Willow warbler
I am not 100% sure if this is a willow warbler or another species of warbler. There are a few species which are very similar.
This is a list of all the species found around lake Kerkini according to the Lake Kerkini Management Authority visitors guide.
Cettia cetti - Cetti’s Warbler
Locustella naevia - Common Grasshopper Warbler
Locustella fluviatilis - River Warbler
Locustella luscinioides - Savi’s Warbler
Acrocephalus melanopogon - Moustached Warbler
Acrocephalus schoenobaenus - Sedge Warbler
Acrocephalus palustris - Marsh Warbler
Acrocephalus scirpaceus - Eurasian Reed Warbler
Acrocephalus arundinaceus - Great Reed Warbler
Hippolais pallida - Eastern Olivaceous Warbler
Hippolais olivetorum - Olive-tree Warbler
Hippolais icterina - Icterine Warbler
Sylvia cantillans - Subalpine Warbler
Sylvia melanocephala - Sardinian Warbler
Sylvia crassirostris - Eastern Orphean Warbler
Sylvia curruca - Lesser Whitethroat
Sylvia communis - Common Whitethroat
Sylvia atricapilla - Blackcap
Phylloscopus orientalis - Eastern Bonelli’s Warbler
Phylloscopus sibilatrix - Wood Warbler
Phylloscopus collybita - Common Chiffchaff
Phylloscopus trochilus - Willow Warbler
If someone knows them better and has another suggestion please let me know.

The common chiffchaff, or simply the chiffchaff, is a common and widespread leaf warbler which breeds in open woodlands throughout northern and temperate Europe and the Palearctic.
comments (4)
It seems to me that Cettia cetti is more brown and dosn't have any yellow like this one has. Posted 10 years ago
But it's most certainly a Phylloscopus, Cetti's warbler is best identified by its greyish cheeks and throat, which clearly stand out from its russet-red back, and its tail, which is cocked most of the time. Posted 8 years ago