You wouldn’t normally think of sewage treatment sites as wildlife havens but they are often sought out by bird ringers because they are indeed very attractive to birds. A particular treatment process, percolating filters, are rich with fly larvae, worms and snails that graze on the bacteria that treat the sewage. Sewage works are particularly beneficial to birds in the winter providing an insect food source during cold weather because the filters rarely freeze due to the warmth of the sewage. Accessing such sites is difficult and requires robust risk assessments and method statements and ringers have to take the unusual steps to wear full personal protective equipment including hard hats and high visibility clothing.
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| Sewage treatment works in the snow. Photo by Lee Barber |
Grey Wagtails are present at most sewage works; ringing studies have shown that many breed and are resident. In the winter Grey Wagtail numbers increase, presumably from birds originating in upland areas and we have seen that with a bird ringed as a nestling in the Welsh mountains that was retrapped at Marlborough sewage works in the winter. In March, we ringed a Grey Wagtail at Marlborough sewage works and this bird was retrapped by a ringer in Belgium in June. This is the first example of a BTO ringed Grey Wagtail to be recovered in Belgium. We contacted the Belgian ringer and he recorded it as a male and thinks it was breeding nearby.
Some years ago, a Grey Wagtail was retrapped at Calne sewage works that was originally ringed in Denmark so ringing shows that our wintering Grey Wagtails come from the continent as well as from Britain or Ireland. In addition to mapping movements, bird ringing can provide information about survival and on 4 December 2016 we retrapped a Grey Wagtail that we originally ringed on 23 January 2010 making it 6 years 10 months 19 days old; just short of the BTO longevity record.
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| Grey Wagtail. Phone by Lee Barber |
Sewage works are also very attractive to Pied Wagtails and Meadow Pipits. Meadow Pipits have shown some level of winter site fidelity at Marlborough sewage works with many retrapped in subsequent winters and some returning for up to five years but we haven’t yet learnt where they are breeding.
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| Pied Wagtail. Photo by Lee Barber |
Chiffchaffs are another bird that are now strongly associated with sewage works. Thousands now winter across the UK and sewage works are definitely the most popular site for them. Birdwatchers seek out these sites to look for Chiffchaffs in the hope of finding a browner looking one that is Siberian Chiffchaff. If we retrap one originally ringed on its breeding grounds that really would be exciting. Personally I have not been that lucky but I have retrapped a Chiffchaff that had originally been ringed in Belgium. We will continue to monitor birds on sewage works and who knows what we will learn next.
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