التفسير التبليغي
(أَوَلَمْ يَرَوْا إِلَى الطَّيْرِ فَوْقَهُمْ صَافَّاتٍ وَيَقْبِضْنَ مَا يُمْسِكُهُنَّ إِلاَّ الرَّحْمَنُ إِنَّهُ بِكُلِّ شَيْءٍ بَصِيرٌ)
Regarding (أَوَلَمْ يَرَوْا إِلَى الطَّيْرِ فَوْقَهُمْ صَافَّاتٍ), what is intended by this is that they sometimes spread their wings and sometimes fold them.
(مَا يُمْسِكُهُنَّ إِلَّا الرَّحْمَنُ) meaning: Nothing holds up the birds, spreading their wings above you, except the Most Merciful.
(مَا يُمْسِكُهُنَّ إِلَّا الرَّحْمَنُ) because He, glory be to Him, is the Sustainer of all things.
Source: التفسير التبليغي