There is no definitive answer to this question as there are many different ways to measure strength. Some birds may be stronger flyers, while others may be able to lift heavier objects. Additionally, different bird species have different levels of physical strength. For example, an ostrich is much stronger than a hummingbird.
One way to measure the strength of a bird is by its wingspan. The Andean condor has the largest wingspan of any living bird, at up to 3.2 meters (10 feet 6 inches). This allows them to soar for hours without flapping their wings and gives them the ability to fly at high altitudes where other birds cannot survive. The Andean condor also has very powerful legs and talons which it uses for hunting prey and defending itself from predators.
Another way to measure the strength of a bird is by its weight-lifting ability. The kori bustard is the heaviest flying bird in the world, with males weighing up to 18 kilograms (40 pounds). These massive birds are found in Africa and can take off into flight carrying heavy loads such as eggs or small mammals in their talons. Kori bustards have also been known to fight off predators such as lions with their powerful kicks and strikes from their large feet.
So which bird is the strongest? It depends on how you define strength!