is there any reason for them to sing?
Yes, a call for breeding
Water pipit
The water pipit is mainly monogamous, although both sexes may deviate from this occasionally. The male has a display flight in which he climbs to , flies in an arc and glides back down, singing throughout. 
what is water pipit? A small passerine bird which specie does it belong to? Motacillidae when you said that the bird is passerine, what did that term mean? A passerine is any bird of the order Passeriformes where did these type of birds originate from? The mountains of Southern Europe and the Palearctic eastwards to China where is the pipit found? The mountains of southern Europe and Asia from Spain to central China, along with the Mediterranean islands of Sardinia and Corsica what colours are these birds? Greyish-brown upperparts, weakly streaked with darker brown, and pale pink-buff underparts fading to whitish on the lower belly and what is the size of these birds? UNANSWERABLE what is special about their voice? Their ability to sing does the male or the female bird sing? Male
wiki:4710615
Water pipit [SEP] Breeding
The water pipit is mainly monogamous, although both sexes may deviate from this occasionally. The male has a display flight in which he climbs to , flies in an arc and glides back down, singing throughout. The female constructs a cup nest from grass and leaves which is lined with finer plant material and animal hairs. The nest is hidden in vegetation on the ground, sometimes in a hollow. The normal clutch is four to six eggs laid from the end of April to early July. Eggs are greyish white with darker grey or brownish speckles mainly at the wider end, and they measure and weigh of which 5% is shell.