The example is given: just like a green bird enters into the green tree. It does not mean that the bird is mixed. No. The bird is keeping its independence as an individual. But it appears to others, those who do not see properly, that it has merged into the tree. It has actually not merged. And because it does not merge, therefore they fall down. They again come out. That is explained in the Śrīmad-Bhāgavatam that ye 'nye 'ravindākṣa vimukta-māninaḥ... Vimukta-māninaḥ. They are thinking, "Now I have become one with the Supreme." But actually, that is a false impression. Māninaḥ. Māninaḥ means actually it is not fact, but he's thinking like that. Vimukta-māninaḥ.
Why he's thinking like that? Aviśuddha-buddhayaḥ. His knowledge is not perfect. Aviśuddha. Viśuddha means perfect, and aviśuddha means not perfect. Unnecessarily he's thinking that "I have become one with the..." I remain the same part and parcel. As Kṛṣṇa says, mamaivāṁśo jīva-bhūtaḥ (BG 15.7).