Phasic and tonic blinks | Winks and Blinks

Characteristics and functions of phasic and tonic blinking in birds

Summary

It is not possible to observe birds at close quarters without noticing their blinks. Yet this commonplace but curious phenomenon has received almost no attention. The aim of this study was to characterize the following features of blinks: a) which of the three structures (upper eyelid, lower eyelid and nictitating membrane) are involved in spontaneous blinking, and b) what is the relationship of each type of blink to sleep, pecking, preening and head movements. Video recordings were made during daylight hours of 311 species from 26 orders and the type of blink, duration and circumstances of the blink documented. Two main types of blink were seen:

1) Phasic blinks. These most commonly involved the nictitating membrane alone, were brief and occurred mainly on shifts in gaze. They also occurred at rest and in some species were observed on pecking. They were seen in 183 species. In 35 species, while the blink involved the nictitating membrane there was also minimal lowering of the medial aspect of the upper eyelid. In 86 species, predominantly from the orders Psittaciformes, Strigiformes and Columbiformes, it was an upper eyelid blink though the nictitating membrane could usually be seen to participate. The three types of phasic blink had mean durations varying from 121-216ms with wide ranges.

2) Tonic blinks. These involved the lower eyelid, were more prolonged than phasic blinks and particularly occurred with drowsiness and on preening. The mean duration of the tonic blinks was 2955ms again with a wide range. They were observed in 85 species across most orders. During such blinks, the nictitating membrane was also deployed though this tended to be obscured by the lower eyelid. In species with mobile upper eyelids, the upper and lower lids sometimes moved up and down in unison while maintaining eye closure; phasic upper lid blinks also occurred during the course of a tonic lower lid blink.

In 44 species, arcades of blood vessels were seen in the nictitating membrane during a blink.

Blinks were also studied in four species of crocodilia, the closest living relatives of birds. These were of longer duration than in birds but again fell into two types, nictitating membrane blinks and more prolonged lower eyelid blinks. The upper eyelid only moved passively with eyeball retraction.

Blinking fulfills a number of different functions and in birds it appears that these are met with two types of blink. The phasic blink maintains the tear film on the cornea and clears debris. It also protects the eyes during pecking. The tonic blink has a mainly protective role in preening and somnolence. As a prelude to sleep, tonic blinks lead to sustained eye closure and exclusion of light. The vascularity of the nictitating membrane may contribute to the maintenance of respiration of the cornea when the eyelids prevent access to the air for prolonged periods. Possible reasons why upper eyelid blinks evolved in some species are discussed.

Introduction

There have been few detailed studies of blinking in birds. In one of the earliest accounts, Owen (1866) noted that, like reptiles, birds have three eyelids. The upper and lower lids move vertically. The third, the nictitating membrane, is drawn horizontally or obliquely across the cornea by the combined action of quadratus nictitantis and pyramidalis nictitantis, muscles which lie behind the eyeball. The membrane returns to its resting position on the medial side of the orbit by virtue of its elasticity. He noted that the lower eyelid rises in sleep. Owls and nightjars blink with their upper lids. The upper eyelid is elevated by levator palpebrae superioris and the upper and lower eyelids are brought together by the action of orbicularis oculi.

In an anatomical study, Stibbe (1923) contrasted the nictitating membrane blink of birds with that of mammals. In the latter, the membrane is not actively drawn across the eye by the contraction of muscles. Rather, it lies in a state of elastic tension out of sight on the medial side of the globe, springing across when the eyeball retracts by the action of the retractor bulbi muscle. In cats and dogs, this only occurs during blinking with the eyelids. The lower lid is depressed by the action of depressor palpebrae inferioris.

Blount (1927) measured the inter-blink interval in a wide range of mammals, some reptiles and amphibia, and in four species of birds. In an Eagle owl, often the blinks involving the eyelids or nictitating membrane were unilateral. Of relevance to the present study, he noted that in a parrot, the lower lid was much slower to return to its resting position than the upper lid. Fish do not have eyelids and Blount surmised that blinking evolved in air-breathing animals as a means of preventing desiccation of the cornea by exposure to air.

Mowrer (1932), struck by the association between blinking and head movements, proposed that blinking may have a role in stabilizing vision by preventing images blurred by rapid head movement from reaching the retina. Yorzinki (2016), in a study of peacocks using eye-tracker technology, reported that their blinks appear to coincide with large gaze shifts during which visual information is already suppressed. Blinks are suppressed under conditions of high alert in the presence of a predator.

Kirsten and Kirsten (1983) measured the spontaneous blink rates of 25 species of birds. They also noted a correlation between blinks and head turns. Nocturnal species such as owls had a lower blink rate than diurnal species.

Curio (2001), in the only paper which addresses specialisation of function of blinks in birds, described tonic and phasic blinks. In the tonic blinks of many orders (Anseriformes, Accipitriformes, Falconiformes, Galliformes, Charadriiformes, Columbiformes and the Oscines suborder of Passeriformes) the lower lid rises during sleep. In Psittaciformes and Trochili (hummingbirds), the upper lid descends in sleep and in Strigiformes and Caprimulgifprmes, both lids are involved. He commented that ‘such information is lacking for most orders, or the handbooks provide wrong or conflicting information’. When awake, birds move one or both eyelids in a ‘phasic, usually swift mode’. This usually happens in response to a threat of injury. Curio (2001) regarded the function of the nictitating membrane as both cleansing and protective and doubted its possible role in filtering out undesirable retinal stimulation. He noted that blinking is a feature of social signaling in some species.

The advent in recent years of high resolution, high speed digital video cameras which work in low light and at high image magnification, and the development of video-editing software which lends itself to frame by frame analysis of what has been recorded, it is now possible to observe and quantify information on blinking unobtrusively and noninvasively.

The aims of this study were to characterize in a large number of species and orders of bird the following features of blinks: 1) which of the three structures (upper eyelid, lower eyelid and nictitating membranes) were involved in spontaneous blinking, and 2) what was the relationship of each type of blink to sleep, pecking, preening and head movements.

A further aim was to see how closely blinking in birds conforms to that of their closest relatives, the crocodilia. With birds, crocodilia are the only living descendants of archosaurs, a group of diapsid amniotes which once included dinosaurs and pterosaurs (Green et al 2014). While birds evolved over the last 200 million years into 10,000 or so species ranging greatly in size, shape and habitat, crocodilians evolved into only 23 different species and have changed little since they first appeared (Schwab 2012).

Information is sparse on blinking in crocodilia. Unlike birds, crocodilia have a muscle behind the eye which retracts the eyeball, the retractor bulbi. Where crocodilia have one muscle which actively draws the nictitating membrane across the eye, the retractor membranae nictitantis (Wedin et al 1953), birds have two. Eye closure in crocodilia is performed by elevation of the lower eyelid (Oria et al 2013), the hinged upper eyelid also being able to fall into the orbit entrance with retraction of the eyeball.

Methods

The study began in 2014. Birds were videoed initially using a Panasonic Lumix DMC-FZ200 Digital Camera (12 megapixels) and later with a Nikon Coolpix B700 (23 megapixels) and a Sony FDR AX53 Handycam. The zoom on these allowed close-up views of the eyes even when the bird was a few metres away. Initially filming was done at 25 frames/second but later at 100 or occasionally at 200fps. In the illustrations, the third frame from the onset of the video sequence done at 100fps, for example, is labelled 30ms though it covers the period 30-40ms; each frame having a duration of 10ms. At 25fps, the duration of each frame is 40ms. As some blinks only lasted 40ms, the move was made to video at higher frame rates to avoid missing data. Higher frames rates were also important to capture the rapid eye movements associated with saccadic oscillations.

Where results from videos done at different rates were pooled for the purposes of analysis, the durations for the higher rates (100 and 200fps) were corrected so that they conformed to having been videoed at the rate of 25fps (which accounted for the majority of results). Thus, a duration of 50ms recorded at 100fps was corrected to 80ms, as it would have fallen in the 40-80ms frame, had the video been recorded at 25fps.

It was possible to confirm the duration of each frame by dividing the duration of the video sequence by the number of frames using Adobe Premier Pro, as long as the play back was at normal speed and not in slow motion (the high-speed recordings from the Panasonic Lumix and Nikon Coolpix cameras only play back in slow motion). VLC Media player was used initially for the measurements and later Adobe Premier Pro (which allow sequences to be run backwards and forwards, frame by frame, making measurements easier to perform).

The duration of a blink was calculated as follows: Blink onset: the frame on which a change in the position of the membrane/lid was first seen in a blink; End of blink: the frame before the one in which the membrane/lids return to the pre-blink position or the last frame before the one which began a sequence of frames in which the position of the membrane/lid did not change (it was not always possible to compare the position of the lid at the end of the blink with its position at the beginning, as the eyelid did not always return to its opening position at the end of the blink); Duration of blink: the number of frames from onset to end including the first and last. When a blink only occupied one frame, the duration was taken as the duration of that frame (this mainly occurred when videoing at 25fps, making the duration 40ms). For each species, at least 3 blinks were analysed, whenever possible. The number of blinks was sometimes less than this – some birds blinked infrequently, others only blinked during rapid head movements which blurred the images.

Birds were videoed for as long as they remained within view unless they were still, when it was done for about a minute. When the opportunity arose, the same bird might be videoed repeatedly. Often, several individuals from the same species of bird were studied.

Wild birds were studied in the Australian Bush and public parks around Sydney, Australia, and to a small extent in public parks in the UK and the Netherlands. No specific permission was required for photography in any of these locations. Captive birds were filmed at Taronga Zoo, Australian Reptile Park, Featherdale Wildlife Park (in and near Sydney); Jurong Bird Park (Singapore); London Zoo, Suffolk Owl Sanctuary (UK); Rotterdam Zoo, Vogelpark Avifauna (Netherlands). The studies involved videoing of the birds without in any way interfering with their activities for example by attracting their attention with bird calls or using lights or flash. As a zoom lens was used, it was possible to film wild birds at a distance. None of these included protected or endangered species. The rules regarding photography varied from zoo to zoo but in no case was photography banned provided that photographs and videos were not published (without permission) for commercial purposes. As neither captive nor wild birds were subjected to any interference beyond that of being videoed from a distance, approval from an Ethics Committee was not sought.

With the exception of raptors, only one eye was usually seen, but where both were visible, it was common to see unilateral or asynchronous blinks.

All filming was done during daylight hours. No time limit was set on how long a blink could last but the end of the blink had to be seen for it to be so designated. Otherwise, it was assumed that the bird had fallen asleep.

Captive crocodilia were videoed at Taronga Zoo, Australian Reptile Park (in and near Sydney); London Zoo and Rotterdam Zoo.

For the purposes of this report, still images taken from the videos were converted into line drawings using Prisma software.

Results

Information was gathered on a total of 312 species of bird from 26 orders.

A) Types of blink

Two main types of blink were observed:

1) Phasic blinks

Phasic blinks are defined as brief and rapid. These were seen mainly on shifts in gaze but also occurred with the head still. In some species they were observed during pecking. Phasic blinks took three forms:

a) Nictitating membrane blinks

These involved the nictitating membrane alone. This was the commonest type of blink and was seen in 183 species and in all but 3 orders (see Appendix 1).

An example is given in Figure 1, in which a captive bush stone-curlew (Burhinus grallarius) was videoed at 25fps during a slight head movement. Each frame represents 40ms. The membrane, which is transparent apart from its pigmented leading edge, first appears in the 40ms frame moving away from the medial canthus. It has covered the cornea by the 80ms frame and then withdraws back to the medial canthus over the next five frames. As described in the Methods, the blink onset was taken as the 40ms frame and the end of the blink as the 280ms frame, 7 frames giving a duration of 280ms.

Figure 1. Nictitating membrane blink in a bush stone-curlew videoed at 25fps with the head turning and lowering.

Nictitating membrane blinks were observed in a domestic fowl (Gallus gallus domesticus) pecking at a layer of dried fruit juice on a vertical piece of corrugated iron bordering a compost heap in a fruit bat rescue compound. The hen pecked frequently and rapidly, and it was apparent that with every peck, the nictitating membrane appeared just before the beak struck the metal. The sequence of events in 7 pecks, which occurred over a period of about 5 seconds, is shown in Figure 2. The nictitating membrane was first seen 30-50ms after the head first began to move. The beak struck the metal 60-80ms after the onset of the peck and the blink ended 90-120ms after the onset of the peck. This appears to be reflex blinking and is not included with rest of the results which are of spontaneous blinks (it might be argued that blinking on gaze shift is also reflex, but it did not occur with every shift of gaze).

Figure 2. Sequence of events in 7 consecutive pecks involving nictitating membrane blinks in a domestic hen videoed at 100fps.

Nictitating membrane blinks just before the point of impact during pecking were also seen in a domestic turkey (Meleagris gallopavo) and a brush turkey (Alectura lathami). Though many birds were seen pecking, the velocity of the head strike precluded a clear sighting of the eyes in most cases, even videoing at 100fps.

b) Nictitating membrane blink with minimal lowering of the upper eyelid

This looked very much like the first type of phasic blink but on close inspection there was minimal lowering of the medial aspect of the upper eyelid at the onset of the blink. It was seen in 35 species and these are listed in Appendix 2.

An example of this type of blink in a captive grey heron (Ardea cinerea) videoed at 200fps is provided in Figure 3. In the 25ms frame, with the nictitating membrane across the cornea, the medial aspect of the upper eyelid has moved down and laterally a little.

Figure 3. Lowering of the medial aspect of the upper eyelid at the onset of a nictitating membrane blink in a grey heron filmed at 200fps.

Slight movement of the upper eyelid had to be distinguished from movements of the eyelids due to the eye moving beneath. This was an issue in birds with relatively preserved eye movements such as hornbills.

c) Upper eyelid blinks

Here, the most striking feature of the blink was lowering of the upper eyelid. The extent of this varied from blink to blink in the same species, but the upper eyelid usually reached the pupil and sometimes covered it. Complete closure of the eye was unusual. In most cases, the nictitating membrane could be seen crossing the eye diagonally as the upper lid lowered.

Upper eyelid blinks were seen in 86 species, mainly from three orders: Strigiformes, Columbiformes and Psittaciformes. A full list of species showing this type of blink is shown in Appendix 3.

In Figure 4, a blink during a gaze shift is shown in a masked owl (Tyto novaehollandiae). In the 60ms frame, the left upper eyelid has lowered a little and an opaque nictitating membrane is visible. In the 100ms frame, the upper eyelid has half covered the cornea and the nictitating membrane has reached almost maximal excursion. Both lid and membrane then start to withdraw, withdrawing fully by 250ms. The right eyelid, which is not visible initially, lags behind the left eyelid; blinking in the two eyes is not synchronous.

Figure 4. Nictitating membrane and upper lid blink during a head turn in a masked owl videoed at 100fps.

In species which only blinked on gaze shifts and in which the head turn was very rapid, it was sometimes impossible to get a clear picture of the blink. Species where this was a problem are listed in Appendix 4

A feature in some species of ground feeding birds was upper eyelid blinking associated with pecking. It appeared to begin immediately after the target was spotted, well before the target was reached (unlike the nictitating membrane blink seen in the hen peck above) and continued until the head was raised at the end of the peck. In Figure 5, upper eyelid blinking in a zebra dove (Geopelia striata) during pecking is shown. The bird appears to make a sighting of the target at 0ms. By 40ms, the eye is half closed and by 80ms fully closed. Yet the target is not reached until the 240ms frame. Assuming that the other eye was also closed, the pigeon was performing most of the peck without ongoing visual guidance. This pecking behaviour was also observed in feral pigeons (Columba domestica) and in painted button-quails (Turnix varius).

Figure 5. Zebra dove pecking, videoed at 25fps.

2) Tonic blinks

These involved the lower eyelid(s) and occurred when the bird was still and appeared to be drowsy, or was preening. The lower eyelid was slow to rise, remained elevated for prolonged periods but could descend quite rapidly at the end of the blink. The duration could be measured in drowsiness but not always during preening as the head disappeared from sight under the wing.

A barking owl (Ninox connivens) was observed dozing on its perch during the day. The lower lids slowly rose and fell repeatedly over many seconds. Full elevation of the lower eyelids is shown in Figure 6.

Figure 6. Barking owl with lower lids fully elevated in a state of impending sleep.

Not uncommonly, when a bird was drowsy and the lower lid was in a state of sustained elevation, the upper lid would suddenly descend and then rise, as though a phasic upper eyelid blink has been imposed on a tonic lower eyelid blink. Sometimes a rapid nictitating membrane blink merged into a sustained lower lid blink.

In species with mobile upper eyelids, the upper and lower eyelids often came together and then moved up and down in lockstep, causing the palpebral fissure to raise and fall slowly over a second or two. This is shown in a Nicobar pigeon (Caloenas nicobarica) in Figure 7. At the start of the sequence (0ms), the lower lid is elevated. Over the next 220ms, the upper lid descends so that the closed palpebral fissure falls. Thereafter, the lower lid starts to rise and by 1440ms, the original position of the two eyelids has been restored. There appears to be synchronous movement of the eyelids in opposite directions.

Figure 7. Nicobar pigeon drowsing, videoed at 100fps. Direction of the movement of the palpebral fissure is shown by the arrows.

A powerful owl (Ninox strenua) was videoed while in a drowsy state (Figure 8). At 40ms after the onset of the blink, the upper eyelids have drooped and the pigmented edge of the nictitating membrane is visible over the nasal aspect of the right eye. At 440ms, the right upper eyelid has descended while the left lower lid has elevated, in both cases while the eyes remain closed. At 880ms, the positions are reversed. The eyelids cease to move at 1080ms though their position has not returned to the pre-blink state. In this and other birds videoed in this series, blinking involving the two eyes was often asynchronous – as one upper eyelid descended, the other rose giving a characteristic ‘see saw’ appearance.

Figure 8. Powerful owl in a drowsy state videoed at 25fps

During preening, the eyes are mainly closed by elevation of the lower eyelid. This is shown in a bush stone-curlew in Figure 9.

Figure 9. Elevation of the lower eyelid during preening in a bush stone-curlew.

In species with mobile upper eyelids, both eyelids may be involved in preening. This is shown in a Barking owl in Figure 10. During preening, the upper and lower lids may also rise and fall together. This is shown in a crested pigeon (Ocyphaps lophotes) in Figure 11.

Figure 10. Depression of the left upper eyelid and elevation of both lower eyelids during preening in a Barking owl.

Figure 11. Preening in a crested pigeon. In the upper frame, the lower lid is elevated and in the lower frame, the upper eyelid has lowered.

A list of 85 species in which the lower eyelid(s) were involved in a sustained or tonic blink is provided in Appendix 5. If the bird was still and appeared to be drowsy, it is recorded as D. Preening is recorded as P.

B. Duration of blinks

The percentage of blinks falling within a given duration for the three types of phasic blink and for the tonic blinks is shown in Figures 12a,b,c &d.

Figure 12a. % of blinks of given duration for phasic nictitating membrane blinks. Figure 12b. % of blinks of given duration for phasic nictitating membrane blinks with slight upper eyelid involvement. Figure 12c. % of blinks of given duration for phasic upper eyelid blinks. Figure 12d. % of blinks of given duration for tonic lower eyelid blinks.

Blinks involving the upper eyelids took a little longer than those which only involve the nictitating membrane. The most common frequency band for the three types of phasic blink was 80-159ms. The three types of phasic blink had mean durations varying from 121-216ms with wide ranges. By contrast, the commonest frequency bands for tonic blinks were 320-639, 640-1279 and 1280-2559ms with a mean duration of 2955ms and an even wider range. Tonic blinks are clearly of longer duration than phasic blinks. A comparison of phasic blinks of all types between 20 orders of bird is shown in Table 1.

Table 1. Percentage of blinks of specified duration in the different orders and total blinks recorded for each order.

The commonest duration time bracket for phasic blinks in most orders was 80-159ms. The blinks of Strigiformes, Caprimulgiformes and Sphenisciformes were of longer duration.

C) Vascularity of the nictitating membrane

A striking feature in some species was vascularity of the nictitating membrane. In Figure 13, prominent arcades of blood vessels are seen aligned at ̴90° to the axis joining the medial and lateral canthi in a black-necked stork (Ephippiorhynchus asiaticus) 400ms after the onset of a blink.

Figure 13. Arcades of blood vessels in the nictitating membrane a male adult black-necked stork videoed at 100fps. The blink lasts just over a second and the head was still.

This feature was observed with varying degrees of ease depending upon how well the eyes were illuminated, the degree of transparency of the membrane and the size of the eyes. The list of species in which this feature was observed is shown in Appendix 6. It was not usually seen in small birds where the head movements were so fast and the eyes so small as to preclude detailed examination of the nictitating membrane.

D Crocodilians

Four species of crocodilian were studied: adult Australian freshwater crocodile (Crocodylus johnsoni); adult, juvenile and hatchling American alligator (Alligator mississippiensis); adult and juvenile saltwater crocodile (Crocodylus porosus); adult Nile crocodile (Crocodylus niloticus).

Again, two main types of blink were observed, one involving the nictitating membrane alone, and the other the nictitating membrane and lower eyelid:

1) Nictitating membrane blinks. These involved excursion of the nictitating membrane, with or without globe retraction. An example in an adult American alligator is provided in Figure 14. At 0ms, the nictitating membrane is visible but unmoving up to that point. At 600ms, the membrane has achieved maximal excursion. By 4000ms, the membrane is back close to its original position and unmoving once more.

Figure 14. Adult American alligator videoed at 25fps. In this case there was no globe retraction. The movement of the nictitating membrane did not depend upon globe retraction, as is the case with mammals.

Blinks were so infrequent as to pose problems in capturing them. An individual might be videoed on the water surface for several minutes, not blink during that time and then submerge or swim out of sight. On land, they tended to doze, slipping in and out of sleep. 15 nictitating membrane blinks were seen during a total video recording duration of 7,117secs, pooling all individuals. This amounted to one such blink captured every 474 seconds of videoing time.

The term ‘phasic’ did not seem appropriate for these blinks. While many only lasted for a second or less, others took many seconds and one lasted 21 seconds.

2) Lower eyelid blinks. These usually lasted much longer than the nictitating membrane blinks. They involved elevation of the lower lid, crossing of the nictitating membrane and retraction of the eyeball. While some movement of the upperlid was sometimes seen, this appeared to be passive and resulting from retraction of the eyeball rather than active closure.

Twelve lower eyelid blinks were observed in 7,117 seconds of videoing, pooling all individuals. These lasted from 0.25-37.8 seconds with a median of 12.58 seconds. In one adult freshwater crocodile, the eye closest to the camera remained open while the other eyebrow (that was all that could be seen) was elevated for 20 seconds, retracted for 2 seconds, then elevated for 7 seconds, retracted for 2 seconds, elevated for 26 seconds, retracted for 17 seconds, elevated for 12 seconds, then retracted for 20 seconds. Whether such alternation between prolonged eye closure and eye opening constitutes blinking becomes a matter of definition.

An example from the American alligator is shown in Figure 15. At 0ms, the nictitating membrane is visible but unmoving. At 800ms, the nictitating membrane has covered the eye. The lower eyelid has risen a little and the upper eyelid has fallen a little – both due to retraction of the eyeball. At 1960ms, the lower eyelid has risen to meet the upper eyelid and closed the eye.

Figure 15. Adult American alligator videoed at 25fps.

The durations of the two types of blink are shown in Figure 16.

Figure 16. % of blinks of given duration for nictitating membrane and lower eyelid blinks of crocodilia.

There was a wide range of duration of blinks of both types but the lower eyelid blinks tended to be longer with a mean duration of 11,347ms compared with 3,202ms for the nictitating membrane blinks.

Discussion

This study confirms the observation of Curio (2001), that blinking in birds appears to be of two types, one that is brief and rapid and the other that is more prolonged and slower to execute. Curio (2001) labelled these phasic and tonic blinks respectively. The terms phasic and tonic derive from Sherrington’s (1906) classical description of reflexes. The knee jerk, a rapid brief contraction of quadriceps muscle in response to tapping the patellar tendon, is an example of a phasic reflex whereas the sustained contraction of quadriceps in response to continuous stretch of the tendon is an example of a tonic reflex. In the present report, only spontaneous blinking has been studied (with the possible exception of blinking during pecks), but the terms phasic and tonic seem appropriate for capturing the differing features of the two types of blink.

By far the most common type was a brief blink involving the nicitating membrane alone. This usually occurred in association with a shift in gaze. Not every gaze shift was accompanied by a blink, and some blinks, even flurries of blinks, were observed with the head still. It is likely that this type of rapid blink would be induced by an approaching object, as a protective reflex to shield the cornea from injury, but this was not part of our study. Phasic blinks were noted during pecking. In a hen this appeared to be reflex blinking occurring 20-30ms before the beak struck the target. In the zebra pigeon it occurred 40ms after the target was sighted, and 200ms before the beak struck the target; presumably this required the bird to memorise the position of the target.

Curio (2001) also noted that the upper eyelid, lower eyelid and nictitating membrane varied in the way they were deployed in blinking depending on the circumstances and the species. While phasic blinks most commonly involved the nictitating membrane alone, in some species there was slight accompanying movement of the medial aspect of the upper eyelid. It is hard to say what this represents. It might simply be a mechanical result of the nictitating membrane unfurling and pulling on its attachment to the sclera and thereby on the upper eyelid. Alternatively, it may be a forme fruste of an upper eyelid blink – a relic of a time when the ancestors of these birds had an upper eyelid blink, or the beginning of the process of developing an upper eyelid blink. In three orders, the upper eyelids lowered to the pupil or beyond, again in company with the nictitating membrane. Tonic blinks in all the orders studied involved the lower eyelid. The nictitating membrane was also involved at times during tonic blinks as was the upper eyelid in species with mobile upper eyelids.

It seems likely that phasic blinks fulfil the function of maintaining the tear film on the cornea which, lacking a blood supply, depends for its respiration on gas exchange with air. Gas exchange is facilitated by the tear film which, in birds, is mainly produced by the Harderian gland at the base of the nictitating membrane. Debris on the surface of the cornea is also swept towards the medial canthus. Tear fluid drains into puncti on the medial end of each upper and lower eyelid and into the lacrimal duct. In addition, phasic blinks have a protective function during pecking.

Tonic blinks occurred when the bird was somnolent, presumably as a prelude to sleep. Closure of the eyes protects them during sleep. It also reduces light stimulation of the retina; that said, a common finding was that the lower eyelids were paper thin when closed. Perhaps this would allow the bird to awaken if, for example, the shadow of a predator fell across the eyes while they were closed. Tonic blinks are likely to be less effective than phasic blinks in distributing tear fluid and removing debris as the eyelids move so slowly.

The lower eyelids also elevated during preening, whether by the individual or its companions. This provides protection from mechanical injury by feathers or bill.

It is pertinant at this point to discuss the muscles of the eyelids of birds. Both rapidly contracting striated muscle and slow contracting smooth muscles are present (Schwab 2012). It might be expected that rapid (phasic) blinks are performed by striated muscle innervated by cranial somatic nerves, while slow (tonic) blinks involve smooth muscle innervated by the autonomic nervous system. There is some support for this proposition. The nictitating membrane is drawn across the cornea by the action of two striated muscles, quadratus nictitantis and pyramidalis nictitantis which are innervated by abducens, the VIth cranial nerve (Baumel et al 1993). The membrane returns to its resting position by elastic recoil. The upper eyelid is elevated by levator palpebrae dorsalis (superioris), a striated muscle innervated by the oculomotor (IIIrd cranial) nerve. The lower eyelid is depressed by depressor palpebrae ventralis which is innervated by the mandibular branch of the trigeminal (Vth cranial) nerve. Thus, phasic blinks involve striated muscles.

In tonic blinks there is slow elevation of the lower eyelid, sometimes in concert with slow depression of the upper eyelid. Orbicularis palpebrarum, a smooth muscle which is innervated by autonomic (probably parasympathetic) nerves, raises the lower eyelid and depresses the upper eyelid (Baumel et al 1993). Tonic blinks of the upper and lower eyelids are probably performed by these smooth muscles. Some blinks may involve both striatal and smooth muscles. Thus, it was not uncommon to see a slow prolonged lower eyelid blink end suddenly with the lower eyelid being pulled back into an ‘eye fully open’ position. The latter action would be performed by depressor palpebrae ventralis, a striated muscle. In species capable of upper eyelid blinks, it is likely that the upper eyelid is kept tonically elevated by the action of smooth muscle innervated by sympathetic nerves, the equivalent of Muller’s muscle in humans – weakness of which is responsible for the ptosis reported in Horner’s syndrome in an eagle owl (Bubo africanus) (Williams and Cooper 1994) and in a red-bellied parrot (Poicephalus rufiventris) (Gancz at al 2005).

As discussed, in land-dwelling vertebrates, the cornea depends for its oxygen on exposure to air (Barr et al 1977). During prolonged blinks and during sleep, the cornea is no longer exposed to atmospheric oxygen. In humans, gas exchange continues during sleep by virtue of the rich arcade of blood vessels on the conjunctival side of the eyelids. If these are also present in birds, gas exchange might be impaired when the nictitating membrane comes between the cornea and the eyelids. It may be significant therefore the nictitating membrane has a rich arcade of blood vessels which might also serve this function.

In this study, lower eyelid blinks and nictitating membrane blinks in birds were similar to those of crocodilia though the duration of blinks was considerably longer in the latter. In most orders of birds, the upper eyelids did not move downwards during a blink - the same was true of crocodilia, though in the latter the upper eyelids did sink into the orbits when the eyeballs were retracted. Upper eyelid blinks were seen mainly in three orders, Columbiformes, Strigiformes and Psicattiformes. The question arises, what were the factors which led to this? Have they perhaps evolved from a common ancestor which blinked with its upper eyelids? According to the most recent analysis of clade genotypes (Prum et al 2015), these three orders are not particularly close. Columbiformes are classified within Columbaves which includes Turacos, which do not blink with their upper eyelids. Strigiformes lie between Accipitriformes and Coraciimorphae, neither of which blink with their upper eyelids. Caprimulgiformes, upper eyelid blinkers and an order often mistaken for owls, are within the group of Strisores which included swifts and hummingbirds. Psicattiformes are classed with Australaves, a group which includes falcons, which do not blink with their upper eyelids. In short, no clear pattern of inheritance emerges. Further evidence that upperlid blinking is not related to clade genotypes comes from the finding that even with orders such as Passeriformes which mostly blink with their nictitating membranes, there were three exceptions. The Eurasian skylark (Alauda arvensis), zebra finch (Taeniopygia guttate) and Gouldian finch (Erythrura gouldiae) blinked with their upper eyelids.

If upper eyelid blinking cannot be readily linked to clade genotypes, could ecological factors account for it? The upper eyelid contributes to both phasic and tonic blinks in these species. Involvement of the upper eyelid, by providing extra thickness, presumably increases the level of protection of the eyes beneath. So, are these birds at greater risk of eye injury than other birds? This may be the case in ground feeding birds like pigeons and quails which peck for seeds in undergrowth and foliage. Some pigeons, as in the case of the Zebra dove described above, close their upper eyelids during pecks. In the case of owls and nightjars, catching live prey capable of injuring their eyes at night may put them at greater risk than other raptors which hunt during the day when visibility is better. But what was the advantage in parrots evolving upper eyelid blinks? There is no obvious answer to this.

It is interesting to consider how blinking in birds and crocodilians has evolved to suit similar as well as differing needs. Both blink with their nictitating membranes to lubricate and clear debris from the cornea. Crocodilians and aquatic birds, such as cormorants, protect their eyes while submerged, without losing all vision, using their nictitating membranes. Birds protect their eyes during sleep and preening by raising the lower eyelids, which are often quite flimsy. By contrast, crocodilians protect their eyes from injury by retracting the eyeballs deep into the orbits. This causes the thickly armoured hinged upper eyelids to fold like trapdoor covers over the orbits. The lower eyelids are also raised. Having larger prey, the requirements of crocodilians for protection of the eyes are likely to be more critical than is the case for most birds. In the evolutionary balancing act of reducing body weight while increasing visual acuity, birds have mostly abandoned orbital muscles which move the eyes and retract them, while increasing the size of the eyes relative to their body size.

In summary, spontaneous blinking in birds is of two types: phasic blinks which lubricate the cornea, remove debris and protect the eyes during pecking, and tonic blinks which protect the eye during somnolence and preening. Some orders of birds have evolved upper eyelid blinks. Blinking in crocodilia has similarities to that of birds without upper eyelid blinks but with the retained feature of globe retraction.

References

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Appendices to the paper.

Appendix 1

183 species with phasic nicitating membrane blinks

Passeriformes (46 species)

Acridotheres tristis, Ailuroedus crassirostris, Anthochaera chrysoptera, Artamus superciliosus, Chalcomitra senegalensis, Cinnyricinclus leucogaster, Copsychus malabaricus, Corcorax melanorhamphos, Corvus corone, Corvus monedula, Cracticus torquatus, Crypsirina temia, Cyanopica cyanus, Entomyzon cyanotis, Eopsaltria australis, Euplectes orix, Garrulax courtoisi, Geokichla citrina, Gracula religiosa, Grallina cyanoleuca, Gymnorhina tibicen, Hirundo neoxena, Lamprotornis iris, Leiothrix lutea, Leucopsar rothschildi, Lonchura castaneothorax, Malurus coronatus, Manorina melanocephala, Meliphaga lewinii, Melloria quoyi, Menura novaehollandiae, Pica pica, Pitta versicolor, Psophodes occidentalis, Ptilonorhynchus violaceus, Oriolus sagittatus, Ramphocelus bresilius, Sericulus chrysocephalus, Sphecotheres vieilloti, Sphecotheres viridis, Sturnus vulgaris, Struthidea cinerea, Tachyphonus rufus, Turdus merula, Urocissa erythrorhyncha, Zosterops lateralis

Falconiformes (6 species)

Caracara plancus, Falco cenchroides, Falco columbarius, Haliaeetus leucogaster, Hieraaetus morphnoides, Phalcoboenus australis

Coraciiformes (9 species)

Coracias cyanogaster, Dacelo leachii, Dacelo novaeguineae, Eurystomus orientalis, Halcyon coromanda, Merops nubicus, Merops ornatus, Todiramphus chloris, Todiramphus macleayii

Piciformes (3 species)

Merops ornatus, Psilopogon pyrolophus, Trachyphonus erythrocephalus

Bucerotiformes (7 species)

Berenicornis comatus, Buceros bicornis, Bucorvus leadbeateri, Bycanistes brevis, Rhyticeros plicatus, Tockus deckeni, Rhyticeros undulatus

Accipitriformes (16 species)

Accipiter novaehollandiae, Aquila audax, Buteo buteo, Cathartes aura, Gypohierax angolensis, Gyps fulvus, Haliaeetus leucocephalus, Haliastur indus, Hamirostra melanosternon, Milvus migrans, Nisaetus nanus, Parabuteo unicinctus, Sagittarius serpentarius, Sarcoramphus papa, Trigonoceps occipitalis, Vultur gryphus

Pelicaniformes (18 species)

Ardea picata, Ciconia ciconia, Ciconia episcopus, Ciconia nigra, Egretta novaehollandiae, Ephippiorhynchus senegalensis, Eudocimus ruber, Leptoptilos crumeniferus, Geronticus eremita, Nycticorax caledonicus, Nycticorax nycticorax, Pelecanus conspicillatus, Pelecanus onocrotalus, Platalea regia, Plegadis falcinellus, Scopus umbretta, Theristicus caudatus, Threskiornis moluccus,

Suliformes (3 species)l

Phalacrocorax aristotelis, Phalacrocorax carbo, Phalacrocorax varius

Sphenisciformes (3 species)

Eudyptula minor, Spheniscus demersus, Spheniscus humboldti

Eurypygiformes (1 species)

Eurypyga helias

Charadriiformes (12 species)

Burhinus grallarius, Chroicocephalus novaehollandiae, Chroicocephalus ridibundus, Esacus magnirostris, Haematopus longirostris, Larus argentatus, Larus dominicanus, Larus fuscus, Larus pacificus, Thalasseus bergii, Turnix castanotus, Vanellus tricolor

Gruiformes (9 species)

Balearica regulorum, Eulabeornis castaneoventris, Fulica atra, Gallinula tenebrosa, Gallirallus philippensis, Grus japonensis, Grus paradisea, Grus rubicunda, Porphyrio melanotus

Cariamiformes (1 species)

Cariama cristata

Musophagiformes (5 species)

Crinifer piscator, Musophaga violacea, Tauraco erythrolophus, Tauraco leucotis, Tauraco persa

Cuculiformes (3 species)

Centropus phasianinus, Eudynamys orientalis, Scythrops novaehollandiae

Phoenicopteriformes (1 species)

Phoenicopterus roseus

Galliformes (15 species)

Acryllium vulturinum, Chrysolophus amherstiae, Chrysolophus pictus, Chrysolophus pictus mut.luteus, Colinus virginianus, Gallus gallus, Gallus gallus domesticus, Lophura diardi, Lophura nycthemera, Meleagris gallopavo, Nothocrax urumutum, Numida meleagris, Pavo muticus, Polyplectron inopinatum, Rollulus rouloul

Anseriformes (23 species)

Aix galericulata, Alopochen aegyptiaca, Anas castanea, Anas platyrhynchos, Anas supercilosa, Anser anser, Anseranas semipalmata, Aythya fuligula, Branta canadensis, Branta sandvicensis, Callonetta leucophrys, Cereopsis novaehollandiae, Chauna torquata, Chenonetta jubata, Cygnus atratus, Dendrocygna arcuata, Dendrocygna bicolor, Netta rufina, Nettapus auritus, Oxyura australis, Sarkidiornis melanotos, Spatula hottentota, Tadorna radjah

Casuariformes (2 species)

Casuarius casuarius, Dromaius novaehollandiae

Appendix 2

35 species with phasic nictitating membrane blinks and slight upperlid involvement

Passeriformes (6 species)

Anthochaera carunculate, Icterus oberi, Lamprotornis regius, Mino dumontii, Oriolus chinensis, Passer domesticus

Piciformes (2 species)

Lybius dubius, Pteroglossus viridis

Bucerotiformes (1 species)

Buceros rhinoceros

Accipitriformes (2 species)

Haliastur sphenurus, Terathopius ecaudatus

Pelicaniformes (9 species)

Ardea alba, Ardea cinerea, Bubulcus ibis, Ciconia abdimii, Ephippiorhynchus asiaticus, Mycteria leucocephala, Pelecanus crispus, Pelecanus thagus, Platalea leucorodia

Charadriiformes (1 species)

Vanellus spinosus

Otidiformes (1 species)

Ardeotis australis

Musophagiformes (1 species)

Musophaga rossae

Phoenicopteriformes (1 species)

Phoenicopterus ruber

Galliformes (9 species)

Afropavo congensis, Alectoris chukar, Alectura lathami, Argusianus argus, Coturnix chinensis or Excalfactoria chinensis, Coturnix ypsilophora, Megapodius reinwardt, Pavo cristatus, Tragopan temminckii

Anseriformes (1 species)

Cairina moschata

Struthioniformes (1 species)

Struthio camelus

Appendix 3

86 species with phasic upper eyelid blinks

Passeriformes (3 species)

Alauda arvensis, Erythrura gouldiae, Taeniopygia guttata

Psittaciformes (30 species)

Alisterus scapularis, Aprosmictus erythropterus, Ara chloropterus, Ara macao, Aratinga solstitialis, Cacatua galerita, Cacatua tenuirostris, Calyptorhynchus banksii, Calyptorhynchus baudinii, Calyptorhynchus funereus, Cyclopsitta diophthalma, Eclectus roratus, Eolophus roseicapilla, Lophochroa leadbeateri, Lorius garrulus, Lorius lory, Melopsittacus undulatus, Nymphicus hollandicus, Polytelis alexandrae, Trichoglossus haematodus, Trichoglossus chlorolepidotus, Neophema pulchella, Platycercus eximius, Platycercus elegans, Polytelis anthopeplus, Polytelis swainsonii, Psephotus haematonotus, Pseudeos fuscata, Psittacula eupatria, Psitteuteles versicolor

Piciformes (1 species)

Picus viridis

Strigiformes (22 species)

Athene cunicularia, Athene noctua, Bubo africanus, Bubo bengalensis, Bubo bubo, Bubo scandiacus, Bubo virginianus, Ketupa ketupu, Ninox connivens, Ninox novaeseelandiae, Ninox rufa, Ninox strenua, Ptilopsis leucotis, Strix aluco, Strix leptogrammica, Strix nebulosa, Strix seloputo, Surnia ulula, Tyto alba, Tyto longimembris, Tyto novaehollandiae, Strix uralensis

Charadriiformes (1 species)

Turnix varia

Caprimulgiformes (1 species)

Podargus strigoides

Columbiformes (26 species)

Caloenas nicobarica, Chalcophaps indica, Columba guinea, Columba leucomela, Columba livia, Columba palumbus, Ducula aenea, Ducula bicolor, Ducula pinon, Leucosarcia melanoleuca, Gallicolumba crinigera, Gallicolumba jobiensis, Geopelia cuneata, Geopelia humeralis, Geopelia striata, Geophaps plumifera, Goura victoria, Lopholaimus antarcticus, Ocyphaps lophotes, Phaps elegans, Ptilinopus magnificus, Ptilinopus regina, Ptilinopus superbus, Sericulus chrysocephalus, Spilopelia chinensis, Streptopelia risoria

Galliformes (2 species)

Coturnix chinensis or Excalfactoria chinensis, Coturnix japonica

Appendix 4

7 species where no blink measurements could be made

Passeriformes (5 species)

Cinnyris coccinigastrus, Euphonia violacea, Lonchura castaneothorax, Ramphocelus bresilius, Sicalis flaveola

Podicipediformes (1 species)

Podiceps cristatus

Galliformes (1 species)

Turnix castanotus

Appendix 5

85 species with tonic lower eyelid blinks (D: drowsy; P: preening; upper lid also involved: UL)

Passeriformes (7 species)

Cinnyricinclus leucogaster, Erythrura gouldiae UL, Hirundo neoxena P, Lonchura castaneothorax P, Stagonopleura guttata UL, Struthidea cinerea D, Taeniopygia guttata P

Psittaciformes (4 species)

Calyptorhynchus banksii P UL, Calyptorhynchus funereus D, Psitteuteles versicolor UL, Trichoglossus chlorolepidotus UL

Piciformes (1 species)

Lybius dubius P UL

Strigiformes (7 species)

Bubo bengalensis D, Bubo scandiacus D UL, Ninox connivens D, Ninox strenua UL, Strix aluco, Tyto alba D, Tyto novaehollandiae UL

Accipitriformes (2 species)

Haliastur indus P, Trigonoceps occipitalis D

Pelicaniformes (6 species)

Ardea alba P, Ciconia episcopus P, Ephippiorhynchus asiaticus UL, Leptoptilos crumeniferus, Pelecanus crispus P, Pelecanus thagus P

Suliformes (2 species)

Phalacrocorax aristotelis, Phalacrocorax carbo P

Sphenisciformes (3 species)

Eudyptula minor D, Spheniscus demersus, Spheniscus humboldti,

Charadriiformes (5 species)

Burhinus grallarius, Esacus magnirostris, Haematopus longirostris D, Turnix varia P, Vanellus tricolor

Gruiformes (1 species)

Grus japonensis

Musophagiformes (1 species)

Tauraco erythrolophus D

Cuculiformes (1 species)

Centropus phasianinus D

Columbiformes (10 species)

Caloenas nicobarica D P UL, Chalcophaps indica Ul, Columba leucomela D UL, Columba livia P UL, Geopelia striata UL, Goura victoria P UL, Leucosarcia melanoleuca D UL, Lopholaimus antarcticus UL Ptilinopus regina D UL, Ptilinopus superbus D UL,

Phoenicopteriformes (2 species)

Phoenicopterus ruber P, Phoenicopterus roseus D

Podicipediformes (1 species)

Podiceps cristatus P

Galliformes (15 species)

Chrysolophus pictus P, Coturnix chinensis or Excalfactoria chinensis D, Coturnix ypsilophora UL, Pavo cristatus P, Pavo muticus P, Turnix castanotus, Gallus gallus D, Chrysolophus amherstiae P, Nothocrax urumutum P D, Tragopan temminckii UL, Colius virginianus P, Alectoris chukar P, Coturnix japonica UL D, Chrysolophus pictus mut. luteus P, Rollulus rouloul P

Anseriformes (15 species)

Aix galericulata D, Alopochen aegyptiaca P, Anas castanea P, Anas melleri D, Anas platyrhynchos, Anas supercilosa D P, Aythya fuligula, Branta sandvicensis D P, Cairina moschata D, Chauna torquata P, Dendrocygna arcuata P, Dendrocygna bicolor P, Nettapus auritus D, Oxyura australis D, Sarkidiornis melanotos P

Casuariformes (2 species)

Casuarius casuarius D P, Dromaius novaehollandiae D

Appendix 6

44 species where arcades of vessels were seen in the nictitating membrane

Passeriformes (1 species)

Mino dumontii

Psittaciformes (2 species)

Calyptorhynchus baudinii, Alisterus scapularis

Falconiformes (1 species)

Falco cenchroides

Coraciiformes (1 species)

Dacelo novaeguineae

Piciformes (1 species)

Pteroglossus viridis

Bucerotiformes (1 species)

Buceros bicornis

Strigiformes (1 species)

Tyto novaehollandiae

Accipitriformes (6 species)

Accipiter novaehollandiae, Aquila audax, Hamirostra melanosternon, Sagittarius serpentarius, Terathopius ecaudatus, Trigonoceps occipitalis

Pelicaniformes (11 species)

Ardea cinereal, Ciconia abdimii, Ciconia episcopus, Ciconia nigra, Ephippiorhynchus asiaticus, Geronticus eremita, Leptoptilos crumeniferus, Mycteria leucocephala, Nycticorax nycticorax, Theristicus caudatus, Threskiornis moluccus

Charadriiformes (2 species)

Burhinus grallarius, Esacus magnirostris

Gruiformes (4 species)

Balearica regulorum, Grus japonensis, Grus paradise, Grus rubicunda

Cariamiformes (1 species)

Cariama cristata

Musophagiformes (1 species)

Musophaga rossae

Galliformes (4 species)

Acryllium vulturinum, Afropavo congensis, Alectura lathami, Argusianus argus

Anseriformes (5 species)

Aix galericulata, Anas platyrhynchos, Anser anser, Cairina moschata, Chauna torquata

Struthioniformes (1 species)

Struthio camelus

Casuariformes (1 species)

Casuarius casuarius