Blinks on pecking | Winks and Blinks

Blinking and squinting on pecking and with drowsiness

A striking feature of some birds is that they blink when they peck. With each peck, just before the moment of impact, the nictitating membrane deploys. Examples are provided in a chicken (Gallus gallus domesticus) and a domestic turkey (Meleagris gallopavo).

Four pecks associated with nictitating membrane blinking in a chicken, played at 10% speed.

130ms after the head begins to move and immediately before the beak strikes the fruit coated surface, the nictitating membrane deploys. The lower lid also rises a little.

Blink on pecking in a domestic turkey. The nictitating membrane appears 200ms after the head movement begins.

Zebra doves (Geopelia striata) also blink on pecking, but using the upper lid as well as the nictitating membrane.

Brief upper lid blink on pecking in a zebra dove. It is first seen in the 40ms frame and has ended after the strike, in the 160ms frame. If you look closely, it is possible that the eyelids retain a small gap between them even at maximal blink.

Upper lid blinking on pecking in a laughing dove (Spilopelia senegalensis). Played at 20% speed.

Crested pigeon (Ocyphaps lophotes) squint on pecking

The upper and lower lids have come together while pecking and the pupil is visible

Zebra dove (Geopelia striata) squinting during pecking

Wonga pigeon (Leucosarcia melanoleuca) squinting while drowsy.

The upper lid descends, then the lower lid rises to meet it, the (palpebral) fissure between them moves up and down. In the bottom left picture where the bird is squinting, it is unlikely that this is improving the depth of focus as the nictitating membrane is across the eye.

White-headed pigeon (Columba leucomela) squinting while drowsy

The upper lid descends, then the lower lid rises to meet it, the (palpebral) fissure between them moves up and down. Again, although the bird is squinting it is unlikely that this is improving the depth of focus as the nictitating membrane is across the eye.

Bush stone curlew (Burhinus grallarius) squinting while drowsy

The nictitating membrane crosses, the lower lid rises, the eye wobbles (saccadic oscillation) then the nictitating membrane withdraws and the squint persists

Tawny frogmouth (Podargus strigoides) squinting while drowsy

The lower lid rises slowly until the palpebral fissure is reduced to a slit. The pupil remains visible. At the end there is an upper lid blink.

Discussion
During pecking, in species such as hens the nictitating membrane covers the eye as the bill approaches its target. This, presumably, protects the cornea to some extent from debris flying off during the peck. In species with mobile upper lids, the upper lid descends and the lower lid rises during a peck. The slit between them may rise and fall, the two lids working in unison. In drowsiness, the same thing may happen and with either pecking or drowsiness, the nictitating membrane may be visible in the slit between the lids. It has been found that often the eyes do not close completely during pecking in pigeons raising the possibility that effectively reducing the vertical aperture of the pupil increases depth of perception (Ostheim et al, 2020). But the evidence for a role other than eye protection is not strong.

Reference
Ostheim, J., Delius, J.A.M. Delius, J.D. Eyelid squinting during food-pecking in pigeons. J. Exp. Biol. 2020, 223, jeb.223313.