There are well over 200 living species of owl currently recognised, ranging in size from the mighty Eurasian eagle-owl (Bubo bubo) with a wingspan of up to 180 cm (6 feet) down to the tiny elf owl (Micrathene whitneyi) with a wingspan of just 27 cm (10 inches). They are grouped into two closely related families; the great majority of species belong in the Strigidae family, with just 20 assigned to the barn owl family, the Tytonidae. While there are some internal differences between the two, the most visible in the living animal is that barn owls, unlike regular owls, have distinctive heart-shaped faces.
Both families have been around rather longer than most mammal families have, with fossil representatives dating back to at least the Early Oligocene, around 35 million years ago. The two families are, however, rather like pigs and peccaries, each others' closest relatives, together forming the order Strigiformes. Since few people would deny that a barn owl is, in fact, an owl, it's this order that's really the scientific counterpart of our everyday understanding of the word.
And the Strigiformes, it turns out, have an even older history than the two families that currently comprise it.
To understand the origin of owls, then, it's helpful to look at these even older species, some of which date back to not long after the extinction of the non-avian dinosaurs. There are, perhaps surprisingly, quite a number of owl fossils of this age. Which sounds good, but there's a problem: they're almost all rubbish.
Well, okay, so perhaps 'rubbish' is a bit harsh, but they're not really what you'd call complete. The problem with flying birds is that they have light and fragile skeletons that don't fossilise well, and ones of this age tend to be rather fragmentary. The identity of the oldest known owl fossil is pretty much a tie between Ogygoptynx from Colorado and Berruornis from Germany, both of which date to the mid Paleocene, around 60 million years ago. Both of these fossils consist of... one bone, from the lower leg.
Which is apparently enough for researchers to be confident that they are, indeed, owls, but tells us very little about what those owls were actually like. It's really only once we reach the following, Eocene, epoch that we find a fair number of species, of which Eostrix, Oligostrix, Minerva, and Palaeoglaux are perhaps the most widely recognised. While the even older species are hard to place anywhere in the owl family tree at all (other than that they're somewhere near the base of it) most of these are placed in the family Protostrigidae. They are also known primarily from a few leg bones, plus part of the wing of one of them and a few bits of toe.
The one exception is Palaeoglaux. This doesn't seem to be a protostrigid, but is unlikely to belong to either of the living families. It's known from a headless, but otherwise mostly complete, skeleton discovered at the Messel Pits near Frankfurt in Germany, along with a few wing and leg bones from Quercy in France. It's interesting because the German fossil includes imprints of the feathers, some of which form what appear to be a crest on either side of the body. Assuming that's what they are (which isn't 100% certain) then they would be similar in form to feather crests used for display in some living birds. And if that's true, then it's odd, because a visual signal would be a surprising thing for a nocturnal bird to have... so perhaps it wasn't nocturnal.
The bird has been named Primoptynx poliotaurus, which translates as "first owl from the Greybull [rock formation]". The fossil isn't quite as complete as that of Palaeoglaux, but still better than others of comparable age. It includes parts of the beak, backbone, and breastbone, most of one of the wings and almost the entirety of both legs and feet.
These show a bird that would be large for a modern owl, but not excessively so - at around 40-50 cm (16-20 inches) in body length, the spectacled owl of Latin America or the tawny owl of Europe and western Asia would both be roughly in the same size range. One of the reasons that that's significant is that at least one of the species of Eostrix was tiny, smaller even than the living elf owl. This means that, even this early on, owls were already diverse, at least in terms of size, and therefore likely had a long evolutionary history. Most likely, although we have no relevant fossils of this age to prove it, owls already existed before the KPg asteroid impact took out the non-avian dinosaurs.
Unfortunately, it isn't possible to say where Primoptynx fits in the owl family tree relative to other birds of similar age. The protostrigids - the only definitely known family of owls from the Eocene - are defined on the basis of the shape of the leg bones (for obvious reasons) and, ironically, the specific bit of the bone that's needed to match it up is one of the few bits of the leg that's missing in this new fossil. So, maybe, maybe not.
The feet, however, are relatively well preserved, and we can see that the claws on the first and second toes are unusually large and hooked, compared with both modern owls and Palaeoglaux. Indeed, there is no particular reason why owls should need large, sharp claws, since, like falcons, they kill prey by severing the neck with their beaks, using their toes only to hold the prey in place.
There are, however, birds that do have extra-large, sharp, claws on exactly these toes, and they are thought to be more closely related to owls than falcons are. These are the hawks, members of the family that includes eagles, as well as sparrowhawks, goshawks, and so on. These typically kill by tightly gripping prey with their feet until they puncture them with their claws. It seems, then, that Protoptynx hunted, not as owls do today, but rather as eagles do.
Given their size, they are unlikely to have used such a technique against mouse-sized animals (and actual mice didn't exist yet anyway). Quite what they did eat is, of course, impossible to say, but the northern goshawk (Accipiter gentilis), which is about the same size, feeds primarily on crows, pigeons, squirrels, and some larger birds, such as pheasants. None of those animals have fossils dating back quite so far - although squirrels come close - but they give an idea of the size range of prey items we're probably talking about.
Indeed, it may well be that the reason that we no longer have large, possibly day-hunting, owls with hawk-like claws is that the actual hawks seem to have made their debut just 5 million years or so after Protoptynx lived. The hawks apparently won out at that particular lifestyle, while those owls that were, perhaps, already shifting towards twilight or nighttime hunting found themselves without competition, eventually giving rise to those we have today.