Goose slide.
Nature provides for birds to lighten the load prior to take-off, so a bird already up in the sky will not need to defecate.
My experience was a bit different. In college some friends and I chose to eat out on the patio at the restaurant by the beach. Seagulls were eating the bits tossed out by people nearby. One of the gulls flew over our table and proceeded to christen my head, shoulder, and plate. Gross. After cleaning up we moved inside for the rest of our meal.
I had several projects at a large bay area refuse processing facility that seemed at any given moment was populated by something like 1/4-1/2 of the seagull population of the San Francisco bay. On occasion they utilized a contract falconer to come through and let a couple hawks put the fear to them. When those seagulls were airborne and that guy released those raptors it would result in a SNOWSTORM of seagull s**t!
Bird slide:
Seagulls are disgusting. Rats with wings.