To remain in the air fixed wing aircraft need to maintain airspeed above their stall speed. I assume this also applies to birds. If so, anyone know the stall speed of a sea gull who sometimes appear standing still? Obviously they are using the wind, but without it how fast do they need to fly to maintain altitude?
Birds are not fixed wing, they can adapt themselves so that they can hover like a kite with minimal effort.
What do you mean, African or European seagull?
To give a less jokey reply, we can try to compare to airplanes. Using US measurements because that is what I have in my airplane spreadsheets. Going to Wikipedia for the stats use a wingspan of 4.5 ft and weight of 1.5 lbs. Looking at pictures estimate the aspect ration of about 10 leading to a wing area of around 2 ft^2.
For a lift coefficient I’ll guess 2.5. This would be very high for an airplane but birds can alter their wing shape a great deal. They can’t actually hover without flapping but can fly very slow.
Using the standard lift=1/2 * density * lift coefficient * area * speed^2, solve for speed in mph, weight in lbs, area in ft^2 and standard sea level density you get speed=19.7 * sqrt(weight/(lift coefficient * area). With the guesses above that comes out to 11 mph. Just a back of the envelope number, but should be in the ballpark.
To fly slower than that they would need to flap to push more air down. You can see birds do that as they come in to land.
Birds can hover in a headwind. That meets the minimum stall speed requirements if the wind is strong enough. For a bird, so lightweight and with the ability to reshape its wings for maximum low-airspeed lift, it doesn’t take much air over the wings to make it happen. A steady breeze can be enough.
I remember some claim that birds can fly as slow as 5-6MPH. However, birds rarely fly straight and level with no wind at slow speeds. Most bird flight involves gaining altitude initially, then using a headwind or thermal for more lift, and using gravity to gain speed in a descent.
ETA: Hummingbird flight is obviously a whole different subject.
And airplanes can take off “accidentally” in strong enough winds, if they’re not properly domesticated.
The flapping of the wings creates airspeed to lift the bird, also. Moving their wings in a rowing motion produces lift on the way up. Also, changing wing shape is like flaps on a plane, changing the stall speeb.
I doubt that this observation changes anything here, but in strong winds, I have watched seagulls hovering, then moving backwards and having to adjust their wings to move forward. They have a lot of control.