When we thought on the songs is at night logical to think about the Olws, but also there are other species of birds that in spring sing at night.
But famous bird singing nocturnal is the Rufous Nightingale, but it is possible to listen to other singers.
The last night at four in the night a Cetti´s Warbler was singing, but with differences on its typical song, song four times and only in one of them the song finalized, in the other three occasions only sang the first part of the song.
The birds are not too many that to sing at night and some of them it is easier to listen to them only at dawn or at dusk. This is a list of the birds that commonly can to listen in the north of Spain at sunset. 
Blackbird
Cetti´s Warbler
Wren
Robin
Rufous Nightingale
Wood Lark
Great Reed Warbler
Reed Warbler
Bluethroat
Grasshopper Warbler
I live in Los Molinos , and am living here for 1 1/2 years
with my Spanish wife , Im very lucky to spend time in my wifes country which I have fallen in love with but we
will sadly go home to Colorado in Nov.
Maybe you can help me as european birds are somewhat new to me
At night around the area of our house is a bird that can make this sound for an hour or sometimes less
ITS one sound ” swrit” slowly over and over no other sound
thank you if you can help
and some of Spains lovely birds
Jeff Sigler
First it is to know, where this Los Molinos, since thus I will be able to know but or less than species of birds they exist is that zone.
It is very difficult to know that species is, although reason for you comment, also is possible that it is bats, since them yes them can be listened to repeated sounds and without variation.
He is high-pitched or low sound?.
From where is emitted the sound, trees, buildings, in flight, river…?
I wait for your answer.
Thanks
Here are some links that I believe will be interested