Athithi is whatever creature is hungry

Athithi is whatever creature is hungry and comes to you at the lunch time, whether it is a human being or a bird, beast or insect. All these seek food. The real athithi that you get, you do not regard as such ... At kakabali (offering food to crows) time take plenty of cooked rice outside the house and leave it there. Do not shout or call for any creature nor drive any away. Whatever the creature that comes to eat, let not that disturb your mind. You get thus the merit of feeding lakhs of guests! 

On the same subject Sri Ramakrishna in Gospel http://www.belurmath.org/gospel/

The Master was conversing with Mahimacharan. He asked him: "Isn't feeding people a kind of service to God? God exists in all beings as fire. To feed people is to offer oblations to that Indwelling Spirit. But then one shouldn't feed the wicked, I mean people who are entangled in gross worldliness or who have committed heinous crimes like adultery. Even the ground where such people sit becomes impure to a depth of seven cubits. Once Hriday fed a number of people at his native place. A good many of them were wicked. I said to Hriday: 'Look here. If you feed such people I shall leave your house at once.' (To Mahima) I hear that you used to feed people; but now you don't give any such feasts. Is it because your expenses have gone up?" (Laughter.)