Bubo virginianus – the call of the silent

     The cool November night creeps down my neck as I continue my search for the silent hunter. I carry my cellphone loaded with a few recorded hoots and a flashlight, along with my camera to prove my success should I spot the bird within the dark forest. I hike out until the ambient sounds of Tallahassee begin to dwindle and silence prevails. Then I play the territorial call of the Great Horned Owl in hopes of drawing out an opponent. “Hoo hoohoo hoo hooo hooo,” the resident replies. Excited I play another call and again the owl answers. So I try a third time, but with no luck. He must have been intimidated by the consistency and volume of my call. . . . or maybe he noticed that I was merely an impersonator, who knows. That’s just the way things go.
     Snooping around the Web of Science led me to a number of interesting articles related to Bubo virginianus, commonly referred to as the Great Horned Owl. The first of which came from Ruggeri et al. discussing the retinal structure of birds of prey.
     Great Horned Owls are nocturnal and prefer hunting at night. Birds and especially birds who hunt under cover of night need powerful eyes to catch prey and so have some unique adaptations to give them the upper edge. In general there are two types of photoreceptors: rods and cones. Rods are highly sensitive and more useful under low light conditions whereas cones require more light and are responsible for color vision. Foveae are areas of the retina associated with high photoreceptor density and therefore high visual acuity. Most diurnal avian species have 2 foveae, one being deep (central) and the other shallow (temporal). An exception to this is the owl which has only one fovea and a higher density of rods compared to cones, an adaptation present in nocturnal species. Optical coherence tomography (OCT) is used to visualize retinal structure. The images produced reveal distinctions between retinas of owls and other birds of prey. The outer segment of the retina in the eyes of birds of prey is much thicker than the inner segment and the difference is most dramatic in owls (Figure 8). This greater thickness suggests a higher density of photoreceptors and in the case of the owl these photoreceptors are primarily rods to aid in nocturnal hunting.
     Now that we know how it is possible for the owl to find its prey we might ask what  they shop for. Woodman et al. looked at owl pellets, regurgitated balls of everything that the bird is unable to digest from its last meal, to gain insight into the diet of a typical Great Horned Owl (Figure 1). Mammals and other birds make up the majority of its diet. Great Horned Owls often tear flesh from the bone leaving behind some or all of the skeleton. They may also remove cumbersome parts of the animal such as the skull or a large wing that would be difficult to digest. Bones that are consumed are usually crushed to aid in digestive breakdown, nevertheless pellets are usually produced because the owl cannot take care of every bone.
     Picking a suitable habitat for hunting and mating is an important part of any bird’s life. The Great Horned Owl prefers a well forested habitat, but urbanization has caused fragmentation of natural habitats and they have been found everywhere from heavy forests to densely populated cities. Grossman et al researched the affect of forest cover on owls and their distribution. The habitat showing the most abundance of Great Horned Owls turned out to be one with between 36% and 65% forest cover and with the greatest length of forest edge. My experiences with the Great Horned Owl have occurred in places similar to the habitats described above, two of them: Tom Brown Park and San Luis Park in Tallahassee are both well forested environments with adequate amounts of forest edge and both are surrounded by urban areas. My observations match the cited research and it seems true that even in populated areas the Great Horned Owl makes good use of the available forests.
     While maintaining a territory it is necessary to communicate with other would be rivals or to locate a mate. Owl vocalizations are inherited and not learned meaning that an owl raised in a human environment should still be capable of its natural types of calls. Karla Kinstler (owl caretaker and director of the Houston Nature Center in Minnesota, her website has a variety of Great Horned Owl audio recordings),  has demonstrated this fact by caring for a permanently injured Great Horned Owl, ‘Alice,’ for nine years. Although their calls are innate there is individual variation, each owl has a voice which may sound the same as the next owl to our ears, but other owls are able to tell voices apart and we may do the same by reading the spectrogram of a recorded call (Figure 2). Hoots are produced while the bill is closed and the gular sac expanded. Territorial hoots are produced by both sexes and are made repeatedly with variable spacing depending on how excited the owl is. Females are the larger sex but have a hoot of higher pitch because their syrinx is smaller than that of a male. Greeting hoots are given when arriving on a new perch or in the case of Alice when seeing her human friend after a period of absence. These hoots are short and quieter with only one or two notes. A staccato hoot often occurs when a pair of mating owls are together. It is a rapid hoot with short equally spaced notes frequently leading into a territorial hoot.
     Territories help to divide up the land amongst the owls to ensure each owl can find adequate prey and attract a mate. There are however some who float around and seem to be non-territorial, an odd strategy for the Great Horned Owl. The question Christopher Rohner asked was: why would an owl not have a territory and not breed?
It is possible that the territorial owl is not letting them breed. Another more likely hypothesis is that during the early part of an owl’s life it does not breed while learning the ropes and recognizing what it takes to maintain its own territory. It may not be a problem of owl overpopulation but instead might be thought of as an investment in future reproductive success. While the owl is non-territorial it is not necessarily non-breeding and it is entirely possible that they may be secretively acting as a secondary mate to an already mated owl and all without the high demands of defending a territory.

References:
Grossman et al. 2008. Responses of Great Horned Owls, Barred Owls, and Northern Saw-whet Owls to forest cover and configuration in an agricultural landscape in Alberta, Canada. Can. J. Zool. 86: 1165-1172. doi. 10.1139/Z08-095
Discusses the habitats of Great Horned Owls.

Kinstler, K. 2009. Great Horned Owl Bubo virginianus Vocalizations and Associated Behaviours. Ardea, 97(4):413-420. doi: 10.5253/078.097.0403.
Source of information on vocalizations and the spectrogram, Figure 2.

Rohner, C. 1996. Non-territorial Floaters in Great Horned Owls (Bubo Virginianus). Animal Behaviour, Vol. 53, Issue 5, p. 901-912. doi: 10.1006/anbe.1996.0381
This paper discusses Great Horned Owl territory and strategies of floaters.

Ruggeri et al. 2010. Retinal Structure of Birds of Prey Revealed by Ultra-High Resolution Spectral-Domain Optical Coherence Tomography. Investigative Ophthalmology & Visual Science, Vol. 51, No. 11. doi: 10.1167/iovs.10-5633.
This paper discusses avian vision and is the source of Figure 8.

Woodman et al. 2005. A Curious Pellet From a Great Horned Owl (Bubo Virginianus). Northeastern Naturalist 12(2):127-132. doi:10.1656/1092-6194(2005)012[0127:ACPFAG]2.0.CO;2.
This paper discusses owl pellets and is the source of Figure 1.

  

A couple of posing Great Horned Owls.
http://commons.wikimedia.org/wiki/Category:Bubo_virginianus (source of the two pictures of Great Horned Owls seen above)

Great Horned Owl pellet figure from Woodman et al.

Figure 2. Spectrograms of the types of hoots from wild and captive Great Horned Owls; origin of owls from Minnesota and Wisconsin, USA.
Spectrogram Figure from Great Horned Owl vocalizations paper by Kinstler.

FIGURE 8. Comparison of the enlarged OCT images of the retina among the short-tailed hawk (A), broad-winged hawk (B), great horned owl (C), and barred owl (D). Images (A) and (B) are close to the deep fovea. Images (C) and (D) are close to the base of the pecten. Image (A) consists of 40 A-lines, and im- ages (B–D) consist of 160 A-lines.  Figure from Ruggeri et al.