Reference for Chullin 281:6

כי קתני מתניתין בנוגע מן הצד:

which is not the case here; and if there were there young birds able to fly or addled eggs, one is bound to let the dam go, for it is written, a nest, that is, any nest whatsoever! [He replied,] If that were so, the verse should have stated: 'And the dam sitting upon them'; why is it written: And the dam sitting upon the young or upon the eggs?