Have you even been out at night and heard a bird calling in the dark? Perhaps you have seen an owl swooping past your bedroom window during the small hours? Ever wondered how it is possible for birds to see through the gloom? Then you’ve come to the right place!
Although all birds have incredibly good eyesight, only some birds are actually adapted to seeing in the dark. These species, such as owls and nightjars, have specialised eyes with features like reflective layers and a proliferation of particular photoreceptors. Traits like these enable nightbirds to glide around and hunt during the darkest of nights!
Today we’ll be exploring bird eyesight, from the anatomy of a bird’s eye to the evolutionary reasons for the outstanding night vision of some species. Let’s gaze deep into the world of bird vision…
Bird eye anatomy
As visual predators and flying organisms, it is essential for birds to possess good eyesight in order to successfully hunt for food and avoid crashing into objects. In fact, birds have the best vision of all vertebrate groups, and the largest eye-to-body-size ratio of all animal groups – very impressive claims!
Although there are some similarities between bird vision and human vision including a comparable internal anatomy, and some of the same structures, there are crucial differences that make bird vision far superior. These include:
- The pecten oculi – a structure unique to birds. This is a group of blood vessels within the eye which helps to sharpen a bird’s eyesight and provide protection against harmful UV rays. (Read about this in more detail here!)
- Four cones in the retina rather than three – this helps the bird to detect differences in color, and more color shades than humans can see. This makes birds tetrachromats, whereas we are trichromats – hence we can only see the base colors red, blue and green.
- A flatter eyeball shape compared to a human eyeball – this helps a bird to focus more quickly and accurately within different visual fields.
- Different lens location – the lens if further forward in a bird eyeball compared to a human eyeball. This increases the size of the image the retina receives.
- Larger pupils – this means more light is able to enter the eye, and is a key reason for greater night vision compared to humans.
- Vastly more photoreceptors – a bird’s fovea (the cells inside the retina) contain, on average, 400,000 photoreceptor cones for every square millimetre (400,000 cones mm-2). Whereas human eyes contain half that amount!
Check this out for some awesome avian eye diagrams, and images comparing human eyesight with bird eyesight.
For an insight into how birds are able to see in darker conditions, we need to take a closer look at their color perception abilities…
Color perception and motion detection
The enhanced ability of a bird to see a greater range of the UV spectrum than most other animals enables them to spot prey and potential predators, and even to differentiate between one another.
In addition to this, they possess magnetoreception – an incredible perception ability whereby they are able to navigate using the geomagnetic field! This helps them to detect locations, directions and altitudes. Magnetoreception helps birds to migrate such as Autumn Bramblings which fly south from Scandinavia and Russia to avoid harsh winter conditions.
Birds can move much faster than humans using their agility to swoop in and catch prey, or to dart away from danger. So it makes sense that a bird’s motion detection abilities are also faster. During continuous movement, birds can see up to 100 Hertz whereas humans can only see up to 60 Hertz of movement. Conversely they are able to see much slower moving objects which the human eye fails to pick up (find out more here).
However, most birds are diurnal – they are active during the daytime – therefore, their vision is adapted more specifically to daylight conditions. Although they are still able to see better in the dark compared to us, it is the nocturnal birds who have the upper hand.
Nocturnal birds with night-vision
As we have so far discovered, bird eyesight is a force to be reckoned with. But what about birds who are active during the night? How does the anatomy of a bird’s eye adapt to dark conditions? This is where the specialist nocturnal species come in!
The main difference between diurnal and nocturnal bird vision lies in the number of rod cells within the retina. Rod cells are the type of photoreceptor cell that are responsible for letting light into the eye and aiding animals in being able to see during low light conditions. They also give animals peripheral vision because they are concentrated on the outer regions of the retina.
Other adaptions in the eyes of nocturnal birds include:
- Larger eyeballs
- Very few or complete absence of cone cells (the other kind of photoreceptor cell, responsible for color and sharpness perception, primarily during the daytime). Cone cells require high light levels to function successfully. Most nocturnal birds have adapted to night-time light levels, and therefore do not require cone cells.
- Ever seen the eyes of a nocturnal creature glowing back at you in the dark? Or even your pet cat with scary, shining eyes? This effect is caused by the tapetum lucidum – a reflective layer on the back of the retina. The tapetum lucidum maximises the amount of light the photoreceptor cells receive, helping nocturnal birds to see in the dimmest conditions.
It is important to note that even nocturnal birds who are most active during the darkest times cannot see in complete darkness. They require at least a little light for their plenteous rod cells to function optimally.
The advantages of night-vision
- Lowered competition – most birds are diurnal, so nocturnal birds do not have such strong competition when hunting for prey
- Less ambient noises compared to the day-time – this can help night-birds listen for their prey, and for danger. It is also an advantage for birds which sing at night; they can be more easily heard at longer distances.
- Avoiding predation – some birds choose to migrate at night to avoid day-time predators. Nocturnal birds can steer clear of day-time hunters like wolves which mostly hunt in the day.
Let’s focus in on a couple of examples of prime nocturnal bird groups…
Nocturnal bird groups
Perhaps the first kind of bird you think of when listing nocturnal birds, owls are a classic night-prowling group. Nocturnal owls are incredibly unique in that they have tubular-shaped eyeballs. This means they cannot move their eyes around like we can, so they make up for it with an impressive range of motion in the neck muscles. Most species can swivel their heads to view a 270° field without moving their bodies!
With incredibly large eyes (3% of their entire body weight! Compare this to the 0.0003% of human body weight our eyeballs account for!) and incomparable far-sight, an owl makes a dangerous night-time predator. Their near-sight is relatively weak, as they specialise in spotting and swooping in on prey like small mammals, reptiles and amphibians from a distance.
When they get close to catching prey, whisker-like hairs surrounding their beaks act as motion-sensors, taking over from their visual proficiency. They can also protect their precious eyes using one of their 3 eyelids! Yes, one fascinating fact is that all birds possess 3 eyelids. One eyelid moves over the eyeball when a bird blinks, one is used when a bird is sleeping, and the other – the nictating membrane – can be drawn over when a bird is in close proximity to its prey. This is vital as owls will swoop to the ground amongst the undergrowth to pin their prey down.
Owl eye color
Usually, a nocturnal owl will have dark brown or black eyes, like the Barn Owl. Eye color does not enhance their night-stalking habits but is an evolutionary adaption which camouflages them against the night.
Some owls are crepuscular – this means that they hunt during low light conditions at dawn and dusk. These species mostly have piercing orange eyes, like the Eurasian Eagle Owl.
Diurnal owls mostly have yellow eyes. Species like the Snowy Owl have incredible hearing as well as advanced eyesight. They specialise in ‘sounding out’ prey in undergrowth or even beneath snow!
This group of birds can be nocturnal or crepuscular. They hunt in the dark, catching night-time flying insects on the wing. During the day, Nightjars are camouflage experts with their squat form and mottled brown plumage.
Nightjars can be found all over the world, but are called Nighthawks in North America, or Goat Suckers due to their mythical reputation that they could swoop in and steal milk straight from a Goat’s teat! This myth is based on their spookily silent flight, and the fact that people noticed they often flew close to livestock. Contrary to popular belief, this is not to steal milk, but to collect the insects that are attracted to livestock!