Are you my mother

Father's here, back at our nest, but his face is frowning, or is it in jest? 'My son,' he said, 'It's about your mother. She's not your mother, and nor your brother your brother.' 'What?' replied son, tearing up fast. 'Mother said she was when I checked last.' So he went to her, balling with tears. 'Mother, have you been lying to me all these years?' 'Why no, I am your mom, as you can plainly see. Look at my long neck, and our smiles of glee.' Son was convinced but you may not be later, when you see that son bird's mom, is an excavator.