Haast’s eagle, New Zealand giant eagle – Largest Eagle ever
Guess what,
Previously I think that the harpy eagle is the largest and strongest of all eagle… however, when I read about Haast’s eagle, definitely I am wrong…
Haast’s eagle’s wingspan is about 2.6 meter for a large female and weight approximately 10-13 kg 
Here is the information from BBC site:
Haast’s eagle, New Zealand giant eagle
Harpagornis moorei
Haast’s eagle was the largest eagle ever to have lived and is the only eagle in the world ever to have been top predator of its ecosystem
Physical Description
Haast’s eagle was a large eagle with a low, narrow skull and an elongated beak. The males were smaller than the females. It had relatively short wings for its size: these were designed for flapping flight not for soaring. Its wing structure also helped it to catch and subdue prey as large as, or larger than, the eagle itself, and was better suited for fast, manoeuvrable flight in dense forest. Because of its large size, Haast’s eagle was approaching the upper limit of size for flapping flight – if it got any bigger it would have had to rely on gliding. Its leg bones were better suited for perching or for gripping prey than for walking about on the ground. The structure of the foot and length of the talons meant that Haast’s eagle could apply much greater force with its feet than other birds of prey. The talons could stab several centimetres into flesh, and often punctured bones as well.
Diet
It preyed upon flightless birds, including various species of moa. Palaeontologists believe that its prey ranged in size from 1kg to over 200kg in weight – the latter being the giant moa (Dinornis giganteus). The most common prey was likely the flightless Finsch’s duck (Euryanas finschi), now extinct. As New Zealand lacked any terrestrial mammals, the Haast’s eagle was top predator.
Behaviour
The Haast’s eagle is unusual, because of the sheer size of many of its prey. Most eagles kill animals that are less than their own body weight. This is because they have to be able to fly while carrying their kill. As there were no terrestrial predators bigger than a tuatara (a reptile about 500g-1kg in weight) in New Zealand, the Haast’s eagle only had to defend its meal from other eagles, and thus didn’t have to carry it to a safe place to eat it. The eagle attacked a variety of flightless birds found in New Zealand including the now extinct moas. It would launch itself from a high perch onto its prey and strike at the moa’s side. Its large talons grasped the hindquarters of the moa, and killed it by inflicting deep crushing wounds that caused massive internal bleeding. The moa perished from shock or blood loss. Over a dozen fossil moa have been found with gashes and punctures from eagle claws on their pelvis. Fossil moa bones show us how the eagle used its beak after it had caught its prey: it used the elongated beak to open up the carcass and reach inside to grab mouthfuls of organs such as the kidneys. When people arrived in New Zealand, the eagle may have mistaken them for moa and thus attacked and eaten them.
History
The Haast’s eagle was found all over South Island during the Pleistocene, but was mostly restricted to the south and east of South Island after the end of the Ice Age. The arrival of people in New Zealand had unfortunate consequences for the eagle: by 1400 AD, most of the forest habitat it used had been cleared by fire, and most of the large flightless birds that it ate had been hunted to extinction. The Haast’s eagle was likely extinct by 1400 AD, although there are a few 19th century accounts of sightings of very large birds of prey in mountainous areas.