Originally without plumes (i.e. conditions) he comes to possess plumes (becomes vested with conditions) in a large tree of golden foliage, representing a world transitory and ephemeral and flies away in different directions (i.e. reborn among several orders of men). The Sloka simply means that the supreme should when vested with conditions becomes the creature-soul. The ascetics perceive that Lord who is Eternal.
हिरण्यपर्णमश्वत्थमभिपद्य ह्यपक्षकाः। ते तत्र पक्षिणो भूत्वा प्रपतन्ति यथा दिशम्। योगिनस्तं प्रपश्यन्ति भगवन्तं सनातनम्॥