If I had been asked about the sandwich tern before I started accumulating tweeted sightings for the Nature of Dorset database I would have said that it was a summer visitor arriving in Dorset in April and leaving to head south in September and that the main location to see them would be the lagoon on Brownsea Island. Now the general thrust of that statement is true but when you look at the detail you see there is more to the sandwich tern's presence here than that. There is indeed a big influx in week 14, the middle of April, and there is an extended period of records in September and October.
To my surprise, however, the weekly reporting chart shows records for virtually every week of the year which means there are sandwich terns here in the winter; not many but it seems that some do not migrate south with their colleagues. That is assuming that these wintering birds actually bred here in the Poole harbour colony ... may be these are birds from colonies that nested elsewhere, further north perhaps, who think a trip to Dorset is far enough to travel. If anyone is aware of any more information on this I would love to hear about it.
The other surprising thing to me is that despite the main nesting colony being on Brownsea the most records come from Ferrybridge, between Weymouth and Portland. At the time of writing there are about four times as many records from Ferrybridge than Brownsea and there even more from Ferrybridge than from all of the disparate sites around Poole harbour. Many of the Ferrybridge records are 'out of season' as sandwich tern do not breed at Ferrybridge, they have a little tern colony there, but why would birds from Brownsea only migrate as far as Ferrybridge for the winter? This adds credence to the idea that these are actually migrant birds from further afield perhaps?
The distribution map shows the two clusters of sightings quite well with sandwich tern being seen at many different locations around, and near, Poole harbour and then numerous locations from along the Fleet from Ferrybridge to Abbotsbury so it is far more widespread in the county than one might think given its preferred breeding location.
If you have never seen a sandwich tern than catch the ferry to Brownsea in June and walk to the MacDonald hide on the Dorset Wildlife Trust reserve and you will see, and hear, the nesting colony just few yards in front of you.
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