In waders (shorebirds), the main moult (molt) usually takes place after the migration that follows the breeding season. Golden Plovers adopt a different strategy, starting wing moult while still nesting. Given that these adult birds are not going to fly anywhere any time soon, this seems like a very efficient strategy. So, why do Icelandic and Scandinavian Golden Plovers moult differently? Is this a reflection of available resources?
The post-breeding moult
Moult is an energetic process, especially the post-breeding moult, which includes a change of all of the wing and tail feathers. To complete the whole process, birds ideally need to find a three-month period when resources are good, climatic conditions are benign and there is no need to migrate. For birds on the East Atlantic Flyway that spend the non-breeding season in Europe, moult typically takes place after the breeding season and before days get shorter and the weather gets colder.
One place with plenty of food-rich mud is the Wash, in eastern England. Here, up to 300,000 waders gather each autumn, including Knot from Greenland and Canada, Grey Plover from Siberia and Curlew from countries such as Finland. A relatively small proportion are juveniles, which will only moult their body feathers, but there are probably at least 200,000 waders in full moult at this time of year – dropping and growing a grand total of perhaps a billion feathers between them. Some populations use the Wash as a feeding station, before moving on to moult in their wintering grounds, but this is a minority. This group includes taymyrensis Bar-tailed Godwits (more about these birds here) and schinzii Dunlin, which will travel further south, to Africa.
There are Golden Plovers spread across the autumn mud-flats of the Wash too, made up of a mixture of birds that have bred in Britain, Europe, Scandinavia and Iceland. Although they end up in the same winter flocks, their moult strategies are different. Recent research by Paula Machín and colleagues has focused upon how breeding season conditions impact upon the moult strategy of two distinct Golden Plover populations, birds breeding in Scandinavia and Northern Russia and others breeding in Iceland. The resulting paper is published in the Journal of Avian Biology.
Conditions at the breeding grounds and migration strategy shape different moult patterns of two populations of Eurasian golden plover Pluvialis apricaria Paula Machín, Magdalena Remisiewicz, Juan Fernández-Elipe, Joop Jukema & Raymond H.G. Klaassen
Icelandic Golden Plovers
Up to one million Golden Plovers arrive in Iceland each spring, mainly from Ireland and western parts of the United Kingdom. This is estimated to be nearly half of the European breeding population. Iceland might seem small, when compared to the vast land-mass of the European continent, but it’s a haven for waders. This status is threatened by the spread and intensification of lowland farming, increased afforestation and by the ‘summer cottage’ industry – but those are stories for another day.
The Heiðlóa (Golden Plover) is a welcome sight and sound at the end of the Icelandic winter. The first migrants appear about 23 March and nesting can commence as early as 26 April. The usual clutch size is four eggs, with both parents sharing incubation duties. Some first nests are lost, due to predation, but females can lay another clutch. Joop Jukema studied Golden Plovers nesting near Selfoss in the Southern Lowlands of Iceland, timing his captures of nesting birds to coincide with the later part of the incubation period. He was able to assess the progress of moult by scoring the growth of the primary feathers and to work out when each bird would have dropped its first primary. The estimated mean start date of primary moult for males was 19 May (95% confidence interval 27 April – 10 June) with females starting an average of 9 days later, on 28 May (95% confidence interval 6 May – 19 June). On average, males started to moult their primary feathers nine days before the start of incubation, while females started to moult at the same time as incubation began. Potentially, hormone changes associated with the stage of the breeding season could be linked to the onset of moult.
Icelandic Golden Plovers complete their moult prior to departure from the country. By making catches of birds in late August and early September, it was possible to show that the primary moult period is about 100 days. No birds were caught in suspended moult, strongly suggesting that Icelandic Golden Plovers do not attempt to cross the Atlantic before they have attained a complete, fresh set of feathers.
Swedish Golden Plovers
Paula Machín’s main study site was in Ammarnäs in Sweden, on roughly the same latitude as Selfoss and hence with the same amount of daylight. Ammarnäs is colder in spring than Selfoss, not benefiting from the warmer waters of the Gulf Stream which wash the shores of southern Iceland. The average start of incubation in Ammarnäs was eight days later than in Selfoss with the commencement of moult being seventeen days later. Males started primary moult three days after the start of incubation, with females starting twelve days into incubation. It is interesting to note that the difference in timing of the two sexes is nine days, just as in Iceland.
Female Golden Plovers left the Ammarnäs breeding territories at the end of July. From observations of females caught on their nests, it seemed likely that individual females were not starting the moult of their outer primaries, typically completing the moult of primary four and not dropping primary five. Males stayed with chicks for an extra fortnight. Given the longer period of time available to males, it is likely that they were able to moult more primary feathers than their partners, prior to departure from the area.
Catching birds during the chick-guarding phase or just before migration is very difficult but Raymond Klasssen and colleagues were able to study birds on similar strategies by catching birds at Lund, in southern Sweden, and in the Netherlands. Here, birds from Swedish breeding areas and further afield gather to moult, recommencing primary moult at the point at which it had been suspended. Inspecting moulting birds in the period August through to November enabled the research team to monitor the second part of the moult of Continental birds. Given that the distance between breeding areas and these staging areas is relatively short and that tracking showed that it could be completed in one or two days, it is possible that adults might be able to migrate while in active moult.
Spot the difference
Overlapping the breeding and moulting period is rare in migratory birds but it makes sense in a time-constrained annual cycle. The research team suggest that Icelandic plovers presumably need to initiate moult early in the season, in order to be able to complete it at the breeding grounds. This is not an option for Continental plovers, as their breeding season is much shorter, due to a harsher climate and an earlier drop-off in the number of arthropods, their main food source. These Golden Plovers cannot delay the start of the moult period until after the autumn migration because there is insufficient time to compete a full moult in areas such as Lund or the Netherlands, prior to the onset of winter frosts. The fact that Golden Plover are largely associated with farmland, rather than estuarine sites, may make them more susceptible to sub-zero temperatures than, for instance Grey Plover.
When incubating and looking after chicks, Icelandic and Swedish Golden Plovers were able to moult at the same rate. However, there were differences in the second part of the primary moult season. Away from their territories, Icelandic birds continued to moult at the same rate as previously but, having moved to Lund or the Netherlands, Continental birds could moult twice as fast as before. The availability of earthworms in these staging areas may make it easier to acquire resources for the energetically-expensive moult process.
Despite the faster rate of moult of Continental birds in the later period of their moult, the total period of primary moult is longer than that of Icelandic birds. Birds completing their moult in Iceland took an average of 100 days to replace their primaries, whilst Swedish-breeding birds took 16 days longer. This difference may be associated with the time taken to complete the first stage of moult, prior to the migratory flight away from the breeding sites.
A key finding of this paper is that splitting the moult period extends the total period of primary moult. For Swedish breeders, this is the best option, however, as there is insufficient time to complete autumn moult close to the breeding grounds or after the breeding season. As the authors conclude, “Meeting the energy demands of breeding, moult, and migration calls for different timing and spacing of these events in their annual cycle, adjusted to conditions at their breeding and stopover sites, and to their migration strategy.”
There’s more from Paula Machín and her colleagues on this blog-site: https://overthetreeline.wordpress.com/
There are more WaderTales blogs about moult:
- The not-so Grey Plover includes general information about wader moult and talks about some of the stresses that may occur.
- Lapwings can moult while on migration. Read more in Lapwing Moult, which also talks about how to study moult without catching birds.
- Bar-tailed Godwit; migration and survival mentions the different strategies of two subspecies of Bar-tailed Godwit, both of which can be found on the Wash in autumn.
I look forward to future papers about moult strategies to add to WaderTales. Here’s a list of the 60+ blogs that are already available: https://wadertales.wordpress.com/about/
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.