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 www.waarneming.nl and ebird.org, 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!

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