Jackdaws are smaller than Crows and have a mostly grey head, a dark grey front and distinctive light/white eyes. Often seen in pairs, they have a strong pair-bond.
Those in the UK/Europe through to mid-Russia/Central Asia are also called the Western or Eurasian Jackdaw, to distinguish from the Daurian Jackdaw in the Far East.
They are gregarious birds which spend much of their time high in trees or on high buildings. They often nest high on churches, cathedrals and similar; also on cliffs.
The flock ("clattering") has a strong social hierarchy. Jackdaws will occasionally carry off and hoard shiny material/objects, as magpies do.
There are four subspecies of Eurasian Jackdaw over its range, that shown being C. m. spermologus in the UK and most of Western and Central Europe. Above with a partial white collar is C. m. soemmerringii, which covers northeast Europe, Western Russia, Turkey through to Central Asia as far as Mongolia and north India. The nominal subspecies covers Scandinavia and Eastern Europe.