In Iceland, young trend-setting Black-tailed Godwits are changing the timing of spring migration
In recent years, earlier arrival of spring migrants has been widely reported in birds as diverse as swallows and waders but it’s not a universal trend; species such as British Cuckoos and Icelandic Whimbrels have not changed their arrival dates. Interestingly, many of the species that have not advanced timing tend to be those that are declining. By thinking about the mechanisms that enable some species to take advantage of earlier spring warming it might be possible to explain how timings and population changes may be linked.
The simplest way for spring migration to advance would be for individual birds to change their arrival dates, arriving earlier now (either because they have departed earlier or migrated faster) than they did in previous years. These changes could be facilitated by changes to weather conditions before, during or after migration. In general, the arrival of short-distance migrant species has advanced more than long-distance species, which has led to suggestions that individual birds are able to assess conditions on their breeding grounds from afar, and to ‘fine-tune’ arrival accordingly. This could explain why long-distance migrants seem less well able to change their schedules than species which have less far to travel.
This pattern of advances in arrival dates, and greater advances in short-distance migrants is seen in birds arriving into Iceland each spring (Gunnarsson & Tómasson). By monitoring the first dates of a range of migratory breeding species to the area around Laugarás, an inland village in southern Iceland, over the period 1988 to 2009, Tómas Gunnarson and Gunnar Tómasson showed that species which spend the winter further south than France showed no change in arrival, whilst those from further north in Europe were returning earlier. The southern group included the only wader in the Gunnarsson & Tómasson study which uses this migration strategy, Whimbrel (see diagram).
One of the species that has advanced spring arrival (by about two weeks in the last two decades) is the Black-tailed godwit. Since 2000, we have been recording arrival dates of individually colour-ringed godwits into coastal Iceland – giving us the opportunity to assess whether individuals have indeed brought forward their time of arrival. By making regular visits to the same sites we have discovered that the dates when we first come across individuals are remarkably consistent. Although the arrival of the whole population is spread over a five or six week period, the window in which a specific Black-tailed Godwit appears is generally predictable, whether he or she is a bird that we tend to first see in mid-April or mid-May. There are annual differences, of course, which appear to be linked to periods of adverse weather during the period of the sea-crossing (Gunnarsson et al 2006), from departure points in The Netherlands, The UK Ireland, France and Portugal, but there is no significant trend.
Although the arrival date for the population has been advancing at ~0.8 days per year (Gunnarsson & Tómasson 2011), there has been no trend in individual arrival dates (not significantly different to zero days per year); Gill et al. 2014.
If individuals are consistent in arrival but the population is advancing, the advance must presumably result from new birds recruiting into the population being earlier-arrivers than recruits from previous years? Fortunately, there is a second long-running set of data that’s available to answer this question, in a large part because of the efforts of Pete Potts and Ruth Croger of Farlington Ringing Group. In the period 1999 to 2014, they organised teams of volunteers to ring Black-tailed Godwit chicks in Iceland, with the support of the Icelandic Natural History Museum. Significant contributions to the total of over 350 colour-ringed chicks were also made by Tómas Gunnarsson and José Alves, while researching the breeding ecology of Black-tailed Godwits for the Universities of East Anglia and Iceland.
Wader chick mortality is quite high and there are further losses in the eighteen-month period between autumn departure from Iceland and the first return trip, eighteen months later, so it was wonderful to have a sufficiently big cohort of marked recruits to look at patterns and trends. For this study, arrival dates for 46 individuals of known hatch year were available for analysis. As can be seen from the graph, arrival dates of new recruits have been getting earlier, with birds hatched in the last decade arrive around two weeks earlier than individuals hatched in the 1990s.
There are several reasons why recent recruits may be arriving earlier than in previous years, but the most likely is that this is a knock-on effect of advances in godwit laying dates that have occurred in recent decades. Icelandic godwits nest earlier in warmer springs, and the frequency of warmer springs has increased. Early fledging may benefit new recruits, by increasing the time available for them to migrate south, locate a good winter site and be in condition to return early when they recruit into the breeding population (Alves et al. 2013 http://www.esajournals.org/doi/abs/10.1890/12-0737.1 http://dx.doi.org/10.1890/12-0737.1). As their arrival date will be consistent thereafter, the overall timing of arrival of the population will advance.
Many other studies of different species in which individuals are tracked during migration are showing similar levels of consistency in individual timing of migration. What then is causing the variation among species in rates of advance? Long-distance migrants typically arrive later on the breeding grounds and breed quite soon after arrival, while short-distance migrants can have quite large time gaps between arrival and laying, depending on conditions for breeding. Short-distance migrants therefore have more capacity to advance laying dates (because they are on the breeding grounds waiting for suitable conditions), while long-distance migrants, such as Whimbrels in Iceland, arrive later and so cannot breed earlier even in a warmer year. Advances in spring arrival dates may therefore result from advances in laying dates and associated benefits of early fledging for recruits, and lack of advance in long-distance migrants may be a consequence of arriving late and hence being unable to take advantage of early, warm spring conditions.
In the Icelandic subspecies of Black-tailed Godwit, which is expanding in both number and distribution, it is clear that young recruits to the breeding population are driving the advance in timing of migration. We only know this because of the long-term programme of chick ringing by volunteers and because we have been able to record the timing of individual birds’ migratory activities over a large number of years. Funding for this work has been provided by the volunteers themselves, NERC , Icelandic Research Council and EU TMR.
This blog is based upon research presented in the following open access paper:
Gill, J.A., Alves, J.A., Sutherland, W.J., Appleton, G.F., Potts, P.M. & Gunnarsson, T.G. 2013 Why is timing of bird migration advancing when individuals are not? Proceedings of the Royal Society B. , 281, 20132161
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.