Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.1.7

10.2.1.7

atha vakrau karoti vakrau hi vayasaḥ pakṣau bhavataḥ sa caturaṅgulameva paścādudūhati caturaṅgulam purastādupasamūhati tadyāvadevodūhati tāvadupasamūhati tannāhaivātirecayati no kanīyaḥ karoti

He then makes the wings crooked, for a bird's wings are crooked; he expands them behind by just four finger-breadths, and contracts them in front by four finger-breadths: he thus draws them out by just as much as he draws them in; and thus, indeed, he neither exceeds (its size) nor does he make it too small.