Visual acuity
There is an increasing body of evidence that supports the display hypothesis, which states that early feathers were colored and increased reproductive success. Coloration could have provided the original adaptation of feathers, implying that all later functions of feathers, such as thermoregulation and flight, were ed. This hypothesis has been supported by the discovery of pigmented feathers in multiple species. Supporting the display hypothesis is the fact that fossil feathers have been observed in a ground-dwelling herbivorous dinosaur clade, making it unlikely that feathers functioned as predatory tools or as a means of flight. Additionally, some specimens have iridescent feathers. Pigmented and iridescent feathers may have provided greater attractiveness to mates, providing enhanced reproductive success when compared to non-colored feathers. Current research shows that it is plausible that theropods would have had the visual acuity necessary to see the displays. In a study by Stevens (2006), the binocular field of view for "Velociraptor" has been estimated to be 55 to 60 degrees, which is about that of modern owls. Visual acuity for "Tyrannosaurus" has been predicted to be anywhere from about that of humans to 13 times that of humans. However, as both "Velociraptor" and "Tyrannosaurus" have a rather extended evolutionary relationship with the more basal theropods, it is unclear how much of this visual acuity data can be extrapolated.