Greylag geese can fly at high altitudes of about 8,000 meters (5 miles). This is in fact quite a difficult task since many creatures cannot breathe at such altitudes because of the lack of oxygen available at high altitudes. This deficiency makes respiration difficult. That is why we can hardly breathe when we climb up a high mountain. Flying at an altitude with a rarefied atmosphere, the bird needs to flap its wings faster. In order to move its wings more, the bird will expend more oxygen, which makes the situation even more difficult for it. However, these birds do not experience any difficulty even while they are flying at altitudes of thousands of meters. That is a result of the special creation of their lungs, which make the most of the rare oxygen at high altitudes.
Functioning in a different way than those of other creatures, these birds' lungs enable them to obtain more energy from the rarefied atmosphere. And this is one of the signs of Allah's perfect creation.