That’s too literal to make a proverb, like saying a goat eats grass.
The idea of kiaga is that, unlike the civilized broiler that eats mash from a trough, the shenji chicken will scrape the hard floor out of habit.
Think of the Isaac Hayes song “I took You Out Of The Ghetto” (but I couldn’t get the ghetto out of you).
Kíría waheaga ríu nyederia.
Is that a proverb? 
exactly, there’s even a kikuyu song whose title and author i forget but extrapolates the dumpsite meaning through the story of the woman given a good life in town but betrays the singer and reverts…a good morning.
That’s not a proverb.
“Muthichi ungi ndamuthuire” is a proverb.
na murigia nakio niwe ucokagia nguo…
Kaindi kaarwa gatitigaga kwaarwa…