Velociraptors on air: the megapode complex.
Deinonychosaurs are, like most dinosaurs, depicted as engaging in extensive parental care. The idea that dinosaurs were good parents has overall become very prevalent in media, as it is the most heartwarming way to show how complex these animals were. And indeed, we do know of several dinosaurs that engaged in extensive parental care, like ornithopods and ceratopsians.
However, the truth is that parental care in dinosaurs was a more complex issue than most people give it credit for. Unlike modern birds, most non-avian dinosaurs had extremely slow growth rates; instead of reaching adult size and sexual maturity in the space of an year or less, the norm was to grow for several years, reaching sexual maturity several years later, and even then keep growing for a few more years. This style of growth rate is more remaniscient of that present in endothermic reptiles, except in that growth stops some time after sexual maturity instead of occuring through the whole of the animal’s life (this style of growth is obviously also present in hominids like us, but for completly different reasons).
This slow grow rate would be severely detrimental for animals engaging in parental care; most animals cannot afford placing so much energy to offspring that develop so slowly. This is why birds and mammals generally have fast growth rates: to kick their offspring out of the nest as quickly as possible. Primates and a few other intelligent mammals are the exception because we rely almost exclusively on the entire group to raise the young, which are not expected to leave until adulthood. This style of prolonged parental care is not observed in birds as equally intelligent such as corvids, which have a much different brain development process. Furthermore, non-avian dinosaurs were far inferior in terms of mental capacity, which would render such a state of extensive parental care a self-destruction mechanism
It is more likely, thus, that most dinosaurs only engaged in parental care through a very brief period of time, which the young leaving very early on their lives. Once independent, their energy would be spent more on day-to-day survival rather than growth, thus ending up with a very slow growth rate. Because they’d be much smaller than the adult animals, these young dinosaurs would occupy a very different ecological niche, not only reducing species diversity, but also ensuring differences between the young and the adults.
Evidence to support this
Modern examples of endothermic animals with similar growth rates and ecological implications are the birds known as megapodes. These primitive Galliformes are unique among Neornithes in that they don’t engage in parental care; while the parents do happen to incubate the eggs in a crocodyllian like fashion (and even then some species like the Maleo merely lay the eggs in volcanic sands and end their bond to the offspring right after), the young are left to fend for themselves. As a result, they are able to fly within days after birth, if not right after birth. The young are obviously much better flyers than the adults; some have been recorded being capable of flying for over 35 km without stopping , and in fact they might be responsible for the megapode success at colonising melanesian and micronesian islands (as well as the entirety of Indonesia).
As a side effect, there are very few megapode species co-existing in the same area. Most of their diversity is due to the fact that these birds colonised a very large number of islands, the vast majority of which with only one or two species residing on them. Their slow growth rates indicate that most ecological niches occupied by fowl are pretty much taken all through the lifetime of these birds; the small, partridge like niches are taken by juveniles, while larger niches akin to those of pheasants are taken by the adults. There’s little room for other species with all availiable niches taken.
A similar situation occurs with most dinosaurs, with a high diversity of large dinosaur species, but considerably fewer smaller species, even in lagerstattë fossil sites. The major exceptions seem to have been ornithischians like ornithopods and marginocephalians, as well as basal coelurosaurs, and even then their adult forms tended to have been medium sized animals. In the Late Cretaceous, this style of niche partitioning seems to have been taken to its logical conclusion; in most ecosystems, the vast majority of the present dinosaurs had gigantic adult sizes, with pratically no medium sized dinosaurs, with the exceptions of a few small ceratopsians, relic non-hadrosaur ornithpods and, of course, deinonychosaurs. For instance, in Maastrichtian North America, Tyrannosaurus rex occupied pratically all large carnivorous niches, leaving only room for species predating on small prey such as the dog sized dromaeosaur Saurornitholestes and the giraffe sized pterosaur Quetzalcoatlus. (Troodontids, being omnivores, were exempt from competition)
This can even be observed in deinonychosaurs. Few taxa actually share the same environments; despiste coming from the same formation, Sinornithosaurus and Microraptor were set apart by 10 million years, for example. And even small specimens like Bambiraptor were usually just the juveniles of larger species. Coupled with the slow growth rates recorded in all tested specimens, it is more likely than not that deinonychosaurs were superprecocial (the proper term for animals able to leave the nest within days or even right after birth), leaving the nest early and occupying various niches across lifetime.
No real arguments against
It has occasionally been stated that deinonychosaurs were not superprecocial because we have evidence of brooding behaviour among troodontids. However, this is a rather poor excuse, considering that incubation =/= parental care.
Like mentioned before, megapodes take care of their eggs (sans for a few examples like the Maleo). The breeding pair builds a mound where the eggs are placed, and for the remainder of the incubation period both parents pay close attention to the temperature of the nest, meeting all requirements for the sake of their offspring. Yet, the parents do not raise the chicks, leaving before the eggs hatch. Thus, we have an obvious example where parental care does not extend beyond incubation.
A more obvious example occurs in ratites. We know that the examples of brooding troodontids are all male, implying that, much like ratites, male deinonychosaurs were the ones that raised the young. Baby ratites are generally not superprecocial, but they are very independent from the parents, relying on them for protection alone; they can find food on their own, thus the parents don’t waste energy bringing nourishment. There is, however, a ratite clade that is, in fact, superprecocial: Apteryx, the kiwis. Like all ratites, the male is responsible for incubating the eggs, but does not waste energy protecting the chick; the baby kiwi is every bit as solitary as the adult animals. In comparation to the other kiwis, this is not something revolutionary, since parental care is kept to a minimum anyway, but it shows that animals that invest a lot of energy in ensuring that the eggs survive will not spend the same amount raising the subsequent chicks.
Thus, parental care in deinonychosaurs was at most very relaxed. The known troodontid embryos show that the brain was considerably well developed, indicating that it was at the very least precocial. If juvenile deinonychosaurs were capable of flight, the need for parents would vanish completly. And even then both scenarios are not exclusive; modern small birds like bee-eaters use larger birds like bustards as perches from which they have good spots to hunt aerial insects by hawking. In species where the ecological niches of the adults would have been different from those of the babies, there’s enough room for symbiotic weirdness.