Can someone plz help, I've asked this question 3 times and have not gotten an answer. How is Milton's description of birds realistic?

Meanwhile the tepid Caves, and Fens and shores
Their Brood as numerous hatch from th' Egg that soon
Bursting with kindly rupture forth disclos'd
Their callow young, but feather'd soon and fledge
They summ'd their Pens and soaring th' air sublime
With clang despis'd the ground, under a cloud
In prospect; there the Eagle and the Stork
On Cliffs and Cedar tops their Eyries build
Part loosely wing the Region, part more wise
In common, rang'd in figure wedge their way,
Intelligent of seasons, and set forth
Their Aery Caravan high over Seas
Flying, and over Lands with mutual wing
Easing their flight; so steers the prudent Crane
Her annual Voyage, borne on Winds; the Air
Floats, as they pass, fann'd with unnumber'd plumes:
From Branch to Branch the smaller Birds with song
Solaced the Woods, and spread their painted wings
Till Ev'n, nor then the solemn Nightingale