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A. Introduction

Avian Of all , rely most heavily on I. General Shape and Structure vision (diurnal excluded)

Flight is very visually demanding but other Highly visual animals – large relative to head behaviors important as well! size

foraging European Starling – 15% of predator detection head mass

mate choice Human – 1% of head mass

Why large eyes??

Large eyes provide - larger and sharper images

Birds have 3 B. Shape • upper Avian eye has different shapes (unlike mammals) •lower „ Flat – majority of birds, similar to shape of • nictitating membrane – lizards, horizontal axis shorter than vertical has own lubricating duct, cleans and protects eye

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„ Globular – Falconiformes, many Passerines, „ Tubular – , some eagles, horizontal axis is axes are similar in length longer than the vertical

C. Internal Structure

„ Aqueous humor – clear thin liquid in anterior „ – transparent tissue covering the chamber, lower osmotic pressure exposed area of the eye „ Vitrous humor –clear jelly-jelly-likelike substance that „ – A thin sheet of striated muscle fibers and fills the posterior chamber of the eye , maintains connective tissue that form a diappghragm in front of , controls the amount of that enters shape of the eye posterior chamber „ Choriod coat –dark middle layer, richly supplied „ – structure at the base of the iris with blood vessels, absorbs radiant energy containing muscle fibers that contract to alter the shape of the lens

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II. Accomodation

„ – a layer composed of tough Altering of refractory apparatus so image falls fibers forming the white of the eyeball, on . supports shape of eye and attachment point for muscles, ossicles present in sclera A. Fish „ Retina – inner surface of the eye, sensitive to Lens moved forward and backward (camera) light, contains photoreceptor cells and is to change focal length continuous with optic nerve Cornea not involved (refractive index similar to water)

Refraction – the change of direction of a ray of light (ability of the eye to refract light so as B. Birds to form image on retina) 2 lens system – cornea and lens (only lens in mammals) SP – ray of light 1. Brucke’s Muscle – operates through ciliary SPL – original direction ligament S Q SPR – refracted ray Contraction causes annular pad to press against lens (increases convexity) QQ - perpendicular

L Q R

2. Crampton’s Muscle – contraction increases insert picture curvature (thus refractive power) Most terrestrial (hawks and owls) muscle causes cornea to become more convex a. CiddjttlCornea provides coarse adjustment, lens provides fine adjustment

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b.aquatic diving birds – cornea can’t be used for C. Lens focusing underwater (similarity of refractory index between cornea and water), muscle reduced. Lens shape varies more among birds than any other vertebrates. c. some diving birds have muscular iris that presses against lens to further increase convexity (mergansers, cormorants)

• flat anterior, convex posterior (Parrots) D. Iris • convex both anterior and posterior (ducks, owls, nighthawks) varies from deep brown (most common) to bright (Red-eyed Vireo, American Coots), benefits not understood white, (Brown Thrashers), and .

May function in species recognition.

III. Retina Retina has 3 types of photoreceptors • rods – black and white vision

Retina is avascular (different from mammals) – • cones – color vision prevents shadows and light scattering. • double cones

Made possible by unique structure - Pecten

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5 major classes of retinal neurons are recognized • photoreceptors • bipolar cells • horizontal cells • amacrine cells • ganglion cells

Nocturnal species have made entirely of Many of cone cells are rod cells, “double cones” Diurnal species have duplex retinas. All vertebrates with exception of placental mammals have this cell type.

Consists of principle cone and accessory Single cones as well as one or both double cones cone may contain an oil droplet – common feature in cones. Consist of lipids in which carotenoid pigments are dissolved

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Droplets can be transparent or clear, pale yellow, Droplets positioned at distal green, orange, or red. end of inner cone segment. Light must pass through Light must pass through droplet to reach visual before entering ppgigments – acts as filter eliminating other photosensitive outer wavelengths segment

• Red oil droplets found in red sensitive cones • Blue and violet sensitive cones have clear oil • Yellow droplets found in green sensitive cone cells droplets, common in aerial feeders that work and in part of the retina used in distant vision. Used against a blue sky as haze filter • UV vision found within birds, suggested that oil droplets screen out UV - so cones without oil sensitive to UV

House Finches have little UV perception and only UV perception 5% of cone cells lack oil droplets • serves signaling function • cue discriminating foods (plants, seeds, flowers Hummingbirds 15% of cone cells lack oil droplets etc.) • may play role in navigation as to coloration of sky. Short-wavelength gradients would vary depending on sun’s angle in sky.

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Bennett et al. 1997 Under any lighting conditions females had repeatable tastes Do females use UV in mate • Without filter, females preferred males that had selection? lowest UV reflectance Each female allowed to assess • With filter, used other characteristics 4 males either behind a UV filter or not. “UV got great feathers baby!” Measured how long female stood in front of each male.

Consequently, birds have greater range of color Some nocturnal species (owls, goatsuckers) have a vision layer at back of eye called “tapetum lucidum”

Acts as mirror & reflects light back through retina making it more likely light will strike sensory cells

Vision greatly enhanced under low light – produces the “eyeshine” of nocturnal animals

C. Areas and Foveas Cone density in fovea Area – region of retina where cones are densely Humans = 38,000/mm2 packed, and no rods – provides greater acuity Hawk = 65,000/mm2 Fovea – is ppyit in retina and may be associated with area Increased is related to structure of the fovea

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Visual acuity – ability to discriminate two points American Kestrel as separate entities from a distance. Recognizes insects (2mm long) from distance of 18 m!

Humans comparable to some birds Passerines, raptors, vultures 2.5 – 3 x that of humans

bifoveal – birds that require good distance afoveal – chicken, California Quail, Crested estimation have both a central and temporal Guineafowl fovea. monofoveal – most birds have single fovea Terns, swallows, swifts, hummingbirds positioned in center of retina Provides good depth perception Owls single fovea temporally located

IV. Pecten B. Function A. Structure Specific function has been elusive for ~ 200 years Found in both birds and Highly vascularized! reptiles Provides blood supply for nutrients and O2 located with base positioned exchange over “blind spot”

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„ shading of retina: But shape is variable! eliminates light scattering so that bright images are in greater contrast „ detection of motion: images emerge between lamellated shadows (rapidly appears, Kestrel Blue Jay disappears) „ orientation and navigation:

Does it have other functions????

V. Monocular vs. Binocular Vision shadow cast by pecten from sun falls on either Eyes of most birds on side of head – the central or temporal fovea accomodates images simultaneously (monocular vision) Using the position of sun could provide means Provides wide ((gRock Pigeons 300E, to orient American Woodcock 360E)

Owls have eyes located in front giving much Trade-off associated with monocular vision – wider binocular field of vision difficult for depth perception 180E field of view overall – most binocular Binocular field of view occurs directly in front (and behind for Woodcocks)

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