the non-breeding season in Ghana does not know that it would have a better annual survival rate and be likely to return earlier to Greenland each spring had it ended up wintering in England or travelled as far as Namibia.
- A bird in Namibia has no idea that it could have saved itself an accumulated migration distance of 37,000 km each year by stopping in England, without affecting its probability of survival.
- A first-year bird that spends its first potential breeding season feeding on the beaches of Mauritania, will be unaware that first-years from Portugal have all travelled north to Greenland.
It’s amazing what colour-ring readers have helped to discover but there is much still to learn about the migration strategies of individual waders.
Let’s hope that birdwatchers will continue to look out for colour-rings, as flocks of Sanderling chase the waves in and out on beaches throughout the world.
Paper in Journal of Animal Ecology
Jeroen Reneerkens, Tom S. L. Versluijs, Theunis Piersma, José A. Alves, Mark Boorman, Colin Corse, Olivier Gilg, Gunnar Thor Hallgrimsson, Johannes Lang, Bob Loos, Yaa Ntiamoa-Baidu, Alfred A. Nuoh, Peter M. Potts, Job ten Horn & Tamar Lok.
Graham (@grahamfappleton) has studied waders for over 40 years and is currently involved in wader research in the UK and in Iceland. He was Director of Communications at The British Trust for Ornithology until 2013 and is now a freelance writer and broadcaster.