By Jason Loghry
Why is it that almost all Spoon-billed Sandpipers recorded in Japan are juvenile rather than adult?
Comments and discussion are welcomed.
Oct92012
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Not only Japan, Korean records of SBS at non-regular sites, (I mean 1 or 2 individual visiting sites, some irregular sites now have almost every year records) are also records of juveniles. It seems juveniles migrating for the first time are having difficulties with finding proper habitats where can supply enough feeding materials and roosting sites. In my opinion these birds will hardly survive throughout migration and might not reach to the wintering grounds. Thus, there is possibility that these populations do not have significant meaning on viable populations.
Dear Mr./ Dr (?) HK Kim, I agree. And I also believe that this is one important contributing factor in the rapid decline in this species which is too often overlooked.
When I lived in Japan (1990-1998), all the Spoon-billed Sandpiper I saw during southward migration were juveniles, and these often seemed to turn up after low pressure weather systems had passed through. All photos I have seen in recent years from Japan during southward migration are also juveniles. Most of these birds were/are not in the kind of habitat we find at the Geum estuary or formerly in Saemangeum.
Have long hypothesised that this is because adults have already experienced migration, and have learned where the optimal habitats/sites are (or else they fail to migrate successfully). For this species, optimal sites are very dynamic, extensive estuarine systems where their bill-structure is perfectly adapted for finding food (during a seasonal period of maximum food abundance). Historically, such estuarine systems were widespread along the coasts of the Yellow Sea, southern China, and in several parts of Japan, as well as southern Sakhalin etc. Now, due to coastal barrages and reclamation, these optimal sites are very spottily distributed.
Once they have found a remaining optimal site, and have staged there successfully, adults then likely try to return to the same site each following year, probably staging for several weeks (undertaking a rather energy-expensive post-breeding moult into non-breeding plumage during September and October).
This is one reason why almost all of the Spoon-billed Sandpiper at Saemangeum in the late 1990s and early 2000s were adults. And why the loss of Saemangeum most likely resulted in the loss of those dependent adult Spoon-billed Sandpiper from the population (adults using the site during northward migration would also have suffered reduced breeding success and increased mortality, of course).
Juvenile shorebirds of many (all?) species, in contrast, migrate on an inherited road map. In the case of Spoon-billed Sands, this might be coarse-scale (as historically, there would have been plenty of good habitat for them anywhere south of the breeding grounds – so just taking an approximate bearing would be enough), or might be programmed more precisely, so that birds “aimed” at particular estuaries. In Japan and elsewhere optimal sites and habitat have both been destroyed in recent decades.
In addition, juveniles are likely also to be more prone to wind-drift during migration, because they have less experience. Not only in Japan: in the ROK too, as you write, it is juveniles that turn up in unexpected localities during southward migration (as did that one in Pohang this year, and in earlier years in Gangneung, on Heuksan and Jeju). Such juveniles often turn up coincident with/just after strong winds associated with low pressure systems.
Arriving in areas with sub-optimal habitat (including sandy beaches or small, simplified river mouths), the juveniles are likely to suffer from relatively high levels of disturbance or interference from other shorebirds, and yet still need also to fuel up for another long migratory flight while undertaking an energy-demanding moult into First-winter plumage (most rapid in early October).
Lacking adequate experience at foraging, these same juveniles also have less experience at migration. For a juvenile that has spent a few weeks in Japan, it then also needs to fly several hundreds of km further to the core of the non-breeding range than those already in the Yellow Sea. For example, it is 3700km from Saemangeum/the Geum to the Bay of Marteban (with staging also possible en-route at Rudong, 600km from the Geum) but it is 4800km direct from Tokyo Bay to the same region. This additional distance, much of it against prevailing winds at that time of year, would be highly energy-demanding. For birds that staged in suboptimal habitat, unable to build up sufficient body fat, this might also quite likely be fatal.
It is in part because of this that our organisation believes that the focus of Spoon-billed Sandpiper conservation needs to be firmly on habitat conservation, especially in East Asia (including restoration of tidal flow in some areas, and enhancement of habitat where possible). If we continue to lose optimal sites used by adults (such as Rudong), and there are too few good sites to be “found” by juveniles, then the species will soon be unable to survive in the wild.
Perhaps too, if poor migratory success rate of juveniles is indeed a key factor in their decline, might headstarting of First-winters (realeasing them in optimal habitat in the bay of Bengal, for example) be more successful than headstarting of juveniles on the breeding ground?