The eyelids of birds
A major function of the upper and lower eyelids in birds is protection of the cornea from injury by air-borne particles, environmental hazards like foliage and by the bill in preening. The lids cover the eyes during sleep. Lubrication and cleansing of the corneal surface is largely done by the nictitating membrane. Though they contain a tough fibrous plate, the lids probably provide little protection from a major blow as from the flailing limbs of an animal prey. In other Classes of animal, the skull takes the brunt of the force as the eyeball retracts into the orbit. But birds have lost the ability to retract their eyes. Instead, in some raptors, protection is provided by having the eyes recessed beneath an overhanging brow.
Some birds have the equivalent of mammalian eyelashes, bristles, which probably have a similar role in protecting the eye by inducing reflex blinking when touched. Birds' eyelid bristles consist not of hair, but of specialized feathers in which the shaft (rachis) lacks barbs. Hornbills in particular have prominent eyelid bristles as seen in this Abyssinian ground hornbill (Bucorvus abyssinicus).
Tapered long eyelid bristles growing at an angle from the upper and lower lids of an Abyssinian ground hornbill. Perhaps these direct water running over the brows away from the eyes. The skin around the eyelids and orbit is bare.
The eyelid bristles also grow at an angle in this female plain-pouched hornbill (Aceros subruficollis).
Eyelid bristles more prominent in the upper lid than the lower, and growing at an angle, in a female plain-pouched hornbill.
The findings are similar in the male plain-pouched hornbill.
Sparse, mainly upper lid eyelid bristles in a male plain-pouched hornbill
As for above, but from a different angle
Long upper lid eyelid bristles are present in the white-crowned hornbill, Aceros comatus.
Sparse, mainly upper lid eyelid bristles in a white-crowned hornbill.
As well as having eyelid bristles, hornbills differ from most other birds in other ways. They have unusually large bills, sometimes with an additional structure, the casque, attached to the upper mandible.
Casque above the bill in a female great hornbill (Buceros bicornis).
Perhaps related to this, they have retained eye movements to a much greater extent than other birds.
Vertical and horizontal eye movements and a nictitating membrane blink in a female great hornbill. Played at one eighth speed.
Most birds change gaze by moving their heads - often very rapidly ('saccadic' head movements). The size and weight of the bill and casque in hornbills limit their capacity to do this and perhaps as a result they have preserved their ability to perform rapid eye movements.
A bird with a similar bill to the hornbill is the toucan. It also has preserved eye movements but no eyelid bristles.
Bare skin around the eye (eye ring), not clearly differentiated into upper and lower lids, though functioning the same in closing the eyes, and with no bristles, in a toco toucan (Ramphastos toco).
The question which remains unanswered, is what were the factors which led to one type of bird evolving eyelid bristles and another with similar morphology and diet (fruit, insects and small animals) to do without. In neither Orders of birds, are the eyeballs recessed, see below. In the case of toucans, the eye-ring is brightly coloured and perhaps serves a purpose in attracting mates.
Protruding eyes in a keel-billed toucan (Ramphastos sulphuratus), on the left, and a female wrinkled hornbill (Aceros corrugatus), on the right.
Hornbills are by no means the only Order of birds with eyelid bristles.
Upper and lower eyelid bristles in a hooded vulture (Necrosyrtes monachus).
A thicket of upper lid bristles with sparser bristles in the lower lid in an ostrich (Struthio carmellus).
In the case of the emu (Dromaius novaehollandiae), what look like bushy eyelid bristles are eyebrow bristles.
What appear to be bushy eyelid bristles.
Eyebrow bristles and rictal bristles (at the base of the bill).
In owls, there may be feathers and/or bristles on the edge and over the rest of the eyelids.
Feathers on the edge and over the surface of the eyelids of a Eurasian eagle-owl (Bubo bubo). There are also rictal bristles.
White bristles on the edge of the eyelid, black bristles over the surface of the eyelids and long feathers around the eyes and base of the bill in a Brown wood owl (Strix leptogrammica).
In owls there are often prominent rictal bristles, providing tactile sensation for these nocturnal predators.
Rictal bristles extending beyond the bill in a spectacled owl (Pulsatrix perspicillata).
Early warning of an object approaching the eye is particularly important in owls, which have prominent eyes.
Birds often have deep corneas, but the corneas in this spectacled owl could be called macro-corneas. Not all individuals in this species have such deep corneas.
Many birds have fleshy caruncles on their eyelids.
Grey caruncles on the eyelids of a cape cormorant (Phalacrocorax capensis) played at 10% speed.
Berry-like eyelid caruncles clustered on the upper and lower eyelids of a Guinea turaco (Tauraco persa).
More subtle are the caruncles on the eyelids of the top-knot pigeon (Lopholaimus antarcticus).
Mauve caruncles on the edge of the upper and lower eyelids of a topknot pigeon. Note also the patch of dark pigmentation on the part of the iris aligned to the bill which may have a role in reducing glare (Morris, JGL and Morris DN, Sectoral heterochromia of the iris in pigeons, Australian Veterinary J, 92: N21-22, 2014).
Caruncles may extend beyond the eyelids.
Yellow caruncles on the edge of the eyelids joining a large red one at the base of the bill in a banded lapwing (Vanellus tricolor).
In some birds, they cover much of the face.
Extensive continuous sheet of caruncle covering the face of a Muscovy duck (Cairina moschata).
Pelicans and parrots have no eyelid bristles.
Red eye-ring with no feathers in a spot-billed pelican (Pelecanus philippensis).
Bare skin with no bristles on the eyelids and in the peri-orbital region in a black-capped lory (Lorius lory). The bumps on the eyelid edges represent caruncles.
In hawks and eagles, there is great variation in the feathering of the eyelids.
Feathers cover the lower eyelid and peri-orbital region in a juvenile bald eagle (Haliaeetus leucocephalus).
In the wedge-tailed eagle (Aquila audax), the skin around the eyes is almost bare.
Bare skin around the eye in a wedge-tailed eagle (Aquila audax). Note the overhanging brow.
Sparsely covered skin and protruding eyes in a black breasted buzzard (Hamirostra melanosternon).
A bird with striking eyelid bristles is the pheasant coucal (Centropus phasianinus).
Eyelid bristles, and bristles interspersed with feathers all over the head and around the bill of a pheasant coucal.
In many birds, the skin around the eyes is brightly coloured, presumably to attract a mate.
Brightly coloured bare skin around the eyes in a male figbird (Sphecotheres vieilloti).
Comment on eyelids of birds
As with blinking, there has been marked diversification in the form which eyelids take across the avian species. And, as with blinking, the factors driving the changes are not always obvious. Eyelid bristles are striking in hornbills, ostriches, and some owls, vultures and cuckoos. Their role is likely to be protective, triggering eye closure when touched. But in most Orders of birds eyelid bristles are sparse or absent. Some have rictal bristles, which also have a protective role, particularly in owls which have poor near vision. Much more common than bristles in most birds are caruncles, often brightly coloured. Bare skin around the eyes, eye rings and irises are also often brightly coloured. Such colouring is easier to understand as it is likely driven by the need to attract the opposite sex.
Extending the comment to the eyebrows, it is a puzzle that while many raptors have deep set eyes protected from a flailing prey by heavy brows, others do not. Owls have protruding eyes, unprotected by brows, but encased, like some dinosaurs of old, in bony sclerotic rings.