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Отпор летењу је сила која делује супротно од правца кретања и узрок губитка енергије приликом лета. Отпор ваздуха се може поделити на две компоненте, ''[[индуковани отпор]]'', који представља инхерентне (неодвојиве) трошкове дизања крила (ова енергија завршава првенствено у вртлозима који настају на врховима крила) и ''[[паразитски отпор]]'' који укључује и отпор трења коже, од трења ваздушних и телесних површина и отпора због облика који делује са предње стране птице. Аеродинамичност птичијег тела и крила смањује те силе.
==Wing Крило ==
[[FileДатотека:Kea in Flight MC.jpg|thumbмини|leftлево|A [[keaКеа]] inу flightлету.]]
TheПредњи bird'sудови [[forelimb]]sптица (the [[wingКрило|крила]]s) су кључ за летење. are the key to flight. Each wing has a central vane to hit the wind, composed of three limb bones, the [[humerus]], [[ulna]] and [[Radius (bone)|radius]]. The hand, or manus, which ancestrally was composed of five digits, is reduced to three digits (digit II, III and IV or I, II, III depending on the scheme followed<ref>Baumel JJ (1993) Handbook of Avian Anatomy: Nomina Anatomica Avium. 2nd Ed. Nuttall Ornithological Club. Cambridge, MA, USA</ref>), which serves as an anchor for the primaries, one of two groups of [[flight feather]]s responsible for the wing's airfoil shape. The other set of flight feathers, behind the carpal joint on the ulna, are called the secondaries. The remaining feathers on the wing are known as [[covert (feather)|coverts]], of which there are three sets. The wing sometimes has vestigial claws. In most species these are lost by the time the bird is adult (such as the highly visible ones used for active climbing by [[hoatzin]] chicks), but claws are retained into adulthood by the [[secretarybird]], [[screamer]]s, [[Heliornithidae|finfoot]]s, ostriches, several swifts and numerous others, as a local trait, in a few specimens.
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