In order to test whether or not pairs of rooks could work together, Seed et al used an apparatus consisting of a flat rectangular wooden tray placed outside the wire mesh "room" in which the rooks were kept. The platform, on which food was placed, had a single piece of string threaded through metal loops at each end, and was just out of the reach.

Thus, each pair of birds could only obtain the food on the tray by cooperating. If only one of them pulled on the string, it would become unthreaded, but if both pulled simultaneously at each end, the tray would move into the "room".

Each pair of rooks managed to pull the wooden tray into the room, spontaneously and, significantly, without prior training. Their performance on the task was comparable to that of pairs of chimps. (Here's a film clip).

However, unlike chimps, individual rooks presented with the food tray, they never waited for a partner to be introduced into the room from a neighbouring compartment before pulling on the string. Furthermore, when tested alone, most of the rooks did not choose an apparatus they could operate singly over one that required cooperation.

This suggests that the cognitive processes underlying cooperation in chimps are more complex than those on which cooperation in rooks is based. The authors speculate that this might be because groups of chimps and rooks are organized differently: whereas rooks form long-term and stable monogomous relationships, the bonds between chimps are highly dynamic, and are both cooperative and competitive.  

Seed, A. M., et al. (2008). Cooperative problem solving in rooks (Corvus frugilegus). Proc. R. Soc. B. doi: 10.1098/rspb.2008.0111. [Abstract]