A map here
http://www.xeno-canto.org/species/Alauda-arvensis 


5 minutes of song
https://m.soundcloud.com/vladimir-arkhipov/skylark-alauda-arvensis 

UK info & about their decline
http://www.rspb.org.uk/....discover/birds/skylarks.aspx
Spring is the best time to hear them but they are definitely not as easy to find as they were forty years ago.