Why do gulls perch on all sorts of structures but never on a tree?

Most gulls do not have feet that are adapted to perching in trees and clinging to tree branches.

Most have slightly webbed feet that help them move about in the water, so when they are on land, they do better walking on rocks or soft surfaces like sand.

Pure probability also puts gulls on the structures of civilization. Birds like gulls and crows have done quite well adapting to human-created environments, and the nice flat surfaces associated with human habitation — pilings, piers, and lighthouses, to name just a few–are ideal places to perch.

Gulls that can nest in trees, like mew gulls, have feet that are not fully webbed. Mew gulls make their nests fairly low, in spruce trees, not deciduous trees, where there is more surface area to cling to.

Many forest birds, in contrast with gulls, have feet with tendons that keep their toes automatically wrapped around branches. For these birds, it takes an effort to unclench.