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A pair of grey wagtails has taken up residence inside the moving gates of a lock on the Thames. It's not ideal - but this room with a view is the best they can do. Wagtails are drawn to locks and weirs because there are plenty of insects. Concrete towpaths aren't an easy place to find accommodation, but all the human activity does keep predators like the mink at bay. Like many river residents, the wagtail has survived by being flexible.