When a flock of Black-tailed Godwit turn up on a Scottish island or lochside, in April or May, it’s probably a sign that the birds have aborted the Atlantic crossing to Iceland.
On 24 February last year, on the Samouco saltpans on the Tagus estuary in Portugal, we saw an Icelandic Black-tailed Godwit wearing four colour-rings: red & lime on the left, green & green flag (RL-GGf) on the right. It had been ringed there on 10 August 2010 by José Alves so it’s not a surprise that it was in the same spot five and a half years later. In between times, on 29 April 2013, RL-GGf was one of 1520 Icelandic godwits counted on the Isle of Tiree, in the Scottish Inner Hebrides, by John Bowler and Graham Todd. Having encountered strong northerlies it had been forced to delay the Atlantic crossing. If you think back to the cold spring of 2013, it is not surprising that strange things happened that year. Northerlies delayed spring arrivals of African migrants in the UK but they also blocked the departure of wintering birds that were trying to fly to Iceland, Greenland and Northern Canada.
RL-GGf is one of our favourite Black-tailed Godwits. It happens to like a small estuary called Grafarvogur in Reykjavik, which Jenny Gill (University of East Anglia) and I monitor daily in the second half of April each year. We’ve seen this bird here in the springs of 2012, 2014, 2015 and 2016, on a total of 13 occasions. We have an arrival date for 2011 too, when he was spotted in Southern Iceland by Tómas Gunnarsson, our Icelandic collaborator, but there’s a gap in 2013, when we left Iceland while he was still on Tiree (see table). There are a couple of early spring records of this bird in the Netherlands, so this is where he probably spends March and early April every year, having left Portugal in February. In the last six years he has set off for Iceland in spring and only in 2013 was he seen in Scotland. He and other godwits that are forced to suspend migration are starting to give us insights into the hurdles that weather patterns can put in the way when birds are trying to travel north (Gill 2015).
Migration in a changing climate
We’ve been monitoring the spring arrival of Black-tailed Godwits in Iceland since 2000, and we’ve shown that colour-ringed birds have their own individual schedules: early birds always arrive early and late birds always arrive late. Differences in the exact date on which each individual arrives may be associated with the weather patterns each year, but individuals are remarkably consistent despite annually variable weather conditions. It therefore appears that individual godwits, like RL-GGf, have a preferred window in which to undertake the Atlantic crossing.
Interestingly, although there is no evidence that individual birds have changed their arrival times in Iceland over the last 15 years, the arrival dates of the population are getting earlier (Gunnarsson et al. 2006). We’ve shown that this advance in migration is being driven by young birds recruiting into the breeding population on schedules which are earlier than those of previous generations (Gill et al. 2015). Ultimately, this is likely to be being driven by warmer springs and earlier nesting seasons. There’s a blog about this paper.
Black-tailed Godwits on Tiree
Over the years, the Isle of Tiree in the Inner Hebrides has proved to be a great place to pick up colour-ring sightings of Black-tailed Godwits. John Bowler, the local RSPB Officer, really enjoys watching out for their spring return when, as he comments, “hundreds can drop in on the loch-sides in full breeding dress”. The very first birds appear at the end of March and numbers increase into April, with often very large flocks occurring at the peak of passage in the last week of April and the first week of May. Birds are usually found on the grazed edges of machair lochs, with numbers declining through May and odd birds lingering to the middle of June. Given that between 1% and 2% of the Icelandic Black-tailed Godwit population wear colour-rings there is a good chance of finding a marked bird. With migration getting earlier, John’s godwit-watching season will probably get longer.
Godwits also appear on Tiree in the autumn but in smaller numbers. The first failed breeders appear in late June, followed by more adults in July and early August and then juveniles in late August through to October, with occasional stragglers in November and December. Young birds often use freshly-cut silage/hay fields on Tiree for foraging, in the same way that many will have done in Iceland as they prepared for the journey south.
Black-tailed Godwits are very site-faithful in every season of the year. However, although 63 different colour-ringed birds have been seen in spring by John Bowler and his colleagues, only one bird has been seen in more than one spring. This low number of repeat between-year sightings on Tiree, where looking for colour-ringed birds is part of the daily routine, very much suggests that birds seen here are dropping in out of necessity, rather than using the site as an annual staging post. The journey from south England or The Netherlands to Iceland is only just over 1000 miles, which is well within the capabilities for migrating waders in non-stop flight – as long as they do not encounter adverse weather conditions (Alves et al. 2012, Alves et al. 2013).
Scottish flocks of migrating Black-tailed Godwits do not only occur on Tiree, of course. On the peak day of 29 April 2013, when 1520 birds were counted on Tiree, 891 birds were also reported at Loch Gruinart on Islay. The 2411 birds in these two flocks constituted about 5% of the Icelandic Black-tailed Godwit population (Gunnarsson et al. 2005). Given that there were other colour-ringed godwits reported in Motherwell and on Benbecula, just how many Icelandic godwits were in Scotland on that day?
The lack of predictability, when it comes to the potential locations of these spring flocks, makes it difficult to monitor patterns across different years. There are simply not enough places at which there are regular counts of birds each spring and too many places where flocks could choose to stop. Fortunately, reports of colour-ringed birds provide a surrogate for flock counts. A quick analysis of the number of colour-ringed Godwits from Icelandic, Portuguese and E England schemes, seen in Scotland between the springs of 2011 and 2016 shows that there were records in every year but with a larger number in 2015, and by far the most records in 2013. None of the birds was seen in more than one spring, emphasising the random nature of these arrivals.
The colour-ring information provided by birdwatchers is making a huge contribution to the migration studies of Black-tailed Godwits. There are now Black-tailed Godwits in Scotland in every month of the year but sightings of colour-marked individuals in April and May are particularly helpful in helping us to identify the influence of weather conditions on spring migration and the migratory routes used by birds from across the winter range. Please report any of these observations to firstname.lastname@example.org who will pass on records to other colour-ring administrators. Details of the godwit work and the publications to which colour-ring observations have contributed can be found in a blog called Godwits & Godwiteers.
This article first appeared in Scottish Birds, published by the Scottish Ornithologists’ Club.
Alves, J.A., Gunnarsson, T.G., Potts, P.M., Gélinaud, G., Sutherland, W.J. & Gill, J.A. (2012) Overtaking on migration: does longer distance migration always incur a penalty? Oikos 121: 464-470. DOI: 10.1111/j.1600-0706.2011.19678.x
Alves, J.A., Gunnarsson, T.G., Hayhow, D.B., Potts, P.M., Sutherland, W.J. & Gill, J.A. (2013) Costs, benefits and fitness consequences of different migratory strategies. Ecology 94: 11-17. DOI: 10.1890/12-0737.1
Gill, J.A., Alves, J.A., Sutherland, W.J., Appleton, G.F., Potts, P.M. & Gunnarsson, T.G. (2014) Why is timing of bird migration advancing when individuals are not? Proceedings of the Royal Society of London B 281: 20132161. DOI: 10.1098/rspb.2013.2161
Gill, J.A. (2015) Encountering extreme weather during migration: individual strategies and their consequences. Journal of Animal Ecology 84: 1141-1143. DOI: 10.1111/1365-2656.12412
Gunnarsson, T.G., Gill, J.A., Potts, P.M., Atkinson, P.W., Croger, R.E., Gélinaud, G., Gardarsson, A. & Sutherland, W.J. (2005) Estimating population size in Icelandic Black-tailed Godwits Limosa limosa islandica by colour-marking. Bird Study 52: 153-158. DOI: 10.1080/00063650509461385
Gunnarsson, T.G., Gill, J.A., Atkinson, P.W., Gélinaud, G., Potts, P.M., Croger, R.E., Gudmundsson, G.A., Appleton, G.F. & Sutherland, W.J. (2006) Population-scale drivers of individual arrival times in migratory birds. Journal of Animal Ecology 75: 1119-1127. DOI: 10.1111/j.1365-2656.2006.01131.x
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.