|No, I'm not very good with Photoshop...|
In our reality, though, they couldn't exist, since they mix and match mammalian and avian features in a way that doesn't happen in natural evolution. Even in the world of D&D they're usually said to be the creation of some long-dead wizard, rather than something natural - although it's worth noting that other hybrid creatures, such as griffons, aren't regarded in the same way. Still, it's at least interesting, for someone like me who writes a lot about real world animals, to consider how such a creature would work if, somehow, it really existed.
Owlbears aren't technically original to D&D, in that they were inspired by some plastic toys that Gary Gygax bought in Hong Kong, but in practice they might as well be. As such, and given that they have become one of the more iconic monsters of the game, they have been much written about, and I dare say my thoughts are no more original than anyone else's. But, sticking just to the primary source material, what do we know about owlbears?
First EditionThe body resembles that of a bear, but with a long, heavy, and apparently muscular tail, somewhat like that of an otter. The head, however, is not at all owl-like, but instead more closely resembles that of an eagle or falcon, with a large beak, a feathered ridge over the eyes, and what appear to be loose folds of hairless skin at the throat. Surprisingly, the beak appears to contain a row of small, but presumably sharp, teeth. Both the fur, on the body, and the feathers, on the head and shoulders, are stated to be brown. Judging from the hit dice, the animal is intermediate in size between a black and a brown bear, but the weight of adult males is said to be over 1,300 lbs (590 kg), approximately that of the largest brown bears recorded in the real world. The creature attacks with both claws and beak, immobilising its prey with a "bear hug" where possible.
Second EditionWe are given more detailed information on the animal's habits and biology in this edition, which I'll look at a little below. In terms of appearance, we are specifically told that the creature is part owl, but the illustration shows a head that is even more clearly eagle-like than before, albeit with large and elongated ears that loosely resemble the decorative ear-tufts of owls. The beak may be serrated, but there is no indication of teeth. The tail is now bear-like, and we are given a height figure for the males that matches that of a large (but not implausibly so) brown bear.
Third EditionAllowing for changes to the rule system, the statistics and physical dimensions of the animal appear much the same, although the hit dice now more closely match those of a regular brown bear. The head no longer has ears, and the beak lacks either fine serrations or teeth, although the cutting surface is jagged. Large feathers now cover the fore-limbs as well as the head and shoulders, making them look almost wing-like. Proportionately speaking, the limbs appear longer than those of a real bear. The eyes are (as in previous versions) said to be red-rimmed, although the illustration shows them as dead white. This broad appearance is retained in Fourth Edition.
Fifth EditionAlthough dimensions are not given, the hit dice, strength, and claw damage of the owlbear all suggest that it may now be significantly larger than a brown bear. It has lost its signature "bear hug" attack, and for the first time, the head actually resembles that of an owl, rather than an eagle. The beak is clearly serrated again, and the feathers stretch down onto the outer aspect of the forelimbs, but are far less extensive than in the previous two editions. The colour of the animal looks to be purplish-grey, rather than brown, but it's likely that this is intended to represent night-time or subterranean illumination, rather than an actual shade as would be seen in sunlight. The eyes are now jet black (which would make sense, if it's in the dark, due to expanded pupils).
After allowing for some minor cosmetic differences, and some artistic license, what does this allow us to say about the owlbear? Looking firstly at the torso and limbs, these are clearly supposed to resemble those of a particularly large brown bear, such as a Kodiak. In the earlier editions, it's clear that the males are larger than the females, as is the case with real bears. Sizes for females are never given, but we can assume that, as with Kodiak bears, a full-grown female probably stands about six feet tall when rearing on its hind legs, and weighs about 1,000 lbs (450 kg).
The head is more problematic. The name of the animal, and some of the text descriptions, clearly imply that it looks like that of an owl, but only in 5E does it actually do so in the pictures. Even then, the beak is unusually large, and, in most versions, serrated. Even apart from that, it's not clearly intended to be any particular real-world owl species, and it's worth noting that 2E says it's the head of a "giant owl" - a species that is itself obviously fictional. Having said that, a screech owl is, perhaps, one of the closer matches.
In terms of the internal anatomy, one can assume that, outside of the head and neck, the organs are those we would expect to find in a mammal. One significant difference between mammals and birds, however, is that the latter do not have teeth, and therefore cannot chew their food. This is a problem for those that eat seeds or other tough plant materials, but not so much for a pure carnivore. As a result, the owlbear probably does, indeed, have a bear-like digestive system, and does not need a grinding, avian, gizzard.
This reinforces the claim that owlbears are more purely carnivorous than true bears, as would the fact that the brain is that of a predatory bird, not an omnivorous mammal. The serrated beak would also help cut up meat, perhaps allowing the owlbear to feed on proportionately larger carcasses than a true owl would.
Owlbears are consistently said to inhabit temperate forests, presumably including both mixed broad-leaf and coniferous types, although one would expect that they must also be quite capable of hunting effectively in more open terrain. Hunting behaviour is likely to be more influenced by the owl part of their natures, partly because the brain and sensory organs are avian, and partly because bears feed on far more vegetable matter than meat. Thus, vision and hearing are likely to be paramount. (From 3E onwards, owlbears are also said to have a keen sense of smell, but birds generally don't, making this something of an anomaly).
The use of a "bear hug" to grapple opponents is interesting, since bears don't actually do this under normal circumstances. Having said that, both bears and owls do, unsurprisingly, try to immobilise their prey before eating it, using body weight or their talons, respectively. It might be that grapples are an adaptation of this used when attacking upright, bipedal, opponents, and not something that owlbears do against, say, deer.
There's also the issue of the mindless ferocity and fight-to-the-death attitude that owlbears are said to display; this can reasonably be taken as a trope of the D&D game, and not something that a real animal would do unless it was cornered.
What sounds does an owlbear make? 2E says they make loud screeches, while the other versions are... ahem... silent on the matter. Interestingly, the voice box in mammals is in the throat - in the bird-like part of an owlbear - while the equivalent structure in birds is in the chest - which is in the mammal-like part. Since it seems hard to imagine that owlbears are silent, there must be some anatomical difference here, and I'd argue that a mammalian voice box (attached, after all, to mammalian lungs) is the more plausible arrangement.
However, if their behaviour is more influenced by the owl parts of their natures, this likely extends to the sounds they make too, so screeches, or even deep hoots, seem more likely than growls and roars. It may also be worth noting that, in 2E, owlbears have a language of sorts, and their intelligence is high enough to do so in 1E as well, although it's not mentioned. From 3E onwards, however, they are just animals, even if the 5E version is marginally more intelligent than a true bear.
One interesting question is what time of day owlbears do their hunting. In 2E, they are said to have a bizarre activity cycle in which they sleep from about midnight to noon, hunting in both the sunshine of the afternoon and the darkness of the first half of the night. It's hard to see why a real animal would do this. 3E improves matters somewhat by stating that owlbears are intermittently active throughout a 24-hour period, while 5E implies that they are mostly nocturnal. The latter makes the most sense, allowing the owlbear to take advantage of the doubtless excellent night vision granted by its large eyes.
Owlbears, so far as we can tell, do not hibernate, nor is there any good reason why they'd need to.
In 2E, we're told that owlbears are said to be highly territorial, with a home range of one or two square miles. In reality, there is essentially no way that they could find enough to eat if this were the case. Although it depends on the local habitat, real bears, which would likely have a similar calorie requirement, regularly inhabit ranges of a hundred square miles or more, and they mostly eat vegetable matter (and fish) which is easier to come across than meat.
The territoriality of owlbears fits with the fact that their brains, and presumably instincts, are those of their avian kin. This also makes sense in view of the claim that they live in mated pairs; adult bears tend to be solitary. Some owl species mate for life, but others only stay together long enough to raise chicks, so owlbears might do either.
This naturally brings us to the question of owlbear reproduction. Different editions all agree that owlbears have up to six young at a time, a figure that's entirely normal for owls, but very much at the high end for bears. What's less clear is how they are born. Both 1E and 2E unambiguously state that owlbears lay eggs, but 3E, by only mentioning "young", at least implies live birth, and 5E does not address the question either way.
Certainly, egg-laying would be surprising, given that that end of the animal is mammalian (and not a platypus or echidna). While the male reproductive organs are presumably those of bears regardless, if owlbears really do lay eggs, many of the female organs must surely be avian.
Owls don't build nests, so the clutch will likely be laid on the ground in some secluded location, perhaps screened by heavy undergrowth. The eggs don't actually need to be particularly large, since newborn bears are tiny compared with their mothers, weighing only around 1 lb (450 g).
Incubation/pregnancy is likely only around two months; it's longer in bears, but only because they have to hibernate. This, in turn, implies an early spring or late winter breeding season, so that the young are reasonably well grown by the time they experience their first winter.
Male bears have no role in looking after their offspring, but we know that owlbears live in mated pairs, and, anyway, the male will have to stay around to feed the female if she has to incubate eggs. The young will hatch (or be born, if they really don't lay eggs) hairless and helpless, although with fluffy down feathers on the relevant parts of the body. Given the ursine body, one would expect the female to provide milk for her young, but their large, sharp beaks would tend to mitigate against that. Possibly owlbears are born with soft beaks that only harden later, or possibly they're active enough to feed on pre-chewed meat, or even small rodents, almost from birth.
[Original images by S. Taheri and Greg Hume, from Wikimedia Commons.]