How can seagulls slide in the wind without flapping their wings?
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.