Table of contents

September 2000 - Volume 84 - 9

Cover image

Cover image expansion

Cover illustration: Your eyes are bigger than your stomach

The new world screech owl, as typified by this eastern screech owl (Otus asio) seen on the cover, is an endearing species widely distributed throughout North America and with close relatives throughout the world (approximately 40 related species). This is the most abundant and perhaps most familiar owl in North America. About the size of a soda can and weighing much less, this attractive little bird (20-23 cm) is a fierce defender of its nest and has the nickname "feathered wild cat". Other species will be vigorously attacked near the nest if the bird fears intrusion. It will often kill animals larger than itself in the name of nest defence, and yet it can be gentle and playful with humans in other situations. The bird is also a fearsome nocturnal hunter. Although the principal foodstuff is insects, this magnificent bird can and will take rodents and even small birds, especially in winter when insects can be scarce.   These birds have tubular eyes as can be seen on the cover. Evolution has eliminated almost all extraocular musculature, to maximising the size of the globes. In essence these birds have sacrificed extraocular motility for improved visual acuity, especially at night. To compensate, most owls can turn their heads with great speed, nearly 270° atop a thin, lightweight spinal column; so quickly that myths of complete 360° cranial revolution have sprung up adding to the mythology of owls. As can be seen in the cranial slice taken at the midline of the cornea above the upper mandible, the eyes are large, both on an absolute and relative basis. They are frontally placed, providing as much binocular stereopsis as possible, as is typical for avian predators. The stereoscopic binocular visual field is approximately 60-70°, although the visual field is larger. The eyes together outweigh the brain as often occurs in raptorial species, especially those that are nocturnal wing feeders. This species is an "eye minded" one with an evolutionary concentration on visual abilities.   The tubular eyes (longer anterior-posterior than equatorial diameter) are asymmetric and structurally not as stable as our own. Consequently, they contain bony plates at the mid-portion of the globes much like girdles to help stabilise the ocular contents. These scleral ossicles are found in the region occupied by the cilary body and pars plana, and are not continuous, since a solid ring of bone could not grow with the eye. The retinal area is surprisingly small with the anterior edge of the retina corresponding to the widest equatorial diameter. The entire eye is outsized with a large lens, a deep anterior chamber, and a long anterior-posterior diameter. This longer "throw" of the image allows for sharper acuity with a larger image spread across the retina. The single fovea in each eye is deep walled, and the retina is duplex, but rod-rich. The pecten is small with fewer pleats when compared with the pecten of other birds and, similarly, accommodation is limited when compared with other avian species.   These owls are usually non-migratory, but may change feeding habits as prey species change with the seasons, as mentioned above. Their prey species have been documented to include flying squirrels, bats, birds, frogs, reptiles, and various invertebrates. Even ducks and poultry have been found in their nests. In essence, this bird is a indiscriminate carnivore. In more northern climates, they tend to remain in their nest cavities relying on cached food in the worst of the weather. Imagine how such a bird with a small crop and stomach must struggle to kill and eat prey species such as a duck (!), which may be nearly its own size and with a bill no less.   Screech owls are heard far more frequently than they are seen. Their melodic whistle beginning in early February in North America, is a sure sign that, despite cold and wintry weather, spring cannot be far behind.- IVAN R SCHWAB, University of California, Davis, Medical School Department of Ophthalmology

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