kiwi source data
Size and Sex in Raptors
In most birds, males are larger than females, but in some birds, such as many shorebirds and birds of prey, the reverse is true. No one is certain why there is this "reversed sexual size dimorphism" in raptors, but a number of interesting hypotheses have been advanced. All are based on a well-established correlation this size difference between the sexes is less pronounced in species that pursue sluggish prey than in those that pursue birds. Vultures, whose prey are least agile of all, show little sexual size difference. Mammal-hunting buteos, such as the Red-tailed Hawk, evolved males that are somewhat smaller than females, whereas in bird-hunting accipiters and falcons, females may be half again as heavy as males.
One explanation for the females' larger size suggests that it protects them from aggressive males that are well equipped with sharp talons and beaks, and the killer instincts to go with them. According to this theory, over evolutionary time, females have preferred to mate with smaller, safer males -- in fact, the female may have to be able to dominate the male for proper pair bonding to occur and for the male to remain in his key role as food provider to both female and young. Such a system would involve sexual selection for smaller size in males. Bird-hunting raptors are assumed to show aggression most suddenly, and to represent the greatest threat to their mates, and they are the ones exhibiting the greatest size difference.
In experimental pairings set up so that male American Kestrels were the larger of the pair, Cornell ornithologist Tom Cade found that the females did not suffer from an avian version of wife abuse. The size difference in kestrels was not very great, however, so this may not be a definitive test of the hypothesis. It seems that while sexual selection may play a role, there probably is more to it.
Another hypothesis proposes that the size differences allow the two sexes to hunt different prey and thus reduce competition for food. Competition is thought to be more severe among bird hunters than among other hawks, since their small agile prey are able to flee in three dimensions and are thus effectively scarcer than, say, carrion or ground squirrels. Indeed, there are data indicating that the hunting success of bird-chasing raptors is only about half that of raptors preying on mammals, and only a sixth that of raptors eating insects. Tom Cade has suggested that, for bird eaters, available food supply in the nesting territory can become limiting, making it adaptive for the male to specialize on small prey and for the female to specialize on large prey. The male feeds the female and young at the beginning of the nesting season; the female becomes an active hunter when the nestlings are larger, and the adults then tend to partition the prey resource in their territory.
But if reducing intersexual competition for food is the reason for the size difference in raptors, why aren't males sometimes the larger sex? One possible reason is that females need to be larger because they must accumulate reserves in order to produce their eggs. Another is that females do not forage for a substantial period while incubating eggs and brooding young. They avoid the risks of the hunt during that time, but they must rely on the small male to feed the entire family. Small fleet prey, aerial or terrestrial, are more abundant than large sluggish prey, so that over time smaller male bird-eating raptors would be favored over larger, less agile ones, because they would be better providers. For species that take more sluggish prey, however, small males would not be so advantageous, which might explain the relationship between prey speed and the male-female size discrepancy.
In most birds, males are larger than females, but in some birds, such as many shorebirds and birds of prey, the reverse is true. No one is certain why there is this "reversed sexual size dimorphism" in raptors, but a number of interesting hypotheses have been advanced. All are based on a well-established correlation this size difference between the sexes is less pronounced in species that pursue sluggish prey than in those that pursue birds. Vultures, whose prey are least agile of all, show little sexual size difference. Mammal-hunting buteos, such as the Red-tailed Hawk, evolved males that are somewhat smaller than females, whereas in bird-hunting accipiters and falcons, females may be half again as heavy as males.
One explanation for the females' larger size suggests that it protects them from aggressive males that are well equipped with sharp talons and beaks, and the killer instincts to go with them. According to this theory, over evolutionary time, females have preferred to mate with smaller, safer males -- in fact, the female may have to be able to dominate the male for proper pair bonding to occur and for the male to remain in his key role as food provider to both female and young. Such a system would involve sexual selection for smaller size in males. Bird-hunting raptors are assumed to show aggression most suddenly, and to represent the greatest threat to their mates, and they are the ones exhibiting the greatest size difference.
In experimental pairings set up so that male American Kestrels were the larger of the pair, Cornell ornithologist Tom Cade found that the females did not suffer from an avian version of wife abuse. The size difference in kestrels was not very great, however, so this may not be a definitive test of the hypothesis. It seems that while sexual selection may play a role, there probably is more to it.
Another hypothesis proposes that the size differences allow the two sexes to hunt different prey and thus reduce competition for food. Competition is thought to be more severe among bird hunters than among other hawks, since their small agile prey are able to flee in three dimensions and are thus effectively scarcer than, say, carrion or ground squirrels. Indeed, there are data indicating that the hunting success of bird-chasing raptors is only about half that of raptors preying on mammals, and only a sixth that of raptors eating insects. Tom Cade has suggested that, for bird eaters, available food supply in the nesting territory can become limiting, making it adaptive for the male to specialize on small prey and for the female to specialize on large prey. The male feeds the female and young at the beginning of the nesting season; the female becomes an active hunter when the nestlings are larger, and the adults then tend to partition the prey resource in their territory.
But if reducing intersexual competition for food is the reason for the size difference in raptors, why aren't males sometimes the larger sex? One possible reason is that females need to be larger because they must accumulate reserves in order to produce their eggs. Another is that females do not forage for a substantial period while incubating eggs and brooding young. They avoid the risks of the hunt during that time, but they must rely on the small male to feed the entire family. Small fleet prey, aerial or terrestrial, are more abundant than large sluggish prey, so that over time smaller male bird-eating raptors would be favored over larger, less agile ones, because they would be better providers. For species that take more sluggish prey, however, small males would not be so advantageous, which might explain the relationship between prey speed and the male-female size discrepancy.
SOURCE DATA: https://teara.govt.nz/en/kiwi/page-2
Kiwi species
It was once thought that New Zealand had three species of kiwi (Apteryx genus). Now it is thought that there are five.
North Island brown kiwiThe North Island brown kiwi (Apteryx mantelli) has reddish-brown plumage and a quick temper. It was once widespread in lowland bush throughout the North Island. By 2002 there were an estimated 25,000 left – about 8,000 each in Northland, the east coast from Hawke’s Bay to the Bay of Plenty, and on the west coast including the King Country, Taranaki and Whanganui. There are also 1,000 birds in the Coromandel. In Northland, brown kiwi have colonised pine forests.
TokoekaOnce thought to be a variety of brown kiwi, tokoeka (Apteryx australis) are now considered a separate species. They are larger, with softer plumage and more communal habits than the North Island brown kiwi. Some 20,000 live on Stewart Island and about 13,000 in Fiordland. Habitats range from high, snowy mountains to sandy coasts. One form, the Haast tokoeka, is a rare, shy bird of South Westland, inland from Haast. In 2020 its population was thought to be about 400 birds.
RowiThe rowi (Apteryx rowi) is the rarest species. In 2015 there were only 450, in the Ōkārito region of Westland. Unlike other brown kiwi, they have greyish colouring and white patches on the face.
Great spotted kiwiThe great spotted kiwi (Apteryx haastii) lives in tough subalpine conditions. It is found in north-west Nelson, the Paparoa Ranges and the Southern Alps between Arthur’s Pass and Lake Sumner. In 2002 it was estimated that 17,000 remained. As its name suggests, the great spotted (roroa or roa in Māori) is the largest kiwi (45 centimetres high). Its grey feathers are mottled with white bands.
Little spotted kiwiAt 25 centimetres tall the little spotted kiwi (Apteryx owenii) or kiwi pukupuku is the smallest kiwi. It was once widespread throughout New Zealand, but suffered severely from predators, despite its aggressive personality. The largest population of about 1,200 survives on Kāpiti Island, near Wellington. Since the 1980s there have been successful transfers to other offshore islands, and to Wellington’s Karori Wildlife Sanctuary (Zealandia).
Kiwi species
It was once thought that New Zealand had three species of kiwi (Apteryx genus). Now it is thought that there are five.
North Island brown kiwiThe North Island brown kiwi (Apteryx mantelli) has reddish-brown plumage and a quick temper. It was once widespread in lowland bush throughout the North Island. By 2002 there were an estimated 25,000 left – about 8,000 each in Northland, the east coast from Hawke’s Bay to the Bay of Plenty, and on the west coast including the King Country, Taranaki and Whanganui. There are also 1,000 birds in the Coromandel. In Northland, brown kiwi have colonised pine forests.
TokoekaOnce thought to be a variety of brown kiwi, tokoeka (Apteryx australis) are now considered a separate species. They are larger, with softer plumage and more communal habits than the North Island brown kiwi. Some 20,000 live on Stewart Island and about 13,000 in Fiordland. Habitats range from high, snowy mountains to sandy coasts. One form, the Haast tokoeka, is a rare, shy bird of South Westland, inland from Haast. In 2020 its population was thought to be about 400 birds.
RowiThe rowi (Apteryx rowi) is the rarest species. In 2015 there were only 450, in the Ōkārito region of Westland. Unlike other brown kiwi, they have greyish colouring and white patches on the face.
Great spotted kiwiThe great spotted kiwi (Apteryx haastii) lives in tough subalpine conditions. It is found in north-west Nelson, the Paparoa Ranges and the Southern Alps between Arthur’s Pass and Lake Sumner. In 2002 it was estimated that 17,000 remained. As its name suggests, the great spotted (roroa or roa in Māori) is the largest kiwi (45 centimetres high). Its grey feathers are mottled with white bands.
Little spotted kiwiAt 25 centimetres tall the little spotted kiwi (Apteryx owenii) or kiwi pukupuku is the smallest kiwi. It was once widespread throughout New Zealand, but suffered severely from predators, despite its aggressive personality. The largest population of about 1,200 survives on Kāpiti Island, near Wellington. Since the 1980s there have been successful transfers to other offshore islands, and to Wellington’s Karori Wildlife Sanctuary (Zealandia).
KIWI LOCATIONS AND SPECIES
Species
GS-Great Spotted
NIBr-NorthIsland Brown
Tok-Southern Tokoeka
Gender
M-Male
F-Female
Weight(kg)
The weight of the kiwi bird in kg
Height(cm)
The height of the kiwi bird in cm
Location
NWN-North West Nelson
SF-South Fiordland
CW-Central Westland
N-Northland
EC-Eastern Canterbury
E-East North Island
StI-Stewart Island
W-West North Island
NF-North Fiordland
Species
GS-Great Spotted
NIBr-NorthIsland Brown
Tok-Southern Tokoeka
Gender
M-Male
F-Female
Weight(kg)
The weight of the kiwi bird in kg
Height(cm)
The height of the kiwi bird in cm
Location
NWN-North West Nelson
SF-South Fiordland
CW-Central Westland
N-Northland
EC-Eastern Canterbury
E-East North Island
StI-Stewart Island
W-West North Island
NF-North Fiordland
More on dimorphism
Sexual DimorphismD.J. Fairbairn, in Encyclopedia of Evolutionary Biology, 2016
What Is Sexual Dimorphism?The term dimorphism denotes a trait that occurs in two distinct forms or morphs within a given species and traits that differ consistently between males and females are sexual dimorphisms. Sexually dimorphic traits may differ so radically between sexes that they can be reliably used to differentiate males from females. Color dimorphisms in many species of birds, lizards, and fishes are familiar examples of this (Figure 1). Other common examples include the presence of antlers or other weapons in only one sex (usually males), adaptations of one sex (usually females) for parental care (e.g., mammary glands in female mammals and brood pouches in many invertebrates), and externally apparent differences in genitalia (Figure 2). However, many sexual dimorphisms are not as extreme as this. Any trait that differs on average between sexes is considered sexually dimorphic, even if the trait distributions overlap considerably between sexes. Height in humans provides a familiar example of this type of sexual dimorphism
Sexual DimorphismD.J. Fairbairn, in Encyclopedia of Evolutionary Biology, 2016
What Is Sexual Dimorphism?The term dimorphism denotes a trait that occurs in two distinct forms or morphs within a given species and traits that differ consistently between males and females are sexual dimorphisms. Sexually dimorphic traits may differ so radically between sexes that they can be reliably used to differentiate males from females. Color dimorphisms in many species of birds, lizards, and fishes are familiar examples of this (Figure 1). Other common examples include the presence of antlers or other weapons in only one sex (usually males), adaptations of one sex (usually females) for parental care (e.g., mammary glands in female mammals and brood pouches in many invertebrates), and externally apparent differences in genitalia (Figure 2). However, many sexual dimorphisms are not as extreme as this. Any trait that differs on average between sexes is considered sexually dimorphic, even if the trait distributions overlap considerably between sexes. Height in humans provides a familiar example of this type of sexual dimorphism