What is the meaning of "كمفحص قطاة" (like a partridge's nest) mentioned in the Hadith: "Whoever builds a mosque for Allah, even if it be like a partridge's nest for its eggs, Allah will build for him a house in Paradise"?

"Mafhas al-Qata" (the place where the sand grouse examines) refers to the spot where it incubates its eggs. It is derived from "al-Fahs" (examination or searching), as the hen and the sand grouse examine the ground with their feet to choose a place for incubation or laying eggs.