Genetic study reconstructes the evolutionary history of Haast’s Eagle and Eyles’ Harrier.
Weird things happen on islands. I am not talking about the series Lost, I am referring to the way some organisms change when they colonize an island. The fossil record has several striking examples of animals that shrink in size, such as the dwarf hippopotamus on Malta. The opposite of this insular dwarfism has also been observed: small animals become giants on the island. Examples of island gigantism include giant shrews on Corsica and giant dormice on Mallorca. A recent study in the journal Molecular Phylogenetics and Evolution investigated two avian examples: the huge birds of prey of New Zealand: Haast’s Eagle (Hieraaetus moorei) and Eyles’ Harrier (Circus teauteensis).
The largest extant eagle, the Harpy (Harpia harpyja) weights about 9 kilograms. The huge Haast’s Eagle was even heavier, estimated at up to 15 kilograms. Its wingspan measured 2.5 to 3 meters. This eagle probably preyed on various species of moa.
Eyles’ Harrier was a bit smaller than Haast’s Eagle. With a wingspan of about 2 meters and a weight of around 3 kilograms it is still the largest known harrier in the world. Its preys were probably smaller species of moa as well as kea (Nestor notabilis), kaka (Nestor meridionalis) and pigeons.
Both species went extinct within the last 700 years, probably due to human-driven habitat change. But where did these giant birds come from? To answer this question, Michael Knapp (University of Otago) and his colleagues sequenced the mitochondrial DNA from fossil material and compared it to extant species.
It turned out that the Haast’s Eagle is most closely related to the much smaller Little Eagle (Hieraaetus morphnoides) from Australia. Similarly, the Eyles’ Harrier is related to a small Australian species, the Spotted Harrier (Circus assimilis).
Molecular dating indicated that the ancestors of Haast’s Eagle and Eyles’ Harrier arrived on New Zealand about 2 million years ago. At this time, the changing climate resulted in deforestation culminating in open grasslands that provided ideal hunting grounds. Because there were no mammalian predators on the island, the raptors could quickly increase in size and establish their position as apex predator.
As mentioned in the beginning, organisms tend to become smaller or bigger on islands. This phenomenon is known as Foster’s Rule. If you are interested in this evolutionary process, I can highly recommend the videos by PBS Eons below. Enjoy!
Knapp, M., Thomas, J.E., Haile, J., Prost, S., Ho, S.Y.W., Dussex, N., Cameron-Christie, S., Kardailsky, O., Barnett, R., Bunce, M., Gilbert, M.T.P. & Scofield, R.P. (2019) Mitogenomic evidence of close relationships between New Zealand’s extinct giant raptors and small-sized Australian sister-taxa. Molecular Phylogenetics and Evolution, 134:122-128.