מילון על חולין 278:4

כי יקרא IF [A BIRD’S NEST] CHANCE TO BE [BEFORE THEE IN THE WAY … THOU SHALT NOT TAKE THE MOTHER WITH THE YOUNG] — If it chance to be, this excludes that which is always ready at hand (in thy court yard) (Sifrei Devarim 227:1; Chullin 139a).