"During the summer of 1964, an expedition from Yale University's Peabody Museum under the author's direction explored exposures of the Cloverly Formation (Early Cretaceous) in Wyoming and Montana in search of fossil vertebrate remains. Among the important discoveries made was that of the spectacular little carnivorous dinosaur described here - an animal so unusual in its adaptions that it undoubtedly will be a subject of great interest and debate for many years among students of organic evolution. Although of modest size, this creature was one of the most unusual of all dinosaurs and provides entirely new insight on the classification of predaceous dinosaurs and on the surprisingly sophisticated capabilities possessed by some theropods. At the very moment of discovery, it was evident from the few fragments on the surface that we had stumbled across something very unusual and quite unlike any previously reported dinosaur."

Habits: A spectacular and exciting find indeed! Although a dinosaurian origin for birds (see above) and endothermic dinosaurs (Owen 1941) had been proposed before, it were Ostrom's (cant help to use this term) visionary 1969 publications that finally gave rise to the general acceptance of both theories amongst most modern palaeontologists. In order to illustrate my use of the term "visionary" I would like to quote a few more sentences from Ostrom's 1969 publication (my comments in round brackets):

"...There can be little doubt that the claw of Deinonychus was well adapted for deep penetration and effective slashing or cutting.... Deinonychus must have been anything but reptilian in its behaviour, responses and way of life. It must have been a fleet-footed, highly predaceous, extremly agile and very active animal, sensitive to many stimuli and quick in its responses. These in turn indicate an unusual level of activity for a reptile and suggest an unusually high metabolic rate. The evidence for these lie chiefly, but not entirely, in the pes (feet).... at least three and perhaps four or five individuals are represented among the Deinonychus remains collected from just a small area at the Yale site. These remains were associated with fragments of only one other species - a moderate-sized ornithopod (Tenontosaurus) that weighed perhaps five or six times as much as Deinonychus. The multiple remains of the latter suggest that Deinonychus may have been gregarious and hunted in packs.... An additional fact that can hardly be explained as coincidental is that Deinonychus-type teeth [but nothing else] have been found associated with skeletons of this same ornithopod at 14 other sites in the Cloverly Formation."

No less then another three, at that time revolutionary, theories and evidence supporting them are presented for Dromaeosaurids in the above:
1. Gregarious habits
2. An active, alert lifestyle
3. Pack hunting behaviour
Since I would like to keep "dinosaur-bird-link" and "endothermic dinosaurs" for the chapters about the Microraptoria and the Troodontidae, lets have a look at what further evidence I was able to find in support of the last three theories.

Gregarious habits: The term gregarious means the tendency of a species to group up with others of it's own kind and form a herd, pack or flock. The fossil record on this part of dromaeosaurid behaviour unfortunatly is extremly poor. I was not able to find any publications (except the already presented above) or mention of fossil sites where more then one individual had been identified with certainty. However, there are some other indicators that may allow the inference that gregarious habbits for dromaeosaurids were likely. One of these indicators is the finding of other, more or less closely related to Dromaeosauridae, dinosaur fossils. A bonebed of Sinornithomimus dongi included no less then 14 individuals of this propably herbivorous theropod (Kobayshi & Lü 2003). "Remains of adults and juveniles found together (at "Jack's birthday site") may indicate that young troodonts enjoyed some parental protection in a family group" (Ben Creisler 1997). Several tracksites document the forming of groups of even large sauropods in order to protect themselves from large predators (Ostrom 1972). Since it is at least very likely that large theropod predators also preyed on the much smaller Dromaeosauridae (Farlow, date unknown), I cant see any good reason why dromaeosaurids shouldnt have taken advantage of gathering in herds or packs and benefit from the additional protection such behaviour provides. However, until undoubtedly "non-predator-trap" (swamps and similar locations where a large prey-animal died and in return attracted a multitude of predators) or otherwise artificial (for example rivers carrying the remains of many dinosaurs to the same location) bonebeds are discovered, such inferences remain speculative!

Active Lifestyle: The ratio of femur (tighbone) to the tibia (lower leg-bone) length indicates that Dromaeosauridae were no fast runners. This doesnt mean they were generally slow-moving animals, but rather that they were not amongst the fastest running dinosaurs! Instead Dromaeosauridae were extremly agile animals. The prezygapophyses (forward pointing extensions on top of the vertebrae) and the chevrons (backwards pointing donward extension of the tail vertebrae) of all but the most anterior caudals (the very last tail vertebrae) were modified into extremly long, double, bony rods reinforcing the tail. In the case of the Dromaeosauridae, the prezagypophyses also have backward pointing extensions. These rods restricted the tail movement (except for the root and tip of the tail!) and turned it into a rod-like structure that served the animals extremly well when they had to perform rapid turns or stops. Although Dromaeosauridae werent amongst the fastest runners, they were able to literally "turn on a dime"!
For such relatively small animals, dromaeosaurids possessed large claws on their hands and feet. These indicate that they were adapted for a "grapple and slash" style of fighting as seen today in cats, tigers and lions. And as if that alone wasnt enough already the claw of the second toe was inordinatly enlarged and could be held above the ground, pointing upwards at an angle of up to about 90°, in order to effectively prevent it from blunting through contact with the ground while the animal was running. Thus one could describe the Dromaeosauridae as the "kung-fu-fighters" amongst the dinosaurs which undoubtedly required them to be able to perform fast, well coordinated movements of both their legs and arms.
The (for dinosaurs) unusually large brains of the Dromaeosauridae were propably further adaptions towards their "high performance lifestyle". Their eyes could at least partially point forward thus allowing stereo-optical (three-dimensional) vision. Although I couldnt find any literature about the nature of the dromaeosaurid ear, the close relationship to the avialae (birds) suggests that their ear resembled that of birds more closely then that of an alligator or crocodile (Alonso et al. 2004). However Barsbold (1983) noted that the middle ear region of dromaeosaurids was propably similar to that of extant reptiles. I found no literature on the sense of smell of Dromaeosauridae, but my guess would be that, as with other animals primarily relying on vision, their olfactory sense wasnt as well developed as it was/is in extinct allosaurids or extant crocodiles for example.

Pack hunting behaviour: Pack hunting doesnt mean gregarious habits by default! If animals occasionally group up in order to hunt prey larger then themselves, that is already pack hunting behaviour. Gregarious habbits on the other hand mean that they do not only congregate to hunt, but live in packs, herds or flocks regardless of their current activity.
As Ostrom already proposed in 1969 such hunting behaviour is at least very likely for the Dromaeosauridae. Tenontosaurus (the ornithopod that was found together with the Deinonychus remains at the Yale site) is estimated to have weighed about 900 kg. Deinonychus in return is estimated to have weighed only about 50 - 75 kg. That would mean (given that Tenontosaurus didnt weigh more then 900 kg and Deinonychus would weigh 75 kg) that these animals hunted prey being about twelve times as heavy as themselves! There can be little doubt that such a task would be very hard to complete for a single individual Deinonychus. However, it has been argued that the predators may have been feeding on the remains of a carcass that was either brought down by a larger hunter or may even have died of natural causes before the Deinonychus found it.
But there is at least one very famous fossil that prooves that dromaeosaurs actively hunted prey much larger then themselves: The "fighting specimen" GI 100/25. This amazing fossil shows a Velociraptor mongoliensis fighting against a Protoceratops andrewsi. Although only a single Velociraptor is preserved in the fossil, the mass relations of the two animals involved in the fight strongly point towards pack hunting. Velociraptor mongoliensis is estimated to have weighed about 7 - 15 kg. Protoceratops andrewsi is estimated to have weighed about 180 kg. Once again the prey would have had twelve times the mass of the hunter. I cant think of any living predator that hunts prey on his own that is coming even close to the dimensions we are looking at here. From a purely logical point of view, there can be very little doubt that Dromaeosauridae were pack-hunters: Since hardly questionable evidence exists that (at least!) Velociraptor mongoliensis actively hunted prey several times it's own weight, what type of "super-predators" would one require those animals to have been in order to support a solitary hunting behaviour?

How did Dromaeosauridae use their "terrible claw"? What may an attack of a dromaeosaurid pack on a large prey-animal have looked like?

Ostrom suggested 1969 that Deinonychus may have grabbed and held large prey with its hands and then (standing momentarily only on a single foot!) slashed at the victim's belly with the sickle claw in order to eviscerate it. He proposed that the tail would be used to support the animal while it was balancing on a single foot.

Barsbold (1974/1983) described dromaeosaurids using the predatory claws of both feet simultanously, either while hanging on the victim grasping it with the forelimbs or falling on the back (as seen today in some members of the cat family). He also stated that "In the unique 'grasping skeletons' of the carnivorous Velociraptor and herbivorous Protoceratops, the position of the hindlimbs of the predator supports exactly this kind of supposition."

In 2002, Carpenter expressed his doubts about Ostrom's 1969 hypothesis as follows: "Furthermore, it seems doubtful that the dromaeosaurid could hop one legged along side of a fleeing prey while sawing through the tough hide and muscle with the claw of its other foot."

In 2005 a new study published by Manning et al. shed further light on the way in which Dromaeosauridae may have used their specialized claw. In an experimental setup several variations of the claw were tested for their penetrating/cutting abilities (P. Manning, 2005, pers. communcation). The results of this study strongly implicate that the degree of sharpness necessary for the claw to act as a cutting/slashing device is beyond what can be considered reasonable within a natural context. It rather appears that the claw was used as a climbing device that aided Dromaeosauridae when attacking prey larger then themselves and possibly in climbing trees. Since larger forms such as Utahraptor ostrommaysorum and Achillobator giganticus also possessed the modified 2nd pedal ungual the inference that this device may have evolved as a "tree-climbing-crampon" appears impropable to me, but I wouldnt exclude the possibility that small Dromaeosauridae may have used their "sickle claw" in such a context.

In the light of latest research the following appears to be outdated to me. However, I will keep it here in order to demonstrate how the picture of a pack of Dromaeosauridae hunting can change in the light of new finds.

If asked about my own opinion, I would state that it appears doubtful to me that Dromaeosauridae were able to hold and at least partially immobilize prey much larger then themselves with their forearms. It's this "grabbing-part" of both hypotheses that actually gives me some trouble. Pack hunting behaviour would certainly make holding a large prey animal more easy! However, the "grabbing- part" holds the potential for another disadvantage: A lightly built predator holding on to a large victim would certainly be in danger of being hurt by the prey. So is there any way for the predator to minimize the risks? I think there is, but it would definatly require pack hunting behaviour!

Once a pack of dromaeosaurs has picked it's victim out of a herd of prey-animals, the first step would be to isolate that animal. Next the predators would encircle the victim from all sides. Those pack members who dont face the prey's "defensive hotspots" (propably the rear and/or the front) would now leap on the victim with all claws pointing towards it and immidiatly afterwards jump back to save distance. This "back-jump" would propably severe the injuries already inflicted by the initial leap while at the same time reducing the actual contact with the prey to a minimum.

The "back-jump" part still holds the risk of being injured, but I think this one is considerably lower then the risk of being hurt by the struggling prey while holding on to it. Interestingly enough some extant raptors (birds of prey) stun their prey during their initial attack and then return to pick it up (Ward et al. 2002). This behaviour maximizes the forces applied to the prey during the initial attack but also minimizes the risk of being injured by a not "perfectly grabbed" prey animal. The style of hunting I tried to describe for Dromaeosauridae above would also fit very nicely within the "high performance lifestyle" picture.

An alternative scenario based on the research of Manning et al. (2005) would be the following, which I like to call "Rodeo scenario" because of it's similarities to that form of entertainment.
The first steps within the "Rodeo scenario" dont differ much from what I tried to describe above. My speculative "dromaeosaur-hunting-party" would also have to isolate the victim from it's herd, and then encircle it from all sides. Once again it would be those members of the hunting-party that dont face the "defensive hotspots" that would carry out the actual attack, while the rest of the pack would try to divert the victim's attention from it's attackers, possibly by carrying out feint attacks right in front of the prey. Now the actual attackers would leap onto the victim from the sides using the claws of both feet and hands in order to maintain a delicate balance between holding on to the prey firm enough in order to inflict wounds (using their sharp teeth!), and keeping their grip loose enough in order to be able to make it back to safe distance if the prey should decide to apply "desperate measures" such as rolling on the ground (to crush the attackers under it's weight). The modified structure of the dromaeosaurid tail may have served them well in order to counteract with the violent moves of the prey which would naturally do it's best in order to shake off the attackers from it's back. The victim would propably be reluctant to roll on the ground, because it would "know" instinctively that the rest of the dromaeosaur-pack would give short shrift to it once it exposes the vulnerable belly to them. After some time, even the strongest victim would suffer fatigue from blood loss and constant bucking in order to shake off the attackers from it's back. Even if the wounds inflicted by the dromaeosaur teeth wouldnt be very serious (and no "lucky shot" hitting an artery or a vein would occur) the agility and their numbers would propably give the dromaeosaurs a clear advantage over their prey, and it would only be a question of time until the final, deadly attack would be made.

The "Rodeo scenario" undoubtedly exposes the hunting dromaeosaurs to a significant risk of being injured during their attack on the prey. However, there apparently exists fossil evidence for the fact that dromaeosaurs sometimes got killed during attacks on large prey animals (P. Manning, 2005, pers. communication)!
I finally dont want to miss the chance to thank Phillip Manning for the kind way in which he replied to my questions regarding the qualities of the 2nd pedal ungual of the Dromaeosauridae, and Kenneth Carpenter for establishing contact between Dr. Manning and myself in the first place.