Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.2.8

10.2.2.8

atha puce vitastimupādadhāti pratiṣṭhāyāṃ tadvīryaṃ dadhāti pratiṣṭhā vai pucaṃ hasto vitastirhastena vā annamadyate'nnāyaiva tamavakāśaṃ karoti tadyatpuce vitastimupādadhātyanna evainaṃ tatpratiṣṭhāpayati tadyattatra kanīya upādadhātyannehyevainaṃ tatpratiṣṭhāpayatyatho etāvadvā idam mitam bhavatyetāvadidaṃ tadyadevam mimīta etasyaivāptyai

To the tail he gives a span: he thus lays strength into the support, for the tail is the support. The span means the hand, and by means of the hand food is eaten: it is thus for the sake of food that he gives it that space; and when he gives a span to the tail, he thereby settles him (Agni) in (the midst of) food; and when he gives less (space) to this (part of the body), it is because he thereby settles him in (the midst of) food. But, indeed, so much does this (the bird's wing) measure, and so much this (the bird's tail), and hence when he thus measures them, it is for the sake of securing for him that (natural measure).