Psalms 103:5 - Coke's Commentary on the Holy Bible

So that thy youth is renewed like the eagle's— Dr. Delaney observes, that it is generally understood of the eagle, when his feathers moult and fall off gradually and new ones succeed, that his youth is renewed with his plumage; and "I am well assured, (says he,) that these feathers fall by pairs, one in each wing; and that those which fall at once are the two feathers exactly corresponding to each other in each wing; and that this pair is renewed by a new growth, before the next pair falls: a plain appointment of providential wisdom and goodness! for by thus preserving the exact balance of the wings, which a very little inequality (the difference of less than half a feather) in either would destroy, the eagle is never utterly disabled from pursuing his prey; though it be evident, he cannot pursue it with full vigour, till all his feathers be renewed; and therefore his youth is then said to be restored." The fact is, that all birds have yearly their moulting-time, when they shed their old, and are afresh furnished with a new stock of feathers. This is most observable of hawks and vultures, and especially of eagles; which, when they are near one hundred years old, cast their feathers, and become bald like young ones, and then new feathers sprout out.