Wednesday, March 8, 2017

South Island Giant Moa (Dinornis robustus)

The South Island Giant Moa (Dinornis robustus) was the largest of all known moa species and the tallest bird yet discovered. It inhabited South Island, New Zealand during the Quaternary.

Skull of the South Island Giant Moa, Dinornis robustus, collected 15 Mar 1992,
Maximus Cave, New Zealand. Field Collection 1986. CC BY-NC-ND licence.
Te Papa (S.028225)

Etymology
In reference to its great size, the name Dinornis is derived from the Geek words deinos, meaning “prodigious” or “terrible”, and ornis, meaning bird. Its common name denotes South Island, New Zealand to which this bird was endemic. Another species, D. struthoides, which was described based on its similar morphology but much smaller size, is now known to have been the male morph of D. robustus as of 2003.

Habitat & Distribution
South Island Giant Moa had a relatively broad range of habitat preferences, occurring in nearly all vegetated habitats on South Island. Widespread and abundant, the population of this species has been estimated to have been around 479,000 individuals prior to the arrival of human settlers.

Physical Attributes
The South Island Giant Moa stands out as the largest of all the moa and the tallest bird known to have ever lived. Fully grown females stood up to 2m tall at the hips, up to 3.6m when the neck was held fully erect, and weighed a maximum of 250kg (550lbs). Among recently extinct birds, only the Elephant Bird (Aepyornis maximus) of Madagascar was more massive, albeit not as tall. Adult male South Island Giant Moa were considerably smaller, at up to two-thirds the height and one-third the weight of the females. This species had a stilt-legged, rangy skeleton which would have made it very agile despite its great size. It differs from the North Island Giant Moa (Dinornis novaezelandiae) in its larger size, having a relatively shorter, stronger neck, and its deeper, less downcurved beak. Preserved soft tissue remains are known from this species, including an intact foot complete with dried skin, ligaments, and muscles adhering to the bones. Known plumage shows that most of its body except for its lower legs was covered in long, reddish-brown hair-like feathers up to 18cm in length.


Ecology & Behavior
Evidence from coprolites and gizzard contents shows that the South Island Giant Moa was a mixed feeder which grazed and browsed on various trees, shrubs, herbs, and grasses within its environment. Their great height further enabled them to feed on vegetation which other moa could not reach, thus minimizing competition for available food taxa. Biomechanical studies have shown that South Island Giant Moa fed by performing a lateral shaking motion of its head when browsing. Aided by an exceptionally broad head and beak, this behavior enabled them to break twigs and branches of considerable width. Furthermore, evidence suggests that a strong sex-related niche stratification existed within this species. Females appear to have browsed within forested environments where they subsisted on lower-quality, fibrous plant matter while males were more likely to feed on herbfields and forest clearings.

The only natural predator to adult South Island Giant Moa was the Haast’s Eagle (Harpagornis moorei), which was large and powerful enough to fell even the larger females of this species, although the eagle may have been more likely to target the smaller juveniles and adult males due to their more manageable sizes. Preferential hunting of males by the eagles may partly explain why female South Island Giant Moa significantly outnumber their male counterparts at natural fossil-bearing deposits. The eggs and chicks of South Island Giant Moa were more likely to have been preyed upon by South Island Adzebills (Aptornis defossor) or Eyles’ Harrier (Circus eylesi) respectively.

Size comparison between Haast's Eagle (Harpagornis moorei) with male and
female South Island Giant Moa (Dinornis robustus). 

Large, white eggs measuring 240x178mm and weighing an estimated 4kg when fresh have been attributed to this South Island Giant Moa. Females laid 1 to 2 eggs in shallow nests constructed by males who would take on sole incubation duties for at least 2 months. Like modern birds which demonstrate high degrees of female-biased sexual dimorphism, females are likely to have mated with multiple males within their home range and would have competed aggressively with each other over nests. Chicks were likely precocial and able to forage on their own soon after hatching and attained their adult size within just 3 years after hatching.

South Island Giant Moa were heavily exploited by the Maori and became extinct in as little as 100 years after their arrival. Their bones are widespread in Maori middens and comprise a disproportionately large number of males and eggs: the opposite of what seems to have been the case in a natural setting where it seems that adult females were more numerous than males at a given locality. The reason for the large number of male birds and eggs found at archaeological sites may indicate that the Maori chose to collect male birds while they were tending to their nests and were relatively sedentary. This strategy may have been easier than hunting the larger and potentially more aggressive females, but in doing so the Maori would have further reduced the already limited number of breeding males and cut the number of moa which would have survived to maturity. This would have resulted in catastrophic population declines from which the slow-breeding birds could not recover.

Mounted skeleton of South Island Giant Moa in
Yorkshire Museum collections. Wiki

References & Further Reading
Attard MRG, Wilson LAB, Worthy TH, Scofield P, Johnston P, Parr WCH, Wroe S (2016). "Moa diet fits the bill: virtual reconstruction incorporating mummified remains and prediction of biomechanical performance in avian giants". Proceedings of the Royal Society of London B 283: 20152043 <Full Article>

Rawlence NJ, Wood JR, Scofield RP, Fraser C, Tennyson AJD (2013). "Soft-tissue specimens from pre-European extinct birds of New Zealand". Journal of the Royal Society of New Zealand DOI:10.1080/03036758.2012.704878 <Full Article>

Wood JR, Wilmshurst JM, Richardson SJ, Rawlence NJ, Wagstaff SJ, Worthy TH, Cooper A (2013). "Resolving lost herbivore community structure using coprolites of four sympatric moa species (Aves: Dinornithiformes)". PNAS 110(42): 16910-16915 <Full Article>

Oskam CL, Allentoft ME, Walter R, Scofield RP, Haile J, Holdaway RN, Bunce M, Jacomb C (2012). "Ancient DNA analyses of early archaeological sites in New Zealand reveal extreme exploitation of moa (Aves: Dinornithiformes) at all life stages". Quaternary Science Reviews 52: 41-48 <Full Article>

Allentoft ME, Bunce M, Scofield RP, Hale ML, Holdaway RN (2010). "Highly skewed sex ratios and biased fossil deposition of moa: ancient DNA provides new insight on New Zealand’s extinct megafauna". Quaternary Science Reviews 29: 753–762 <Abstract>

Huynen L, Gill BJ, Millar CD, Lambert DM (2010). "Ancient DNA reveals extreme egg morphology and nesting behavior in New Zealand’s extinct moa". Proceedings of the National Academy of Science 107(37): 16201-16206 <Full Article>

Wood JR, Rawlence NJ, Rogers GM, Austin JJ, Worthy TH, Cooper A (2008). "Coprolite deposits reveal the diet and ecology of the extinct New Zealand megaherbivore moa (Aves, Dinornithiformes)". Quaternary Science Reviews 27: 2593–2602 <Abstract>

Turvey ST, Green OR, Holdaway RH (2005). "Cortical growth marks reveal extended juvenile development in New Zealand moa". Nature Letter 435 doi:10.1038/nature03635 : 940-944 <Abstract>

Gemmell NJ, Schwartz MK, Robertson BC (2004). "Moa were many". Proceedings of the Royal Society B doi: 10.1098/rsbl.2004.0234: <Full Article>

TH Worthy (1990). "An analysis of the distribution and relative abundance of moa species (Aves: Dinornithiformes)". New Zealand Journal of Zoology 17(2): 213-241 <Full Article>

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