Final answer:
Birds achieve flight to higher altitudes without flapping their wings through aerodynamic lift, utilizing air thermals. Modifications like hollow bones and contour feathers aid in this efficient form of flight.
Step-by-step explanation:
The process that enables birds to fly to higher altitudes without flapping their wings, while riding on air thermals, is aerodynamic lift. This phenomenon occurs as birds take advantage of rising air currents, or thermals, caused by the uneven heating of the Earth's surface. Birds circling and rising within these thermals receive lift due to the currents of warmer air moving upwards. This lift is explained by the Bernoulli principle, where a difference in air pressure above and below the wing generates an upward force. Birds also make several modifications, such as having hollow bones and fused skeletal parts to reduce body weight, which aids in maintaining flight with minimal effort when soaring on these thermals.
Concerning the other question about the type of feathers that helps to reduce drag during flight, the answer is d. Contour feathers. These feathers streamline a bird's body, helping to minimize wind resistance and therefore enabling more efficient flight.