It may be quite obvious from the English binomial name, as is evident from the Latinate equivalent (Passer), that Tree sparrows are related to House sparrows. In fact, they are very similar but for the slightly smaller stature of the Tree sparrow and for the chestnut brown patch on the cheek, the main distinguishing feature, and one the eagle-eyed among us use to tell the difference.
As for its exterior features, the Tree sparrow can be identified by its chestnut-brown head and nape, which is grey on a House Sparrow, as well as its tail, whose cocked position is almost permanent.
It is also an incredibly active bird, flitting and lively, yet interacts far less with humans than its more common cousin. Despite this, the mainland European variety of the Tree sparrow is more willing to nest in buildings, and therefore sometimes behaves in a similar fashion to Passer domesticus.