In fact: There are four opposing forces on a moving object that determine its ability to fly: weight, lift, thrust, and drag. Generally speaking, thrust is a force pushing an object forward; drag is the friction holding it back. When thrust is stronger than drag, the subject can move forward. Weight, or gravity, is the force downwards towards earth. It’s determined by the mass of the object. Lift is the upward force allowing the subject to get higher in the air. If the lift force is stronger than the force of gravity pulling it down, a bird can be airborne. Therefore, if a bird’s thrust (forward force) and lift (upward force) are stronger than gravity and friction, they can fly.
MIT
“In order for an aircraft to rise into the air, a force must be created that equals or exceeds the force of gravity. This force is called lift. In heavier-than-air craft, lift is created by the flow of air over an airfoil. The shape of an airfoil causes air to flow faster on top than on bottom. The fast flowing air decreases the surrounding air pressure. Because the air pressure is greater below the airfoil than above, a resulting lift force is created.“
StudyFlight
“Lift is the aerodynamic force that is perpendicular to the flight path and is crucial for keeping the aircraft airborne. It’s generated primarily by the wings of the aircraft through a difference in air pressure. When air flows over and the under wing, it travels faster over the top surface, creating lower pressure compared to the higher pressure under the wing. This pressure differential produces lift, effectively countering the weight of the aircraft and allowing it to rise”
Medium
“Birds flap their wings to generate lift and thrust, when they stop flapping and keep their wings stretched out, the wings actively produce lift ( a vertical force sustaining them) not thrust (force in the direction of motion). Lift is the component of aerodynamic forces perpendicular to the flow direction. The explanation for the lift generation is based on Newton’s second and third laws of motion. The net force of an object is equal to the rate of momentum change. To every action there is an equal and opposite reaction.”
Lets Talk Science
“If an aircraft is being pulled down toward the Earth by gravity and its own mass, how does it stay in the air? The answer is the second force, lift. Lift refers to the force that an object needs to overcome its weight. Lift is an upward force caused by air moving over a wing.”
flerf.org 