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 1E53 Cost Mom Strael 
 RochwlBf. Htm rork i 
 (716) *e2 -0300- WiQfi 
 (Tie) 288-8969 -Fa. 
 
Th£ E^-ES OF THE BURROWING OWL 
 
 WITH SPECIAL MTERENCE TO THE FUMDUt OCUU 
 
 ■j^ WOOD, M.D. 
 Chicago. III. 
 
 REPRINTED FROM 
 >NTRIBUT10NS TO MKHCAL AND BIOLOGICAL RBSBARCH, 
 DEDICATED TO SIR WILUAM OSLBR, IN HONOUR OF 
 . }JI8 SEYENJIBTH BIRTHDAY, JULY I J, 1919, 
 
 i a ii • ■''■■'- ■ ■ ■ '-• ' — ■-^- ■— ^^ — ^ 
 
liR. t Asey WOOD 
 
 HUHrnrri I'nlvaniir P. «.. 
 <iilir»rnU 
 
 THE EYES OF THE BURROWING OWL 
 
 WITH SPECIAL REFERENCE TO THE 
 FUNDUS OCULI 
 
 BY 
 
 CASEY WOOD, M.D. 
 
 Chicago, III. 
 
 REPRINTED FROM 
 
 CONTRIBUTIONS TO MEDICAL AND BIOLOGICAL RESEARCH, 
 
 DEDICATED TO SIR WILLIAM OSLER, IN HONOUR OF HIS 
 
 SEVENTIETH BIRTHDAY, JULY 12, 1919, BY 
 
 HIS PUPILS AND CO-WORKERS. 
 
I 
 
 THE EYES OF THE BURROWING OWL 
 
 WITH SPECIAL REFERENCE TC 1ME FUNDUS OCULI 
 
 By Casky a. Wood, M.D., Chicago. III. 
 
 OF all the Strigiformes there is none so interesting from the 
 standpoint of the visual apparatus as that widely distrib- 
 uted New World group— the Burrowing Owls. These birds 
 are found (as the typical species, Sptotyto cuniculnria) through© it 
 the oan-.piis regions of Central South America and w.casionally far- 
 ther south; and are well known, as e subspecies, in Florida (Spiofyto 
 c. floridana), the West J-idia Islands, and on the plains and in the 
 valleys of North America (Sptotyto c. bypogga) as far north as 
 British Coliimbia. 
 
 Of the numerous subspecies may be mentioned also the small, 
 pale-brown, insectivorous, Short-Winged Burrowing Owl (Speotyto 
 c. bradyyttra) inhabiting th^ island of SanU Margarita, Venezuela; 
 and the Haitian form— 5p««yto c. dominicmsis. The Florida Owl 
 is also seen in the Bahamas. Although smaller than the typical 
 spocies, it has larger feet and bill; the plumage is, on the whole, 
 darker, v.ith clear white spots. 
 
 All Burrowing Owls are comparatively small (less th.n 1 1 inches 
 in length), but this peculiarity k .lot so noticeable on account of 
 their unusually long, bare legs. Their h.ibits are said to be mainly 
 diurnal, but observations of the North American species by the 
 wr:ter incline him t the belief that they are essentially nocturnal 
 animals, like most owls. Burrowing habits seem to be common to 
 all the species,. 
 
 Burrowing owls have a dull-brownish, spotted, and barred 
 1 nage; the middle of the chest is white and is partly encircled by 
 a plainly marked buff-brown collar. The liead is rather flat and small, 
 the facial disks are not well defined, the bill is short, and the wings 
 are relatively undeveloped, sc the bird is able to fly only short 
 distances. 
 \-T\^4\ 8.8 
 
Big 
 
 THE EYES OF THE BURROWING OWL 
 
 The food of these owls consists almost entirely of mice, gophers, 
 and similar mammals, as well as of small reptiles and insects. Of 
 these a family of owls, commonly ten in number, will consume 
 enormous quantities; each member disposing of his own weight 
 of pabulum in twenty-four hours! 
 
 According to Knowlton and Ridgway (i) the Burrowing Owl is 
 more or less migratory, and after a return to its usual habitat mates 
 (probably for life) and then arranges its underground nest. The 
 contour of these nesting burrows varies; they are usually about 15 
 inches wide and from j to 10 feet in length. They enter the ground 
 in a diagonal direction for a few feet and then turn at an angle either 
 to the right or to the left. The nesting chamber (la to 18 inches in 
 width), placed at the highest part of the burrow, is mostly lined 
 with dry dung, but sometimes with hair, feathers, or dried grass. 
 The eggs are six to twelve (generally eight) in number; their 
 color, when unsoiled, is glossy white. 
 
 It must be remembered that while the great majority of these 
 owls dig their own burrows, many of them, especially the northern 
 varieties, make use of the abandoned holes of certain mammals, 
 notably of the prairie dog, fox, badge>, ' jnk, and ground squirrel. 
 Doubtless in the latter instances the Burrowing Owl may enlarge 
 or otherwise alter the size, length, and othei dimensions of the newly 
 acquired hole. 
 
 The Burrowing Owi is not over-clean in his habits; the nesting 
 chamber and the remainder of the burrow are often filthy and foul 
 smelling. 
 
 The writer has studied the habits of the North American and 
 Florida subspecies, with special attention to the eyes and eyesight 
 of the bird. From these observations he concludes that this owl, 
 like all the others, is a true night bird, adapting itself with but slight 
 success to daylight conditions. In spite of the fact that Bendire and 
 Hudson refer to the animal as a diurnal owl, their accounts of its 
 habits really bear out the writer's contention of a nocturnal animal 
 with fairly good ('ly vision, yet distinctly embarrassed, uncertain, 
 and confused when the eyes are exposed to bright sunlight. Stress 
 is laid by a number of observers upon the fact that this owl is seen 
 at all times of the day standing guard, often on a little mound of 
 earth in front of his burrow entrance, forgetting that, as a much 
 
Th* Fundus OcuU of the Burrowing Owl-.S>„(„(„ cumculurh hijiM^mt. 
 
! 
 
 THE EYES OF THE BURROWING OWL 
 
 820 
 
 of t^! ,W T'' °^ ^ 'f ''" ^"'^"^'^^ ^*"''y °f the visual apparatus 
 him that the bird ever performs an act requirinR distinct dZ.7j 
 vision. Certainly the northern bird is decidedly noctj nXoct on 
 ally using h.s eyes, but at a disadvantage, during daylight hou ? 
 
 This conclusion ,s confirmed in a noteworthy fashion by a com 
 pa .son of the fundus oculi of this owl with the same pLure Tn 
 owls entirely nocturnal in their habits, and indeed with certa n 
 other evidence (especially that they all show orange or redS 
 fundi) constantly found in night animals. 
 
 These facts have been fully stated by G. Lindsay Johnson (2) 
 as regards he mammalia; and by the writer (3) for the alTeye 
 
 As m all owls, the eyeballs are set well in front and surrounded 
 
 S^St:Z:u^- '-" ''' - °^ '■>« '•^'^-dtn^d 
 Stngiform eyes more closely than those of any other order r, 
 
 reWon toXrT' T' '''' "T"^' ^^ '" -"-'^- "h-amj 
 ^InTn LI^ ^/ ! °ir r'' "' ^° P'''^"' ^^ to obtain binocular 
 vision .n/ron« Structurally, of course, birds' eyes are quite different 
 especially in the morphology of the eyeball, in'^the possession by the 
 owl of a pecten instead of retinal vessels, in the coCed optic neve 
 
 sLTt"'' M " Tr^""" "^'^ '" " "°t proper to spedfy her? 
 
 Slonaker (4 and the writer (5) have pointed out that all rh^ 
 
 owls are exceptions to the rule that the retinal area 0?^ " sL„ 
 
Sai 
 
 THE EYES OF THE BURROWING OWL 
 
 is in birds with a single fovea placed above and towards the nasal 
 aspect of the optic nerve entrance. The owls possess a single, deep 
 fovea encircled by a round, sharply defined area located above 
 and on the temporal side of the optic disk. This arrangement closely 
 approaches the binocular macule of man. 
 
 The writer gives to the owls a class by itself in describing these 
 ares and affirms that the temporal monomacular fundus is found 
 almost exclusively in the owls. 
 
 In conjunction with Arthur W. Head, F. Z. S., the South Ameri- 
 can Burrowing Owls in the London Zoological Gardens were closely 
 studied and examined with the ophthalmoscope. In addition, the 
 interior of the eyeball , 
 both of that type and 
 ofseveral North Amer- 
 ican individuals were 
 examined by the writer 
 in prepared specimens. 
 
 The ophthalmo- 
 scope in particular 
 shows the fundus oculi 
 of Speolyto cuntcu/ario 
 to be that of a typical 
 nocturnal animal. The 
 picture of this bird's 
 background is well shown in the accompanying colored plate, 
 painted by Head and faithfully reproduced here. The ocular 
 fundus of this species is irregularly round, as in all the owls, and 
 in prepared specimens these details show distinctly. 
 
 The single, temporal, oval macular region lies above and about 
 a disk length and a half from the upper end of the papilla. In the 
 center of the macula is the fovea— a dark pigmented spot with fine 
 granules arranged cap-like above it. Outside this, again, is the ovoid 
 circumference of this region, incompletely edged with fine dots. 
 These are more numerously distributed below the macula than 
 above it. Connected with the macular region is a light-colored and 
 rather broad band that extends horizontally to the center of the 
 visible background. It is unevenly divided into two strips by a 
 parallel arrangement of minute pigment dots. 
 
 Lateral View of the Pecten of the Burrowing Owl- 
 Speolyto cuniculana bypogxa. x 9- 
 
THE EYES OF THE BURROWING OWL 
 
 822 
 
 Seen from above, the relatively small perten closely resembles a 
 disarticulated, acuminate leaf, the stem representing the spinous 
 projection immediately above the lowest terminal convolution. The 
 light double folds of the marsupium slope backwards and cover most 
 of the optic entrance; they meet above in a uniform, very narrow, 
 slightly undulating crest whose posterior end projects half the height 
 of the underlying coil well into the vitreous cavity. An extension 
 upwards of the long axis of the disk cuts the retinal band at the 
 junction of the inner and second fourth, making an infulapapillary 
 angle of, perhaps, 40°. 
 
 This interesting owl is especially subject, like other Strigiformes, 
 to pathological variations in the fundus picture after confinement 
 and domestication. Both Head and the writer examined a number 
 of individuals that undoubtedly exhibited choroidal disease and 
 other pathological changes. Rejecting these, the general color of 
 the fundus of this species is found to be dull-orange, mottled and 
 blotched in its upper half with deep orange-red. Choroidal vessels 
 are plainly visible, covering all the lower part of the eyeground, just 
 as in the Tawny Owl. The well-defined macular area is seen within 
 the outer half of the fundus, a little above the upper extremity of 
 the optic disk. It is distinguished from the surrounding choroid by 
 a collection of minute pigment granules or dots with a bright, white 
 spot in their center. 
 
 The optic disk is white and of oblong shape, slightly rounded at 
 the ends. From its edges run a few short nerve fibers that form a 
 complete fringe about the visible papilla. 
 
 The pecten is decidedly larger in proportion to bodily measure- 
 ments than one finds it in most of the larger owls, especially larger 
 than in the Tawny Owl. It extends well forward into the vitreous, 
 and its lower half appears very massive and of a dark brown color. 
 The pectinate convolutions are plainly seen and the anterior or 
 upper half is more delicate in structure, being perforated where it 
 joins the disk. Here it forms a dark network on the surface of the 
 nervehead, where, also, a few red granules mingle with the chocolate- 
 brown texture of the pecten. 
 
8a3 THE EYES OF THE BURROWING OWL 
 
 BIBLIOGRAPHY 
 
 I. "Birds of the World," p. 537. 
 1. Phil. Tr., Lond., 1901. 
 
 3. "Fundus Oculi of Birds," Chicago, 1917. 
 
 4. Jr. Morpb., 1897, XIII, 44;. 
 
 5. Am. Encyc. Opbtb., 1914, IV, 2JI9. 
 
 Paul B. Hoebek, 67-69-71 East sgrH Stkeet, New York.