Re: stall (v)
5. a condition of an aircraft in flight in which a reduction in speed or an increase in the aircraft's angle of attack causes a sudden loss of lift resulting in a downward plunge
stall: the condition of an airfoil or aircraft (or, in this case, a seagull) in which excessive angle of attack causes disruption of airflow with attendant loss of lift
The wing isn't helping the flying object stay aloft any longer.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.