**8. Variations on a theme**

While all owls share these unique adaptations, the evolutionary process of adaptive radiation has produced several variations within the many species [3]. These variations have been influenced by a combination of habitat, prey selection, and activity rhythm (nocturnal, crepuscular, or diurnal).

While all owls share the same ocular morphology, there is a limited variation in iris colour between the various species; either yellow, orange, or black/brown. A recent study has shown that dark eyes are to be found in 71 species belonging to 14 genera, whereas 135 species belonging to 20 genera were classed to have bright eyes (yellow or orange). Dark irises are more frequent amongst strictly nocturnal owls (41 out of 70 nocturnal species [59%]) than amongst owls that have diurnal or crepuscular activity rhythms (37 out of 131 diurnal or crepuscular species [28%]) [35]. The results of the study provided strong support for the existence of an evolutionary correlation between iris colouration and activity rhythm in owls. Beyond that correlation, the study did not find any clear evidence that dark eyes are more likely to evolve in species presenting strictly nocturnal habits than in diurnal species. However, it did find that the most likely explanation for the found patterns would be that dark eyes might be less conspicuous at night and help the owl in avoiding detection by predators or prey.

One of the most distinctive features of the owl is the facial disc (**Figure 5**). However, two groups of owls, Fish Owls and Fishing Owls, have evolved less defined facial discs (also completely lacking the facial disc ruff); almost to the point of being non-existent in the Fish Owl species. There are four species of Fish Owls, the huge Blakiston's Fish Owl (*Bubo blakistoni*), the Brown Fish Owl (*Bubo zeylonensis*) (**Figure 13**), the Tawny Fish Owl (*Bubo flavipes*), and the Malay Fish Owl (*Bubo ketupa*) while there are three species of Fishing Owl, Pel's Fishing Owl (*Bubo peli*), the Rufous Fishing Owl (*Bubo ussheri*), and the Vermiculated Fishing Owl (*Bubo bouvieri*). Fish and Fishing Owls have recently been moved from the Genera *Ketupa* and *Scotopelia* respectively to the Genus *Bubo* in the taxonomic listing because of their shared skeletal and phylogenetic characteristics with Eagle Owls [36].

#### **Figure 13.**

*Brown fish owl (*Bubo zeylonensis*). Photo: Manojiritty. Source: Creative commons: https://creativecommons.org/ licenses/by-sa/4.0/deed.en*

Fish Owls and Fishing Owls are nocturnal and crepuscular hunters and generally search for their aquatic prey from rocks or low hanging tree branches close to the water's edge or wade through the shallow water itself [37]. The less defined facial disc and the lack of the disc ruff (to enhance the acoustic locating of prey) suggest that these characteristics, which are common amongst most other owl species, do not increase the efficiency of hunting aquatic prey visually [37]. As well as lacking the distinctive facial disc found in other owl species, the Fish Owls and the Fishing Owls more or less lack another of the owl's unique adaptations, the comb-like leading edge of the flight feathers (fimbriae) which contributes towards the silent flight in owls [37, 38].

However, it is not just Fish Owls and Fishing Owls that lack the serrations on the leading edge of the flight feathers. A small number of other owl species also lack or have very much less developed fimbriae. These species tend to be primarily diurnal in their activity rhythm [39] and largely insectivorous; species such as the Little Owl (*Athene noctua*), Burrowing Owl (*Athene cunicularia*), Elf Owl (*Micrathene whitneyi*), and Northern Pygmy Owl (*Glaucidium californicum*). Fish and Fishing Owls have no tactical need for silent flight because sound does not travel well between air and water, while the diurnal species similarly have little need for silent flight as they are visible to prey. This would suggest that the vastly reduced fimbriae in these birds are an evolutionary holdover that lacks current function [23].

Further variations can be found in the hind limbs of owls. The extent of feathering on the legs and feet of owls varies from an almost bare tarsus and entirely bare toes to densely long-feathered tarsus and toes. The extent of this variation between species is dictated by geographic location and habitat [40]. An example of this would be to compare the sparsely feathered legs and feet of the grasslands and desert-dwelling Burrowing Owl (**Figure 14**) to the densely feathered legs and feet of the Great Grey Owl of the northern taiga/boreal forests (**Figure 15**).

In 1936, American ornithologist Leon Kelso identified and categorised five types of leg and foot feathering amongst owls, associating each type to a variety of Climatic zones [40]:

**Figure 14.** *Legs and feet of a burrowing owl (*Athene cunicularia arubensis*). Photo: Global owl project.*

**Figure 15.** *Densely feathered legs and feet of a great Grey owl (*Strix nebulosa*). Photo: Jari Peltomäki.*

1. *Toes and part of tarsus bare*. Tarsus is bare of feathers all the way around for part or all of its length. Toes entirely bare of feathers—associated with the humid, warm environment of the Tropical, Subtropical, and Temperate Zones; example of species: Cuban Bare-legged Owl (*Gymnoglaux lawrencii*).

*Designed for Darkness: The Unique Physiology and Anatomy of Owls DOI: http://dx.doi.org/10.5772/intechopen.102397*

2. *Toes bare*. Tarsus fully feathered and at least half of sides and the upper surface of toes bare of feathers—associated with the humid, warm environment of the Tropical, Subtropical, and Temperate Zones; example of species: Tropical Screech-Owl (*Megascops choliba*) and Northern Barred Owl (*Strix varia georgica*).

3. *Toes sparsely feathered or bristled*. Feathers or bristles somewhat thinly distributed over most of upper surface and sides of toes—well represented in all but the colder zones. In the Tropical, Subtropical, and Temperate Zones this type of feathering is more frequently associated with the arid parts of the zones; example of species: Barn Owl and Eastern Screech Owl (*Megascops asio*).

4. *Toes densely short-feathered*. The density of feathering is much greater than in the preceding type, sufficient to hide most of the upper surface of toes from view. Feathers short in comparison to the size of the bird, not tending to conceal part of claws—represented in all the life zones but includes a slightly higher percentage of the owl order in those zones which present a cooler environment, while in zones of greater heat and humidity it constitutes low percentages, example of species: Great Horned Owl (*Megascops asio*) and Short-eared Owl (*Asio flammeus*).

5. *Toes densely long-feathered*. Feathers long in comparison with the size of the bird, tending to conceal part of claws—associated with the colder and less humid environment of the Arctic, Hudsonian, Canadian, and Transition Zones; Example species: Great Grey Owl (*Strix nebulosa*) and Snowy Owl (*Bubo scandiacus*) [39].

Extremes in foot feathering in owls seem to be associated with zones that have extremes of climate and humidity. A perfect example of this is the extremely long and

**Figure 16.** *The underside of the densely feathered foot of a snowy owl (*Bubo scandiacus*). Photo: Roar Solheim.*

dense feathering of the feet of the Snowy Owl, which gives this ground-nesting owl perfect insulation against the cold Arctic climate and the frozen tundra (**Figure 16**).

The long-legged Burrowing Owl, which lives in an arid climate, has extremely sparse feather covering on its legs and feet, with the density of this covering varying between the subspecies. Generally, however, the female usually has a slightly heavier covering of plumaceous feathers on their upper leg than the male; the reason for this possibility is that the female spends more time than the male in the much cooler environment of the burrow chamber during the nesting season. The Burrowing Owl also has an extra adaptation to its hind limbs, giving it a longer step length and potentially faster limb movements for terrestrial locomotion and possibly for digging [41].
