Between June and September each year, the majority of the northwest European shelduck population migrates to these sites to moult and then simultaneously regrow their primaries, secondaries and tail feathers. This is called a catastrophic moult.
During this period they are flightless, and thus relatively defenseless. They congregate in estuaries which offer them safety from terrestrial predators and have abundant food to provide the energy and nutrients required to regrow their new feathers.
After the moult is complete shelduck migrate to non-breeding areas to spend the cool winter months (Sep – Feb) in estuaries with abundant food, and at water bodies that do not freeze over. A reasonable proportion of the shelduck that breed on the continent migrate to the UK during this period, where the chance of estuaries freezing over is smaller.
There are 32 sites within the UK that routinely host >1% of the northwest European shelduck population during the non-breeding season. Shelduck using these sites are protected by law, and the impacts of developments that may impact them must be assessed.
I am using stable isotope analysis to understand how moulting and non-breeding sites are connected by the individuals moving between them.
The prey items in each moulting estuary have a distinct compositon/ratio of carbon and nitrogen isotopes within them. The shelduck eat these prey items during the moulting period, and these isotopes get stored in the new feathers.
I have conducted bulk carbon and nitrogen stable isotope analysis on the feathers collected from each moult site, and identified the unique ratio of these isotopes for each site. I can then compare the stable isotope ratios of feathers collected from birds in non-breeding sites, and compare these to the moult site signatures to identify where those birds grew their feathers a few months previously.
Moult site feathers
Since 2021 my team and I have collected >400 shelduck feathers (secondaries) from moult sites across northwest Europe. They’ve identified the Dee Estuary and Iceland as moult sites that weren’t previously recorded, and updated our understanding of moult progression and timing.
Through bulk carbon and nitrogen stable isotope analysis I have characterised the data clusters for each moult site. With the exception of the Dee Estuary, each moult site has relatively well clustered carbon and nitrogen values. Nitrogen values seem to seperate sites by the sea they are adjacent to (North, Irish, Celtic), and the carbon values seperate sites within this.
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Non-breeding feathers
Between November 2023 and March 2024, volunteers across the country collected feather samples from shelduck in non-breeding areas.
I conducted the same bulk carbon and nitrogen stable isotope analysis on these feathers, to see if the data would match those obtained from the moult site feathers.
Broadly the patterns match, with three clusters seperated by nitrogen value, and variation within these between carbon values.
However the values do not align perfectly, so I will now undertake further research and analysis to understand the ecological drivers of the variation in these data, and what I can reasonably do to align the moult site and non-breeding site data better. This will likely involve analysing a larger sample size, and trying some unsupervised clustering analysis methods to differentiate the moult site clusters more cleanly.