Tyto Robusta

''Tyto robusta'' was a prehistoric barn-owl. It lived at what is now Monte Gargano in Italy, and was an island throughout much of the Neogene when sea levels were higher. The owl's remains date back to the Miocene-Pliocene, giving a total length of about 50–65 cm for ''T. robusta''. This owl provides an interesting case study of evolution and insular gigantism.

Evolution, taxonomy and systematics

It was found sympatrically with the gigantic relative, ''Tyto gigantea'', which at some 220% of the size of the barn owl grew to some 70–85 cm in length—larger than a Eurasian eagle-owl and maybe up to twice as heavy. Like other apex predators they can be presumed to have occurred at low population densities. Being apparently endemic to Gargano Island and perhaps some other islands in today's Apennines, where one or two huge eagles and at least at times a normal-sized barn-owl also occurred (though no large mammalian carnivores), these owls raise the question how minimum viable populations could be maintained long enough for them to live and prosper.

It is extremely commonplace for larger predatory birds such as Falconiformes and Strigiformes to evolve differently-sized sexes. Namely, the females being considerably larger with little or no overlap in size between sexes. This allows them to hunt for different-sized prey, allowing a larger, more resilient population to live off a land. But this was apparently not the case here, as there is a reasonably large sample of bones of each of the 3 owl taxa to display the natural and sexual variation within each.

In the oldest fossil samples, no giant owls were found to accompany them, whereas the latest seemed to a range of owls attributed to all 3 species. The small owls supposedly were replaced by or evolved into ''T. robusta'' from some point onwards, and ''T. gigantea'' appeared later, supposedly alongside it as well as a recolonization by Continental European stock. That this was by no means a very parsimonious assumption was already noted in the original descriptions of the remains assigned to ''T. robusta'':

"If one assumes that the large barn-owls on Gargano would have arisen from multiple colonizations .. ''T. robusta'' would have to be separated into two distinct species. The medium-sized form of the first colonist cannot be the same as the middle-sized form of the second colonist. As there are no morphological findings to support this assumption, it was refrained from separating he ''T. robusta'' material intotwo species. In how far one can actually expect morphological differences in forms so closely related is debatable."

More recently, it was noted that the smaller barn-owl of Gargano was not ''Tyto sanctialbani'', which inhabited Europe north of the Alps in the latter Miocene, but its Mediterranean relative ''Tyto balearica'', which was a sister species or chronospecies of or a southward divergence from ''T. sanctialbani'' and decidedly more robust. In addition, it appeared as if the ''Tyto'' of the youngest samples fell into 2, not 3 size groups.

Presumably, the island was initially colonized by ''T. balearica'' whose core range eventually extended from the Apennines westwards to Iberia; they might have become cut off from the main population when the Mediterranean reflooded at the end of the Messinian salinity crisis 6 million years ago. The entire population evolved into larger ''T. robusta'', which were still close enough in size to exclude

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