The augment suṭ is placed before ka , when the verb kṛ comes after apa and the sense is 'the scraping of earth by four-footed animals or birds'.,
Thus अपस्किरते वृषभो हृष्टः, अपस्किरते कुक्कुटो भक्ष्यार्थी; अपस्किरते श्वा आश्रयार्थी = आलिख्य विक्षिपति ॥ Why do we say 'four-footed animals or birds'? Observe अपकिरति देवदत्तः ॥
Vart:- This rule applies when the scraping is through pleasure, or for the sake of finding food, or making a resting place. Therefore not here:- अपकिरति श्वा ओदनपिण्डमाशितः ॥ And it is in the above senses of scraping through pleasure &c, that the root takes Atmanepada affixes; otherwise the Parasmaipada affixes will follow. See 1.3.21. Vartika,