Owls possess large forward-facing eyes and ear-holes; a hawk-like beak; a flat face; and usually a conspicuous circle of feathers, a facial disc, around each eye. The feathers making up this disc can be adjusted in order to sharply focus sounds from varying distances onto the owls' asymmetrically placed ear cavities. Most birds of prey have eyes on the sides of their heads, but the stereoscopic nature of the owl's forward-facing eyes permits the greater sense of depth perception necessary for low-light hunting. Although owls have binocular vision, their large eyes are fixed in their sockets—as are those of other birds—so they must turn their entire head to change views. As owls are farsighted, they are unable to see clearly anything within a few centimeters of their eyes. Caught prey can be felt by owls with the use of filoplumes—like feathers on the beak and feet that act as "feelers". Their far vision, particularly in low light, is exceptionally good.
Owls can rotate their heads and necks as much as 270 degrees. Owls have fourteen neck vertebrae compared to seven in humans which makes their necks more flexible. They also have adaptations to their circulatory systems, permitting rotation without cutting off blood to the brain: the foramina in their vertebrae through which the vertebral arteries pass are about ten times the diameter of the artery, instead of about the same size as the artery as in humans; the vertebral arteries enter the cervical vertebrae higher than in other birds, giving the vessels some slack; and the carotid arteries unite in a very large anastomosis or junction, the largest of any bird's, preventing loss of blood supply from being cut off while they rotate their necks. Other anastomoses between the carotid and vertebral arteries support this effect.[1][2]
The smallest owl—weighing as little as 31 grams (1 oz) and measuring some 13.5 centimetres (5 in)—is the elf owl (Micrathene whitneyi).[3] Around the same diminutive length, although slightly heavier, are the lesser known long-whiskered owlet (Xenoglaux loweryi) and Tamaulipas pygmy owl (Glaucidium sanchezi).[3] The largest owl by length is the great grey owl (Strix nebulosa), which measures around 70 cm (28 in) on average and can attain a length of 84 cm (33 in).[3] However, the heaviest (and largest winged) owls are two similarly-sized eagle owls; the Eurasian eagle-owl (Bubo bubo) and Blakiston's fish owl (B. blakistoni). These two species, which are on average about 2.53 cm (1.00 in) shorter in length than the great grey, can both attain a wingspan of 2 m (6.6 ft) and a weight of 4.5 kg (10 lb) in the largest females.[3][4][5][6][7]
Owls possess large forward-facing eyes and ear-holes; a hawk-like beak; a flat face; and usually a conspicuous circle of feathers, a facial disc, around each eye. The feathers making up this disc can be adjusted in order to sharply focus sounds from varying distances onto the owls' asymmetrically placed ear cavities. Most birds of prey have eyes on the sides of their heads, but the stereoscopic nature of the owl's forward-facing eyes permits the greater sense of depth perception necessary for low-light hunting. Although owls have binocular vision, their large eyes are fixed in their sockets—as are those of other birds—so they must turn their entire head to change views. As owls are farsighted, they are unable to see clearly anything within a few centimeters of their eyes. Caught prey can be felt by owls with the use of filoplumes—like feathers on the beak and feet that act as "feelers". Their far vision, particularly in low light, is exceptionally good.Owls can rotate their heads and necks as much as 270 degrees. Owls have fourteen neck vertebrae compared to seven in humans which makes their necks more flexible. They also have adaptations to their circulatory systems, permitting rotation without cutting off blood to the brain: the foramina in their vertebrae through which the vertebral arteries pass are about ten times the diameter of the artery, instead of about the same size as the artery as in humans; the vertebral arteries enter the cervical vertebrae higher than in other birds, giving the vessels some slack; and the carotid arteries unite in a very large anastomosis or junction, the largest of any bird's, preventing loss of blood supply from being cut off while they rotate their necks. Other anastomoses between the carotid and vertebral arteries support this effect.[1][2]นกฮูกน้อยที่สุด — ชั่งน้ำหนักเพียง 31 กรัม (1 ออนซ์) และวัดบาง 13.5 เซนติเมตร (5 นิ้ว) ซึ่งเป็นนกฮูกเอลฟ์ (Micrathene whitneyi) [3] รอบขนาดความยาว แม้ว่าจะหนักกว่าเล็กน้อย มีน้อยรู้จักเค้ายาวหัวโขน (Xenoglaux loweryi) และ Tamaulipas นกฮูกแคระ (Glaucidium sanchezi) [3 นกฮูก]ที่ใหญ่ที่สุดตามความยาวดีเทานกฮูก (เสียงที่สมจริง nebulosa), ซึ่งมาตรการประมาณ 70 ซม. (28) เฉลี่ย และสามารถบรรลุความยาว 84 ซม. (33) [3] อย่างไรก็ตาม ที่หนักที่สุด (และปีกใหญ่) นกฮูกมีสองขนาดคล้ายนกอินทรีนกฮูก นกอินทรีนกฮูก (เนื้อหาเนื้อหา) และของ Blakiston ทึดทือ (B. blakistoni) สองสายพันธุ์เหล่านี้ ซึ่งมีเฉลี่ยประมาณ 2.53 ซม. (1.00) สั้นยาวกว่าเทาดี สามารถทั้งสองบรรลุปีก 2 ม. (6.6 ฟุต) และมีน้ำหนัก 4.5 กิโลกรัม (10 ปอนด์) ในเพศหญิงที่ใหญ่ที่สุด [3] [4] [5] [6] [7]
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