I would feed the chicken some of the grain.
I would plant some of the grain to grow more chicken feed.
I would be very nice to the chicken and hope for eggs.
[Any chance of a rooster as well so I can have a flock?]
How much grain have I got with me on the island? Because if I run out I’ll have a hungry chicken and no eggs! Hopefully I’ve got lots, in that case I’d feed the chicken and gets eggs for a long time. Although I’d need to eat a lot of eggs to survive.
What about water? I think water would be a high priority for me, and my chicken!