One of the most common misconceptions regarding any branch of the animal kingdom is that “all birds can fly.” Apterygidae, commonly known as kiwi birds (referred to as simply ‘kiwi’), is one of the families of birds that is incapable of flying. Along with this trait, kiwis possess many others that are indicative of both their highly terrestrial lifestyle, and mammal-like tendencies. The culmination of their characteristics makes them a very unique family in the avian world, and in the animal kingdom in general.
“Ratite” is the clade of flightless birds within the avian class, to which kiwis belong. For years, the evolution of ratite birds, specifically the loss of the bird’s ability to fly, was believed to have been the result of the large continent, Gondwana, breaking up during the Cretaceous period, leading to vicariant speciation. If this hypothesis was correct, molecular evidence should show an alignment between kiwi and ostriches, which are considered to be the the basal component of ratite lineage. But recent genetic research has shown that this is not the case, and instead, it is now believed that loss of flight is a secondarily derived characteristic, due to the fact that now, the closest known relatives of kiwi are the extinct elephant birds of Madagascar, Aepyornithidae. With that being said, the ratite lineage must have exhibited flighted dispersal in order to arrive at secondarily derived flightlessness, considering the fact that kiwi are found only on the isolated continent of New Zealand, while their closest relatives were previously only found on Madagascar (another isolated landmass), before they came extinct.1, 2 In terms of the taxonomic divisions within the family, there were originally five recognized specie (dating back to the 1800s, although molecular and other biological evidence brings to light the potential for there to be at least three distinct lineages of the Brown Kiwi, as a result of geographic divergence on separate parts of the isolated continent. Overall, scientists believe that there may be at least 9 species within Apterygidae at this time.3,4
Often compared to the size and general shape of a common farm chicken, kiwis have a round, bulbous body and a small head. Their tough, leathery skin is covered with feathers which look more like hair rather than the feathers that are commonly associated with birds. Furthermore, strong, stocky legs afford them the ability to run at high speeds, in order to quickly evade predators, since they are unable to fly away when threatened.5 The largest species of kiwi, Apteryx haasti can reach heights of 45 centimeters, while the smallest kiwi species, Apteryx owenii, reaches a maximum height of 25 centimeters.6
Unlike many other species of birds which have hollow bones in order to lighten their body weight and make them more apt for a lifestyle involving flight, kiwi have marrow within their bones, much like that of mammals. Furthermore, most birds have a raised central keel on their breastbone which is the attachment point for the muscles that are primarily used in flight — kiwi’s do not have this. Instead, they have a flat breastbone, yet another indication of their flightless lifestyle, closely mimicking that of mammals.6
Compared to all other birds, kiwis have the lowest basal metabolic rate, and thus regularly function at temperatures well below most . These low metabolic rates can be associated with the minimized amount of muscle, especially in the pectoral region of kiwi, since prominent musculature in that area is not needed for flying. Additionally, lower metabolic rates in kiwi have also been deemed another point of similarity with mammals — it is thought that these birds are able to combat low temperatures during the night when they are most active, due to their ability to facilitate energy conservation as a result of their highly developed, overlapping feathers (as previously mentioned, the feathers are very similar to mammalian fir). In terms of delineating rates between males and females, research has shown that females do in fact exhibit increased metabolism during mating season in order to account for the massive parental burden they experience7. Kiwi’s breeding season runs approximately from August-September, during which time they lay approximately 1-2 eggs. Another unique characteristic of the Apterygidae family is their capability to lay eggs that are the largest in relation to their body size of any species of bird, up to 15% of the female’s body weight.8,9 Interestingly, one the female lays her eggs, they not only require the longest incubation period of any species of bird, but the incubation is done by the male. It is thought that kiwi females leave their partners nest-bound because they are polyandrous, meaning that they will go and mate with other males during the same breeding season — this is an indication that an uneven sex ratio exists within species of kiwi, with there being more males than females.10 Males will incubate the eggs for 11-12 weeks, after which time the chicks hatch, are independent after 14-20 days of parental care, and reach full maturity after 20 months.11
One of the most unique characteristics of Apterygidae is that they are the only bird to possess nostrils at the end of their long, drinking straw-like bill, affording them one of the greatest smelling capabilities of birds.12 Kiwi’s feed primarily on invertebrates and fruit that has fallen to the ground, and they use their smelling ability as well as their protruding bill to sniff around for their food and probe the ground to excavate their food out.7 For many years, it was believed that the olfactory capability in all species of birds was greatly reduced, due to the sense being an overall unimportant facet of the organism. Recent work done within the last decade has proved this belief to be largely incorrect for Apterygidae. Genetic testing in kiwi has shown there to be a large number of olfactory receptor genes, which in other species of vertebrates, are associated with improved ability to detect odors. Additionally, behavioral observation reported kiwis inhaling air through their nostrils, aligning their body in the direction of the scent which they are following, and moving in that direction (similar to how mammals use their nostrils), again indicating the the olfactory bulb is most likely highly developed within this family. Finally, kiwi also possess a uropygial gland (located next to the cloaca) which is thought to produce chemicals that are subsequently deposited within the feces and left at specific locations in order to be used as social markers since they are detected via smell.13
All members of Apterygidae possess a generalized vocal behavior in the form of repeated shrieking, although there are different, species-specific calls. Both male and female kiwi show clear changes in the rate at which they produce vocalizations depending on the time of year, due to the fact that they are largely used for mating and breeding purposes, and on the time of day, considering their nocturnal lifestyle.14
In modern times, Apterygidae have had an interesting relationship with humans. Considering their nocturnal tendencies, many people in New Zealand never really see kiwi. Despite this, kiwi have been deemed the national bird of the country and serve as a significant point of pride amongst New Zealanders. With that being said, humans are also one of the most serious threats to kiwis. Researchers estimate that over the last 100 years, the number of kiwi’s on Earth has reduced upwards of 90%. Each of the five main species of kiwi are to some extent threatened. Kiwi are threatened by many species that are considered to now be invasive, originally brought into the country by Polynesians and Europeans — these organisms include cats, dogs, ferrets, pigs, opossums, and weasels. Although kiwis largely evolved in the presence of native competing mammals, they do not have the appropriate responses and/or adaptations in order to fend off the invasive species. The eggs, as well as the full grown birds are at risk for predation by these other species. 4 The Kiwi Recovery Program, which was launched in 1991, is specifically focused on managing kiwi populations such that they do not go extinct. Including managing the extent of human deforestation throughout New Zealand, predator control is also a major point of focus15. A reasonable degree of success in the realm of kiwi conservation has been found in creating sanctuaries throughout New Zealand. The primary goals of these sanctuaries is to identify given areas of land over which they attempt to reduce overall invasive mammalian predation, removing the eggs and potentially the young offspring when they are born to safer places away from potential predation instances, and then returning the matured adults once they have developed the senses necessary in order to be successful within their environment.16
Overall, Apterygidae is one of the most unique families within the avian class in that the similarities to mammals are striking: their overall body structure, with strong legs as a result of their secondarily derived inability to walk; low basal metabolic rates and ability to productively conserve energy; and their highly developed olfactory bulb and smelling capabilities, are some of the most prominent characteristics of this family and within this comparison. Since kiwi are only found on the isolated land mass that is New Zealand, there is a high degree to which they are threatened from invasive predators, in addition to the human deforestation that has been an ongoing issue since the beginning of the 1900s. With that being said, these organisms need to be protected, and there are conservation efforts already in place attempting to accomplish that. Further research needs to be done on this family to not only identify the potential different species that exist, but also clarify taxonomic delineations in terms of their distinct similarity to mammals and understanding the underlying propellers to the many secondarily derived characteristics.
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