A transition toward flightlessness in Mallards, Indian Runner Ducks, and their hybrid offspring

Many studies have explored wing function in volant birds, and how flight is gained over ontogeny and evolution. However, flight is often secondarily reduced or lost, and how birds transition from wing- to leg-dependence is unknown. Most losses occurred long ago, so stages of wing reduction have never been functionally evaluated or even observed. Here, we assess the anatomical changes and locomotor consequences of a transition toward flightlessness in a unique group: flying Mallards (Anas platyrhynchos), flightless Indian Runner Ducks (Mallard-derivative), and their experimentally produced hybrid offspring. Based on wild birds, we hypothesized that shifts in development would lead to wing reductions in Hybrids and especially Runners, with compensatory increases in leg size and performance. Our results show that Runners indeed share many features with wild flightless birds, and that Hybrids are intermediate. However, the assembly of these features is surprising. Wings become relatively smaller due to increases in body size rather than reductions in wing area or muscle, which are conserved. Legs, in contrast, are labile: Runners invest proportionally less in their leg muscles as juveniles but more as adults than Mallards. This contrast between wing conservation and body and leg peramorphosis suggests that for at least some flight losses, bodies and legs change before wings, which get proportionally smaller before being reduced. Given that juveniles with incipient flight and adults with reduced flight both engaged their wings and legs cooperatively, wing-leg coordination may facilitate shifts along a flightless-to-flying spectrum, exemplifying how specialized structures can be gained or lost.