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There are a lot of seagulls in a park near where I live. Sometimes I can see them somehow "sliding" forward or backward diagonally when flying against the wind, however, their wings aren't flapping. What are the flight mechanics behind this?

Can gliders mimic this style of flight?

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I've never heard of other birds than albatrosses do dynamic soaring, but since this is over land, it can be simpler ridge soaring—as the sea breezes hits the shore (and obstacles on it), it turns upwards and the birds simply fly in this updraft. They are flying against the wind and gliding down relative to it, but when you add the speed of the wind, they may be stationary relative to the ground—or zig-zag from side to side if the wind speed is a bit less than the forward speed the birds need to efficiently glide.

Gliders can do the same, but since they are bigger, they need a mountain range where the seagulls can do with a several feet embankment.

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If your perception is that the seagulls are pointing straight into the wind but moving sideways to the wind, I suspect that you are mistaken. When the wind is strong and the groundspeed is very low, it only takes a slight difference between aircraft (or bird) heading and wind direction to produce a large crosswind component in the ground track. This is certainly true for gliders (and airplanes) as well as for birds.

So "sliding sideways" in relation to the wind is accomplished by steering the aircraft (or bird) to point slightly to the left or right of where the wind is coming from. In strong wind, an aircraft or bird can move perpendicular to the wind direction, with no "forward" progress, while still pointing almost directly into the wind with wings level. Note that this does not actually involve any sideways component to the airflow over the aircraft (or bird).

If the seagulls you observed are maintaining altitude while doing this, without flapping their wings, then they are undoubtedly in slope lift. Slope-soaring on a windy day often involves pointing almost straight into the wind, even while the ground track follows the line of the slope, which may be perpendicular to the wind.

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Some birds 'harvest' energy from the wind gradient. Perhaps it's the case of the seagulls that you have observed.

https://en.wikipedia.org/wiki/Dynamic_soaring

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    $\begingroup$ I'm sure the more probable answer is ridge lift. $\endgroup$ – Peter Kämpf Apr 24 '17 at 19:05

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