What kind of feather is crucial for supporting new growth during molt?

The process of molting in birds is essential for replacing old feathers with new ones, and certain types of feathers play a critical role during this period. Old feathers, especially those that are intact, provide structural support for the new feathers that are emerging. The shedding of old feathers creates space for the new growth, and the presence of healthy, intact old feathers can assist in protecting the delicate new feathers as they grow. These established feathers may also help with temperature regulation and aerodynamics during the transitional phase.

Newly grown feathers, while they represent the future of the bird's plumage, are still developing and do not provide support during the molting process. Impinged feathers, which are intentionally inserted into the shafts of broken feathers for repair, are not a natural part of the molting process and do not contribute to new growth. Lastly, while all remaining external feathers have their importance, they do not specifically facilitate the support needed for new feather growth during the molt, making the structural integrity of the old feathers more significant during this phase.