Small and very lively leaf warbler, smaller and tinier than a house sparrow, about 10 cm in length, olive-green above and off-white to yellowish below, and a well defined yellowish supercilium. So lively that it's a nightmare to photograph, but a real pleasure to watch... It is also a nightmare to ID, because of its resemblance with Wood and Willow warblers (P. sibilatrix and P. trochilus), and Icterine warbler, too.
This is a common eurasian leaf warbler; here seen in a canopy of protected wetland, an exceptional site managed by Pro Natura Geneva. The marsh in question is a small one, and surrounded by agricultural fields and urban settings; but is a part of a chain of protected habitats/wetlands on a migration route.