The invention relates to a leading edge mobile flap (16) for a main wing of an aircraft, this flap including an aerodynamic skin (18) that has a bird impact-sensitive frontal area (24), and a rear skin (28) integral with the aerodynamic skin (18), the flap also comprising a plurality of ribs (34) spaced out along a leading edge longitudinal direction (X'). According to the invention, the flap additionally includes, between two directly consecutive ribs, a single rigid bird trajectory-deflecting wall (42) anchored to the skins (18, 28). Furthermore, in a cross-section taken along any plane orthogonal to the direction (X'), the wall (42) forms with a geometric chord (26) of the flap an angle (.alpha.1) with a value of less than 45.degree..