The dog took me running out at Berthem this morning and I heard my first Belgian cuckoo (I heard my first cuckoo of the year in Italy over the Easter break). That’s late and, as this article in last Sunday’s Observer explained, the lateness and the singularity of the call are both potentially worrying. Some European cuckoo populations are declining and nobody knows why. There was a theory that in some regions the bird was running out of nests in which to lay its parasitic eggs but populations of the reed warbler, its favourite victim, are on the rise. Although the experiment the newspaper article described involved just five birds it was nevertheless revealing. Cuckoos, like swallows, winter in Africa but nobody knew exactly how they got there or back. The answer, in part, is that some head down to the toe of the Italian isthmus and then cross the Mediterranean and the Sahara before reaching the Congo. But others traverse Spain and the Gibraltar straits and follow the African coast down to Senegal before turning inland towards the Congo. There may be other routes. In this particular experiment, one of the five birds died whilst still in Africa and the four others were returning late. Again, nobody knows why. Still, when I heard the cuckoo this morning I was a little more knowledgeable about how he probably got there.