**9. Shared survival mechanisms and behaviour**

While these six unique adaptations combine to make the owl the highly efficient nocturnal predator that it is, the owl also shares several other adaptations and habits which contribute to its survival. Owls have cryptic coloured plumage made up of a mixture of soft browns, greys, black, and white and arranged in subtle markings of streaks and spots which serve to break up the bird's outline, rendering it almost invisible against its background [42]. Some species, such as the Eagle, Owl, and the Longeared Owl, also have feathered head adornments (ear, tufts, or horns), which help to break up the distinctive round shape of the owl's head [43], while some *Glaucidium* species have developed false eye markings, or an *occipital face* on the nape of their necks, made visible by the fluffing of the head feathers and tilting the head forward [44] (**Figure 17**). Studies have shown that these false eye markings are an effective countermeasure against daytime avian mobbing [45]. Overall, the camouflage of owls is incredibly effective against predators, against being mobbed by other birds during the day, and against being spotted by potential prey at night.

Roughly one third of all owl species are subject to colour polymorphism (colour morph), existing in genera, such as *Strix*, *Tyto*, *Megascops*, *Otus*, *Psiloscops*, *Lophostrix*, *Glaucidium,* and *Bubo*. The pigment melanin is responsible for many observed cases of colour morph, in which there is a great deal of variation within owls and while there are a number of hypotheses surrounding colour morph, the exact mechanisms which drive these variations remain unresolved [46]. One hypothesis, that apostatic selection drives colour morph in owls, where intraspecific colour variation should be promoted in predators by prey forming an avoidance image for the more common colour morph has been proven highly unlikely [47]. A more likely explanation is the niche variation hypothesis, where the species with broader ecological niches should be more variable compared with those with narrow niches because of the action of disruptive selection [48] and that it is an adaptive character likely maintained by the selective advantage of morphs under different environmental conditions via disruptive selection mechanisms [46].

Climate-related colour morph can be seen in the Eastern Screech Owl where individuals exhibit rufous, intermediate, or grey colouration that is likely caused by relative amounts or concentration of black or rufous melanin subtype (eumelanin and pheomelanin, respectively). This species exhibits clinal variation in morph prevalence; the rufous morph predominates in warm climates while the grey morph dominates in a less humid and colder environment (**Figure 18**).

The rufous morph of the Eastern Screech Owl is rarely seen in the northern areas of its range as the mortality rate is greater than that of the grey morph variant in conditions of extreme cold. It is also noted that females of the rufous phase have a

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

greater survival rate in much lower ambient temperatures than their rufous-coloured male counterparts [12]. This greater survival rate in females is probably due to reverse sexual dimorphism (RSD), where the female owl is larger and has more bulk than the male and can capture larger prey.

#### **Figure 17.**

*The occipital face of Ridgway's pygmy owl (*Glaucidium ridgwayi*). Photo: Bruce Marcot.*

#### **Figure 18.**

*Two colour morphs of the eastern screech owl. (*Megascops asio*) L – Rufous and R – Grey. Photo: Dick Daniels (http://carolinabirds.org/) https://creativecommons.org/licenses/by-sa/3.0/legalcode*

It can also be hypothesised that colour polymorphism in owls is an adaptive character likely maintained by the selective advantage of camouflage under different light regimes or in terms of physiological adaptation to environmental conditions via disruptive selection mechanisms. Under this hypothesis, climate change could bring about a dramatic change in the colour polymorphism of some northern species. The Tawny Owl (*Strix aluco*) is a colour polymorphic species with a grey and brown morph resident in the Western Palearctic. Studies in Finland have shown that in winter, the grey phase helps to avoid avian mobbing and predators more efficiently than the brown morph and therefore has a higher survival rate in snowy environments. However, as winters are getting milder and shorter in this species range due to climate change, the selection periods promoting grey colouration may eventually disappear [49].

Although some species of owls are specialist feeders, such as Fish Owls and Fishing Owls, and some have a definite preference for certain prey, such as the Barn Owl and the Short-eared Owl with voles, most owls are fairly generalist feeders, with prey as varied as rodents, birds, amphibians, insects and other invertebrates and, in a few opportunistic cases, even bats [50]. Not too long ago, because of their acknowledged diet of live prey, consisting of small vertebrates and invertebrates, it was widely accepted that owls did not scavenge, and any reported observation of this uncharacteristic behaviour was taken as an anomaly. However, recent studies have shown that carrion feeding by owls may be far more prevalent than once thought. In the past, because of their mostly nocturnal activity, dietary information had come mainly from pellet analysis while any observations of scavenging behaviour in owls have been rare and poorly documented. Today, however, with the increasing use of passive infrared wildlife camera traps, baited with a variety of carcasses, including roadkill, a surprising number of owl species have been recorded engaging in this behaviour in Europe, North America, South America, Asia, and Australia (but none, to date, in Africa) [51].

Species recorded scavenging include Barn Owl (*Tyto alba*), Eurasian Eagle Owl (*Bubo bubo*), Tawny Owl (*Strix aluco*), Great Horned Owl (*Bubo virginianus*), Snowy Owl Owl (*Bubo scandiacus*), Ural Owl (*Strix uralensis*), Powerful Owl (*Ninox strenua*), Western Screech Owl (*Megascops kennicotii*), Northern Hawk Owl (*Surnia ulula*), Long-eared Owl (*Asio otus*), Little Owl (*Athene noctua*), and Barred Owl (*Strix varia*). The recorded carrion ranged from feral pigeon to sheep and deer, and there is even a recorded case of a Brown Fish Owl scavenging on the carcass of a crocodile [52–54].

Owls hatch their eggs asynchronously as a survival mechanism against prey shortage. Incubation starts with the laying of the first egg, unlike many other birds that begin incubation with the laying of the last egg of the clutch. With asynchronous hatchings separated by anywhere from a few hours to several days, this gives the older nestlings a distinct advantage in begging for food. British ornithologist David Lack identified asynchronous hatching as an evolutionary adaptation to unpredictable changes in the food supply; if food declines abruptly during the nestling period, the youngest nestlings would die first without endangering the survival of the whole brood [55].

A small number of species, such as the Burrowing Owl, Short-eared Owl, Longeared Owl and the Eurasian Scops Owl have become seasonally migratory [56]. In North America, Northern Burrowing Owls enter primarily in the southern United States from California to western Louisiana, much of Mexico, and scattered sites southward into Central America [57]. While the soft, lightly oiled feathers of owls are not equipped for long flights over large bodies of water, in Europe, the Eurasian Scops Owl crosses the Mediterranean Sea on its long migration south of the Sahara Desert in Africa, while the Long-eared Owl and the Short-eared Owl regularly fly across the

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

North Sea from Northern Europe and Scandinavia to winter in the British Isles where they swell the numbers of resident birds [58].

Snowy Owls make nomadic winter movements and are also subject to irruptions; cyclic events triggered by fluctuations in rodent prey population levels [12]. These irruptions can be on such a large scale after a successful breeding season that huge numbers of young owls spread out from the Arctic Circle into southern Canada, Northeast America, and beyond. Although strong fliers, it seems that Snowy Owls are not averse to seeking any advantage in their dispersal. In October 2001, as many as 60 Snowy Owls boarded a ship near Deception Bay, North Quebec, during a severe gale, while a further three landed on another ship east of Newfoundland. Both vessels were heading for the port of Westerscheldt, on the Belgium/Netherlands border. A number of these owls remained on board for the trans-Atlantic crossing, with one individual eventually making it to Felixstowe in England [59].

Owls hitching a ride on ships is nothing new. In 1903, H.W. Henshaw sighted a Short-eared Owl landing on a ship 500 miles (800 km) northwest of the Hawaiian Islands, while in 1901, W.A. Bryan reported a short-eared owl boarding a steamer which plied between Honolulu and Puget Sound, while it was '680 miles off the mainland' [60]. The British Royal Navy has its own bird watching society, the Royal Navy Bird Watching Society (RNBWS), and since its formation in 1946, the RNBWS has been compiling a database of all birds recorded on British Navy vessels around the world. This database includes the names of the vessels and the geographical positions (longitude & latitude). In 2007, Lt. Cdr. Stan Howe, R.N. very kindly extracted a list of all the owls recorded by the RNBWS around the world from its database for me. There were 242 individual sightings listed, which included species, such as Barn Owl (*T. alba*), Eurasian Scops Owl (*Otus scops*), Striated Scops Owl (*Otus brucei*), Collared Scops Owl (*Otus bakkamoena*), Great Horned Owl (*B. virginianus*), Eurasian Eagle Owl (*B. bubo*), Spotted Eagle Owl (*Bubo africanus*), Snowy Owl (*B. scandiacus*), Tawny Owl (*S. aluco*), Barred Owl (*S. varia*), African Wood Owl (*Strix woodfordii*), Northern Hawk Owl (*S. ulula*), Little Owl (*A. noctua*), Spotted Owlet (*Athene brama*), Burrowing Owl (*Athene cunicularia*), Tenmalm's/Boreal Owl (*Aegoleus funereus*), Brown Hawk-owl (*Ninox scutulata*), Moluccan Hawk-owl (*Ninox squamipila*), Jungle Hawk-owl (*Ninox theomacha*), Long-eared Owl (*A. otus*), Short-eared Owl (*Asio flammeus*), and Marsh Owl (*Asio capensis*). The Long-eared Owl (*A. otus*) and Short-eared Owl (*A. flammeus*) are the most recorded species and there are a number of unidentified species listed only as 'Strigidae'. Anyone wishing to access the RNBWS database should visit the RNBWS website: https://www.rnbws.org.uk/science.

With a combination of its six unique adaptations and all these shared survival mechanisms, the owl is indeed one of nature's cleverest survivors.
