Four years after Jac. P. Thijsse had founded Natuurmonumenten, the first purchase took place on Texel: 7ha of land was purchased in the Waalenburg polder (in the year 1909). It happened at a time when the birdlife of the polder was threatened, because people wanted to lower the water level. At the time, Thijsse wrote about Waalenburg: 'it was teeming with lapwings, clods, black-tailed godwits, redshanks, ruffs, common terns, black-headed gulls, black terns and a slate heron spawned in 1907'. In the course of the 20th century, the polder developed into an important nature reserve where many birds, such as black-tailed godwits, breed and the harlequin and broad orchid occur in large numbers.