A.T. Robertson Commentary Mark 4:32

"yet when it is sown, groweth up, and becometh greater than all the herbs, and putteth out great branches; so that the birds of the heaven can lodge under the shadow thereof." — Mark 4:32 (ASV)

Groweth up (αναβαινε). Matthew 13:32 When it is grown (οταν αυξηθη).

Under the shadow thereof (υπο την σκιαν αυτου). A different picture from Matthew's

in the branches thereof (εν τοις κλαδοις αυτου). But both use κατασκηνοιν, to tent or camp down, make nests in the branches in the shade or hop on the ground under the shade just like a covey of birds. In Mt 8:20 the birds have nests (κατασκηνωσεις). The use of the mustard seed for smallness seems to have been proverbial and Jesus employs it elsewhere (Matthew 17:20; Luke 17:6).