Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.1.1

10.2.1.1

prajāpatiḥ svargaṃ lokamajigāṃsat sarve vai paśavaḥ prajāpatiḥ puruṣo'śvo gauravirajaḥ sa etai rūpairnāśaknotsa etaṃ vayovidhamātmānamapaśyadagniṃ taṃ vyadhatta so'nupasamuhyānupādhāyodapipatiṣatsa nāśaknotsa upasamuhyopadhāyodapatattasmādapyetarhi vayāṃsi yadaiva pakṣā upasamūhante yadā patrāṇi visṛjante'thotpatituṃ śaknuvanti

Pragâpati was desirous of going up to the world of heaven; but Pragâpati, indeed, is all the (sacrificial) animals --man, horse, bull, ram, and he-goat:--by means of these forms he could not do so. He saw this bird-like body, the fire-altar, and constructed it. He attempted to fly up, without contracting and expanding (the wings), but could not do so. By contracting and expanding (the wings) he did fly up: whence even to this day birds can only fly up when they contract their wings and spread their feathers.