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V-shaped and echelon formations help migratory birds to consume less energy for migration. As the case study, the formation flight of the Northern Bald Ibises is considered to investigate different effects on their flight efficiency. The effects of the wingtip spacing and wingspan are examined on the individual drag of each Ibis in the flock. Two scenarios are considered in this study, (1) increasing and (2) decreasing wingspans toward the tail. An algorithm is applied for replacement mechanism and load balancing of the Ibises during their flight. In this replacing mechanism, the Ibises with the highest value of remained energy are replaced with the Ibises with the lowest energy, iteratively. The results indicate that depending on the positions of the birds with various sizes in the flock, they consume a different level of energy. Moreover, it is found that also small birds have the chance to take the lead during the flock.