Locustella species all have brownish plumage, pointed bills, flat foreheads, and really broad, rounded tails. They are major skulkers of low vegetation, making them often difficult to see; they're more likely to scamper away on the ground than fly out. But fortunately, 4 of the 5 species reportedly found in Europe (those listed above, and Lanceolated Warbler, a rare vagrant to Europe, usually summering in the Siberian Taiga) give some unique variation on a monotonous, mechanical, insect-like call, which are usually heard at night and in the early morning.
Common Grasshopper Warbler
Of the three species under comparison, this is the most likely to be heard, as they are most widespread; not being as picky with habitat preference as the other two. At Xeno-canto, you'll find that most of the recordings posted have sonograms looking similar to this:
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| XC134381: (Common) Grasshopper Warbler (Norway by Ingar Bringsvor) |
They all have this monotonous, dry ringing or insect-like whirring. Maybe the pitch (along the vertical axis) varies a tiny bit among individual birds, but always higher than Savi's (see below).
Savi's Warbler
By comparison, Savi's Warblers have higher tempo (frequency) and lower pitched calls: more like a fine buzzing than a whirring. Here is a good example of a sonogram from a typical call:
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| XC129525: Savi's Warbler (Poland by Matthias Ritschard) |
River Warbler
Now comes the species of the group with (what I find) the greatest repertoire, though some of the calls indeed are probably rarely heard, but perhaps they could be different populations, too.
A typical sonogram looks like:
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| XC83760: River Warbler Typical/'Type-1' (Czech Rep. by Kamil Cihak) |
Here is a link to listen: http://www.xeno-canto.org/83760
Pitch can vary a bit, but usually between Grasshopper (higher) and Savi's (lower). The main distinction of Rivers are the slower frequency which makes it sound like a powerful sewing machine. However, there are also some unusual variations on the modulations, such as this (let's say) "Type-B" sonogram:
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| XC129502: River Warbler 'Type-2' (Poland by Matthias Ritschard) |
Here is a link to listen: http://www.xeno-canto.org/129502
Such saw-tooth modulations create in the ear an uncanny resemblance to an antique (low-powered) car engine! Such recordings have been submitted to xeno-canto from Poland and Sweden. Finally, I'll leave you with 2 more untypical songs I stumbled across just to illustrate the vast variety in this Warbler's repertoire:
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| XC101139: River Warbler Untypical (Poland by Tomek Tumiel) |
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| XC80979: River Warbler (Norway by Ingar Bringsvor) |
Here is a link to listen: http://www.xeno-canto.org/80979
Lanceolated Warbler
Just for comparison I have decided to add this species at the end. It has a song that most similarly resembles Common Grasshopper Warblers, and according to Svensson, Mullarney and Zetterstrom (SMZ), some rare, aberrant Grasshoppers have vocalizations that are "very similar" to Lanceolated: so if you are in Europe and think you have heard a genuine Lanceolated, listen carefully (or better yet, get a visual and take a photograph to document it). They are regular (annual?) vagrants to the northeast, but extremely rare elsewhere.
That being said, Lanceolateds' song has slightly higher pitch and higher tempo than Grasshoppers'. Supposedly the verses are shorter as well, rarely lasting more than a minute each (sometimes Grasshoppers can continue a verse 5-10 minutes!) Well here is a prime example of a Lanceolated song:
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| XC83260: Lanceolated Warbler (Sweden by Lars Krogh) |
Here is also the link to listen: http://www.xeno-canto.org/83260
As pointed out in SMZ, it has a quality closer to the shuttling River Warbler's than the uniform, dryer reeling of the Grasshopper Warbler.
If you ever have a weekend day with horrible weather, and no other pressing responsibilities, I recommend playing around with Xeno-canto's brilliant site. ;)
Good birding and listening!
Steve
