The UK has a huge variety of warblers with a wide range of differing songs and they are in full voice in May. Most stay in the UK right through the summer and they are generally twitchy little birds constantly hopping from branch to branch and singing intermittently. They are also quite shy, spending a large amount of their time concealed from view in amongst the undergrowth. This is especially true of the grasshopper warbler (above) which likes to stay on the ground in amongst tussocks and tall grass. Grasshopper warblers are small brown birds which are usually heard before they are seen. The best time to record them seems to be sunrise and sunset but they will sometimes sing on and off right through the day.
I recently went to a spot I had heard them around this time last year and sure enough around 5.30am I heard one singing from a patch of tussocks. I have found the best way to record them is with either a shotgun mic or parabolic reflector on a monopod or tripod as they sing for long periods and don't mind being approached if you are reasonably quiet and careful. I generally like to record individual species in mono and record habitats in stereo this gives you the freedom of making composite recordings by adding the sound of a particular species to a nice stereo recording of its habitat. Above is the mono recording I got and a picture of a grasshopper warbler by Jim Almond who kindly agreed for me to put it on my blog. Please check out his other pictures at http://shropshirebirder.co.uk/. I think the grasshopper warbler's song sounds a lot like a fly fishing reel or a tiny pneumatic drill! Its amazing quite how mechanical it sounds.
In a patch of trees very nearby there was also a willow warbler singing. These are slightly lighter color than the grasshopper warbler and are green rather than brown with a pale yellowish breast. Again the best approach seems to be to locate them by sound and then setup your mic on a tripod at a suitable range (around 5-10m with a reflector), set your levels, and wait for a nice passage of song. Below is the recording I got along with another picture by Jim Almond.
The two warblers I would like to try and record next are the wood warbler which has a song that has been described as sounding like a coin being spun on a marble slab, and the blackcap whos song can be heard if you click the play button on the right of this page: http://www.rspb.org.uk/wildlife/birdguide/name/b/blackcap/index.aspx
For info about other warblers please go to: http://www.rspb.org.uk/wildlife/birdguide/families/warblers.aspx