Bhashya
अथ यत् सगववेलाया गवां रश्मीनां सगमनं सगवो यस्या वेलायाम्, गवा वा वत्सैः सह, सा सगववेला तस्मिन्काले यत्सावित्रं रूपम्, स आदि भक्तिविशेष ओकारः ।तदस्य वयांसि पक्षिणोऽन्वायत्तानि ।यत एवम्, तस्मात् तानि वयांसि अन्तरिक्षे अनारम्बणानि अनालम्बनानि, आत्मानमादाय आत्मानमेव आलम्बनत्वेन गृहीत्वा, परिपतन्ति गच्छन्ति, अत आकारसामान्यादादिभक्तिभाजीनि हि एतस्य साम्न ॥
In plain words
Now, the twilight hour when the cows and sunbeams gather together—or when cows are with their calves—is the sagava time. The solar form in that hour is the first section, Oṃ. Upon it the birds depend. Therefore, those birds fly through empty space without any physical footing or support, taking hold of themselves alone as their support, soaring through the sky; and because of this likeness in form, they truly share in the opening part of this sāman.
Close to the Sanskrit
Now, what is the cow-time, the meeting of cows and rays when cows are at that time, or together with calves, that is the sagava time; at that time whatever is the solar form, that is the first bhakti-division, Oṃ. Upon that birds are dependent. Because of this, therefore those birds, ungrounded and without support in the atmosphere, taking themselves and grasping the self alone as their support, fly about and go, hence indeed because of similarity of form they share in the initial bhakti of this sāman.