Ms Debby Doodeman of Dutch birding organisation FOGOL today published a blog post with many photos on black terns and other wildlife at De Kreupel; seen during boat expeditions on 1 August and 6 August 2016.
The black terns are preparing for their autumn migration to Africa. They are already partly in their winter plumage, more whitish than their summer plumage. It is better to have a whitish belly than a blackish belly if you want to catch fish by surprise. In spring and early summer, they eat many insects. In winter, mainly fish. In late summer here at De Kreupel, they eat both fish and mosquitoes.
This picture, by Dutch ornithologist Jan van der Winden, shows migration of an adult male black tern named Han from the Netherlands to Angola in Africa and back.
This 6 August 2016 video shows black terns resting on De Kreupel; while a coot passes.