The Curlew sometimes called Peewit or Plover call makes the two note song of this well known marsh bird

This whistle does require a nack to how to produce the sound.
There are instructions on the pack to follow as to how to use.
The trick is to the timing on putting your figure on and off the stophole.
As a hint rather than just taking the finger on and off with a small gap created in the sound, continuously blow and slide the finger off as a continuos movement.
(do not blow as 2 seperate notes, make it a continuous sound as you blow on and off)