The Short Answer for a Leghorn Chicken
A Leghorn chicken stays in behind height plus a cover, not height alone. Build the run fence around six feet high in sturdy mesh, or cover a lower run completely with well-fixed netting, because a Leghorn is a light, athletic flyer that clears ordinary backyard fencing with a short run-up. Treat the cover as part of the fence from the first day, since adding it after the flock has learned to fly out means retraining birds that already know the exit. Secure the bottom in predator-proof mesh at the same time, because a run that keeps Leghorns in but lets a fox reach through has solved only half its job.
Why Leghorns Fly Where Heavy Hens Stay
A Leghorn differs from a heavy laying breed in body and temperament. This Mediterranean chicken is slim, alert and quick to flush upward, and a startled Leghorn can gain a fence top, a coop roof and a tree branch in a few beats. Leghorns also range widely and investigate, so they find the one low section, gate gap or unnetted corner that a placid flock would never test. Perches inside the run become stepping stones when they sit near the boundary, and a hen that reaches the top rail will walk it looking for the drop on the far side. Designing for the Leghorn's flight, rather than hoping this flock stays calm, is what makes the fence reliable.
Mesh Choices From Ground to Top
Build a Leghorn run in layers. Use hardware cloth with small openings on the lower walls, fixed to a frame, so predators cannot reach in and hens cannot push a head through and become trapped. Above that, welded mesh can continue to the full height as long as the joins are tight and the top edge meets the cover without a hen-sized gap. Netting overhead must be pulled taut and attached all around, since a sagging cover leaves a funnel at the edge where a Leghorn squeezes out and a hawk pushes in. Bury or skirt the base outward to stop digging, and make the gate frame as tight as the walls, because a Leghorn studies the moving part of its pen most closely of all.
Roosts, Roofs and Staging Points
A Leghorn rarely flies six feet from flat ground in one effort; it stages the climb. A feeder, waterer, roost bar, stump or coop roof near the fence gives this breed the halfway platform that turns a safe height into an easy hop. Arrange the run so tall objects sit in the middle or against the secure house wall, never along the boundary, and keep outside lures in view of the same rule: a compost heap or woodpile stacked against the exterior also works as a landing stage. After rearranging anything in the Leghorn run, stand back and trace the staircase a bird would see, from ground object to rail to cover edge, and break any route you can follow with your eye.
Keeping Flighty Hens in Over Time
Leghorn containment drifts if the flock learns new tricks faster than the pen is checked. Re-fix a net corner lifted by wind before hens find it, replace a rusted lower panel before it flexes, and watch for the single hen that regularly appears on the roof, because she is mapping a route the others will copy. If a Leghorn is found outside, note where she perches and how she likely staged the climb instead of simply raising the whole fence. Log the cover fixings, staging objects to keep away from the walls and any known escape artist in KinsLe using Family Sharing, so the whole household keeps the Leghorn run arranged the way this breed requires.