Deinonychus preferred small plant eaters for prey when hunting (Desmond, 1975), and individually they would have been incapable of killing larger dinosaurs. However, if they had gathered into packs they would have been able to pull down larger prey (Thulborn, 1990).
Those that roamed as solitary individuals would have likely subsisted on smaller prey such as insects and lizards, and supplemented with eggs, fruit and carrion (Thulborn, 1990).
Ostrum, the founder of Deinonychus, has suggested that they hunted in packs because of the remains of five individuals preserved with the fragmentary remains of a single, medium-sized Ornithopod, which was thought to be Deinonychus's prey. It should be noted though that just because their remains are found together that does not mean that the congregated together in life. An example of which is Ghost Ranch, New Mexico where there is a massive bone bed of fossilized organisms which in death have congregated together, whereas in life they lived solitary lives. Other evidence must be used to confirm whether Deinonychus was a pack hunter (Carpenter et al, 1990).
There are other fossil sites where more than one skeleton of Deinonychus has been found with the broken skeleton of a herbibore. An example of which is where three Deinonychus skeletons w3ere found with a dismembered skeleton of a Tenontosaurus. This indicates that during the hunt three Deinonychus's were killed during the attack on one Tenontosaurus which was also killed and consumed by the survivors. The bones were found in a fine overbank deposit that could not have been transported by fluvial processes (Farlow et al, 1997). Therefore, it seems evident that Deinonychus was a pack hunter.