New paper: migratory divide in Red-necked Phalaropes

In October 2015 at the 2nd World Seabird Conference in Cape Town, South Africa, an international collaboration was started to combine geolocator tracking data of Red-necked Phalaropes from breeding sites across the Western Palearctic. Now, a joint paper is published in Frontiers in Ecology and Evolution, showing a migratory divide between populations from Greenland, Iceland and Scotland migrating to the eastern Pacific, and populations in Fennoscandia and Russia migrating to the Arabian Sea. Especially the western route to the Pacific is amazing – a migration route not shared with any other Western Palearctic species. Phalaropes using the two routes and wintering areas differ in migration strategies and also in the amount of movements within the wintering area. Only few studies have shown such diversity in movement strategies among subpopulations within a species.

It has been a tremendous job to collect all this data, especially because Red-necked Phalaropes are far less site-faithful than, for example, Long-tailed Skuas, albatrosses or other seabirds. Out of 10 geolocators deployed, on average 3 were resighted and recaptured. Even if recaptured, things can go wrong: some loggers failed prematurely and others were lost. Considering the effort required for each and every logger, the paper is also a testament to the willingness of people to share their data and the power of international collaboration!

Hence, thanks to all collaborators, in particular Yann Kolbeinsson, Olivier Gilg, Jose Alves
Malcolm Smith, Aleksi Lehikoinen, who led the fieldwork at the other sites.

Please read the full paper here.


New paper out on primary moult in northern hemisphere skuas

A paper I wrote with Rohan Clarke, Peter Pyle and Kees Camphuysen has been accepted for publication in The Auk, the journal of the American Ornithological Society! It can be downloaded here and is titled “Timing and duration of primary molt in Northern Hemisphere skuas and jaegers”.

The paper presents estimates for primary moult timing and duration in all four northern hemisphere skuas: Great, Pomarine, Arctic and Long-tailed Skua, including multiple age-classes (for as far as they are identifiable…). Moult data is difficult to obtain for oceanic wanderers like skuas, as they perform their moult at sea where fieldwork is costly and capturing seabirds is ‘challenging’, to say the least. Recently, people realized that molt data can be obtained from (digital) photography. With a growing popularity of bird watching, bird photographing, pelagic birding trips, and the sharing of large numbers of images over the internet, a very comfortable and cheap way to collect data has emerged. Over the past years, we collected photos of nearly 2000 individual skuas, photographed at sea or near-shore by about 600 photographers. Many photos were found at online sighting portals like and, but we were also greatly helped by many people sharing their pictures through e-mail. From these pics, we inferred the primary moult score and subsequently estimated three moult parameters: the mean start date of moult, the standard deviation in this start date, and the mean moult duration.

What we found in this study is that, as expected, larger species took longer to moult, but this also meant that larger species had a hard timing in avoiding temporal overlap between moult and migration. In general, migratory birds avoid overlap of moult and migration, but we found that Great Skua moult throughout its autumn migration! A rare strategy, possibly facilitated by a very low migration speed through an area with good food availability. Another interesting find is that among the three smaller species, their very first moult cycle took longer than later ones. As soon as they started migrating, moult cycles would run more or less parallel to adults. We suggest that this shorter moult cycle reflects a time constraint set by migration.

This study would not have been possible without all photographers willing to share their pictures. A huge thanks to them!


Moulting adult Pomarine Skua Stercorarius pomarinus, off Mauritania in November 2016 (Rob van Bemmelen)

New paper out on Northern Shoveler x Blue-winged Teal hybrids

Back in May 2014, my friends Niels van Houtum and Martijn Renders found an aberrant duck at the Westerplas, Schiermonnikoog. Its identification caused some discussion, and long story cut short, it turned out to be a Northern Shoveler x Blue-winged Teal hybrid – a first for the Netherlands. This record was the incentive for a paper on the identification and occurrence in the Western Palearctic of this duck hybrid, co-authored by Jörn Lehmhus, Steven Mlodinow, published in the latest issue of Dutch Birding magazine. Northern Shoveler x Blue-winged Teal hybrids show a set of fairly constant characters, allowing confident identification.

The occurrence of Northern Shoveler x Blue-winged Teal hybrids in the Western Palearctic is not really a surprise, given that (vagrant) Blue-winged Teal often associate with Northern Shovelers, and hybridization is common among ducks. Records of this hybrid appear fairly regular in the Western Palearctic, with most records in spring. Interestingly, records show a pattern suggesting migration along a SW-NE line, with records during spring and summer occurring at increasing latitudes as the seasons progress. In the paper, we argue that (most) Northern Shoveler x Blue-winged Teal hybrids likely originate from mixed pairs formed in Europe rather than in the Nearctic.

20140504 02 Rob van Bemmelen

The hybrid Northern Shoveler x Blue-winged Teal from Schiermonnikoog, 4 May 2014.


At least in the Netherlands, it seems that some hybrids are ‘on the radar’ of keen birders and can even entice small crowds of twitchers. These are also routinely accepted by the Dutch rarity committee. For example, there are nine records of Tufted Duck x Ring-necked Duck on the Dutch list. On the other hand, other hybrids have undoubtedly occurred in the Netherlands, but have never been admitted to the Dutch list. The best example of this is Eurasian Wigeon x American Wigeon. Although surely some hybrids likely have a captive origin, I would argue that at least these hybrids are (likely) offspring of genuine vagrants and thereby demonstrate the ‘desperation hypothesis’.

The Northern Shoveler x Blue-winged Teal from Schiermonnikoog has been accepted by the CDNA as the first for the Netherlands, and we hope our paper is an incentive to find and document more records of this handsome hybrid. Coincidentally, the second for the Netherlands (if accepted) was found by Diederik Kok just before our paper came out!

If you are interested in reading the paper, please drop me a line, or browse research gate.

New paper out on consistency in Long-tailed Skua movements

One of the aspects of migration that continues to fascinate people, is the ability of individuals to return to the same spot (either a wintering or breeding site), year after year. Such site-faithful individuals can be relatively easy to record. However, recording site INfidelity has always been a major difficulty: if a bird doesn’t turn up where it was last year, you didn’t detect it, it moved or it died. Tracking studies do not suffer from these problems, and now that tracking studies are producing data from individuals over multiple years, we can finally study the degree of consistency in routes and site selection over the entire annual cycle.

Recently, we published a paper reporting on this subject in Long-tailed Skuas. The paper is published in a theme section on ‘individual variation in migration and foraging in seabirds’ of the journal Marine Ecology Progress Series and can be accessed here. Don’t forget to click on the supplement, which contains a nice animation of the tracks!


Over half of the geolocator data used for this study was obtained from the Long-tailed Skuas in Ammarnäs, with the other part coming from colleagues working in Greenland and Svalbard. Together, we mapped nearly a hundred annual migrations of 38 individual Long-tailed Skuas. Several individuals were tracked over four or five years.

The emerging pattern is remarkable. While Long-tailed Skuas are generally faithful to their route from the previous year, they occasionally choose to go somewhere completely different. The highest consistency is found in early winter, around the time when they arrive at the main wintering area. After mid-winter, individuals start to deviate from their previous route. Such deviations can be huge: one individual went to the Benguela Current in four years, but after arriving in the second and fourth year, it simply flew 5200 km across the Atlantic to spend the remainder of the winter in the Falkland Current, near Argentina! In both occasions it used virtually the same route to get there and ended up in the same area. These patterns suggest they first check out local conditions before deciding where to go subsequently: either continue on their usual route, or on some alternative.

We also showed that individuals can show both ends of the spectrum: individuals showing ‘flexibility’ (deviations from their previous route), are not necessarily always flexible, they can also follow the same route twice.

Request for pictures of moulting skuas

I am looking for photographs of all four northern hemisphere skuas (Great, Pomarine, Arctic and Long-tailed Skua) to study their primary moult. Within my PhD project, I would like to link biomarkers in primary feathers to locations as recorded by geolocators. However, detailed knowledge about primary moult is lacking for these species. This is mainly because they moult at sea, where it has is difficult to them. Recently, digital photography has been proposed to study moult in seabirds, and this is exactly what I am trying with the skuas. However, most of the moult cycle is completed at the wintering grounds, were few birders and photographers venture…

I have been collected photos online (observadowaarnemingebird, flickr, etc), received some from other seabirders, and taken many pictures myself off Mauritania, but still sample size is still rather small and clustered in space and time (a more even spread would be better). Remarkably, this is especially true for the species with the widest non-breeding distribution: the Arctic Skua…

If you have any photographs of skuas in active primary moult and are willing to share these with me, please let me know! They don’t have to be razor-sharp, but I would like to know the exact date and the region (e.g. ‘Benguela Current’, ‘Mediterranean’) where it was taken. Note that pictures of juveniles are of no use as they are not yet moulting.

Any help will be much appreciated and will be acknowledged in any paper coming out of this.