+ Vortices generated the wing tip of the leader moves downstream forming a pair of opposite rotating line vortices. 
+ These vortices are generally undesirable because they create a downwash that increases the induced drag on leaders wing. However, this downwash is also accompanied by an upwash that can beneficial to the followers wing flying behind the leaders one, namely a favorable lift for the followers wing. 
+ How much contributions of the leaders wing to the followers wing in the V formation flight is determined by the strength of tip vortices generated by the leaders wing which is influenced by its position and shape including incidence angle, dihedral angle, aspect ratio and taper ratio.

"Effects of Leader’s Position and Shape on Aerodynamic Performances of V Flight Formation", http://arxiv.org/abs/0804.3879