Most wild birds and animals don’t survive long if they go blind. Not so, our kiwi. The discovery of otherwise healthy, but blind wild kiwi living successfully on the South Island’s West Coast has revealed just how little kiwi rely on their visual senses. For kiwi, it’s all about smell, hearing and vibration.
Most birds have good eyesight. They need it for flying and for spotting predators. But kiwi are nocturnal, their eyes are small and their eyesight is poor at the best of times. For researchers, the discovery of a bird that still has some eyesight, but doesn’t particularly need it, isn’t just surprising – it also gives an insight into the evolution of vertebrate vision.
But back to those blind kiwi – what’s their story?
“Through an intensive conservation management program, an ophthalmologic survey was performed on 160 free-living Okarito kiwi (Apteryx rowi) in their natural habitat in New Zealand. Initial brief examinations revealed that about one-third of the birds had ocular lesions in one or both eyes.”
A veterinary ophthalmologist performed a detailed examination on 11 of the kiwi with these lesions.
“Common abnormal findings included corneal opacification (clouding/scarring of the cornea), shrunken fibrotic globes (shrunken non-functional eye caused by injury or infection), among others. Such ophthalmic findings are prevalent in captive avian populations, but are rare in free-living prey species with reports being limited mostly to raptors (i.e., eagles, hawks, owls, and other predatory birds). This is likely due to the profound negative impact on survival that any reduction in the quality of visual information can have on visually dependent species, which is shown here to not be the case for kiwi, who may depend on other sensory systems.”
Three of those kiwi were completely blind.
“Surprisingly, these three individuals were found in good physical condition, assessed by complete physical examination and body condition scoring by a veterinarian with advanced training in avian medicine. The ocular lesions were also chronic in nature, suggesting that these birds had survived for months to years without vision.”
Radio transmitters were then used to follow the progress of these three blind birds.
“Radio transmitter tracking revealed that the three blind individuals survived for at least 4 more years, and one of the birds had pair-bonded with a visual bird but it is unclear whether or not they successfully mated.”
So what about the evolutionary implications? Having lost the power of flight, are kiwi evolving to be both flightless and sightless?
“The avian visual system is widely recognized for its superior performance. Birds are broadly considered to have an unmatched dependence for high quality visual information compared to other vertebrate classes, largely due to a unique set of ecological demands (e.g., flight, foraging behavior, and sexual behavior). The typical avian eye is relatively conserved in that regard, stamped with several distinctions that afford high performance, such as large size and densely packed retinal neurons.”
In this, as in many other ways, our kiwi are far from ‘typical’ birds.
“Although strict nocturnality in birds is uncommon, most avian species active in low light levels have specializations within their visual systems that facilitate nocturnal performance (e.g., large pupillary aperture for gathering light, rod-dominated retinae for high light sensitivity). However, kiwi (Apteryx spp.), a nocturnal and flightless group of birds, do not follow this general pattern. They possess the smallest eyes relative to body mass of any avian species, have underrepresented visual brain regions, and have the smallest visual fields among birds.”
So what can the ‘average’ kiwi see? Not much, apparently.
“It is likely that the visual system of kiwi is only able to coarsely resolve objects within its visual field in a nocturnal environment. This surprisingly mundane set of visual characteristics for a bird becomes understandable when one considers the kiwi’s energetic devotion and input to other sensory systems: auditory, olfactory, and tactile somatosensory systems with a uniquely positioned set of mechanoreceptors at the tip of their long bill.”
So is eyesight even necessary for kiwi if they can successfully survive without it?
“The lack of reports of blind free-living birds in good health is a testament to the typical bird’s dependence on vision and raises questions regarding the role of vision in kiwi. Despite clear visual adaptations to maintain sight in a nocturnal activity pattern (e.g., rod-dominated retina, the robust health status of the three blind kiwi described here suggests that vision is not necessary, at least not in the ecological niche these three kiwi occupied.”
All kiwi may be on an evolutionary path to blindness?
“Kiwi visual specializations may be remnants from a common ancestor that relied more heavily on vision for survival (e.g., moa), and thus we may be witnessing an example of adaptive regressive evolution. Unlike the kiwi, all vertebrate examples of known regressive evolution of the visual system inhabit areas devoid of light and have completely lost vision. Kiwi could represent an intermediate stage of adaptive regressive evolution where the cost for maintaining a large eye is not well spent for what can be gained in low luminance on the forest floor. Perhaps kiwi eye size and brain visual centers have adapted more readily than the retina and have thus diminished in their relative importance while leaving the retina relatively specialized.”
The kiwi, of course, is not New Zealand’s only flightless, nocturnal bird – so what of the kakapo?
“There has only been one report of a blind, free-living bird: a kakapo (Strigops habroptilus). Additionally, a North Island brown kiwi (Apteryx mantelli) has been described to be blind, but was a captive bird. Both cases, however, were limited to a single individual, and both the ophthalmic status and physical health were not reported in detail. Currently the health status of the endangered kakapo is closely monitored for conservation purposes, and no living kakapos are blind.”
It’s possible that the kiwi’s eyesight is useful for little more than telling them whether it’s night or day.
“Like some nocturnal mammalian species with olfactory specializations that forage on the forest floor, kiwi may use vision to detect periodicity of day and night as a means of determining ideal activity time for foraging.”
The other finely-attuned senses of kiwi are well-adapted to helping them forage without relying on eyesight, but better eyesight might help kiwi better detect their ‘new’ enemies – introduced predators.
“An alternative to adaptive evolution, for which direct evidence is currently lacking, to be considered is that perhaps the evolution of kiwi in the absence of natural mammalian predators has driven sensory allocation away from predator detection and towards sensory systems being more directed at nocturnal ground foraging and social interactions. Predator detection is an unrelenting challenge faced by most bird species and is undoubtedly a major reason why profound ocular lesions in free-living birds are rare.”
The needs and visual trade-off of foraging and predator detection have ‘shaped’ the visual fields in most birds.
“Species that use non-visual cues while foraging tend to have wide visual fields (small blind areas, small binocular fields) for greater allocation toward the detection of predators. In contrast, species that are heavily dependent on vision for foraging tend to have narrow visual fields (larger blind areas behind and above the head) but wider binocular areas. The kiwi is the only known avian species that does not exemplify this pattern, having not only narrow binocular fields but also the narrowest visual fields (large blind areas) of any bird, presumably due to having evolved under little risk of predation. The eyes of kiwi have also not been reported to have significant degrees of movement, which is positively correlated with anti-predator behavior.”
Kiwi don’t need good eyesight to find food, but both blind kiwi and even those with the poor vision of ‘normal’ kiwi, may not be particularly good at detecting predators.
“While it is apparent that kiwi are able to support themselves nutritionally in the complete absence of vision, we do not fully understand how kiwi utilize their visual system. Whether we are witnessing an intermediate stage of adaptive regressive evolution or a consequence of sensory drive due to a unique ecological niche is yet to be determined. Being a rare example of a flightless, nocturnal species of bird having evolved with no consistent natural predators, the kiwi represents an excellent opportunity to study the ecology and evolution of the visual system from a unique perspective.”
The full report of this kiwi study is published in BMC Biology and is freely available online:
Blind free-living kiwi offer a unique window into the ecology and evolution of vertebrate vision (2017)