In The Name of Allah, The Most Merciful, The Bestower of Mercy.

Allah, The Most High, said:

وَتَفَقَّدَ الطَّيْرَ فَقَالَ مَا لِيَ لَا أَرَى الْهُدْهُدَ أَمْ كَانَ مِنَ الْغَائِبِينَ

لَأُعَذِّبَنَّهُ عَذَابًا شَدِيدًا أَوْ لَأَذْبَحَنَّهُ أَوْ لَيَأْتِيَنِّي بِسُلْطَانٍ مُّبِينٍ

He inspected the birds, and said: “What is the matter that I see not the hoopoe? Or is he among the absentees? “I will surely punish him with a severe torment, or slaughter him, unless he brings me a clear reason.” (An-Naml 20-21)

“What is the matter that I see not the hoopoe? Or is he among the absentees?”

Meaning: “Is it that I cannot see him because of me being unaware of his presence due to it being hidden among these numerous different groups, or because – with neither my permission nor command – it is absent?”

Thus, he got angry and issued a threat of punishment, and said:

لَأُعَذِّبَنَّهُ ۥ عَذَابً۬ا شَدِيدًا

“I will surely punish him with a severe punishment” without killing it.

أَوۡ لَأَاْذۡبَحَنَّهُ ۥۤ أَوۡ لَيَأۡتِيَنِّى بِسُلۡطَـٰنٍ۬ مُّبِينٍ۬

“or slaughter it, unless it brings me a clear reason”– Meaning, “(brings) a clear proof for being absent”.

This is from the perfection of his Wara [1] and equity because he did not merely make an oath to punish or kill it, as that can only be done if a sin or wrong doing was committed. It may be that there was a clear excuse for its absence, therefore he made this exception because of his Wara and sound perception. [An Excerpt from “Tafseer As-Sadi]

[1] What is (الوَرَعِ)? Al-Manaawee, may Allah have mercy upon him, said about (الوَرَعِ – Al-Wara): “It said that it is to abandon what causes you to doubt, negate what brings you blame (disgrace), and adhere to what is most reliable (certified).”

Source: at-Tawqeef Alaa Muhimmaat at-Ta’aareef. p. 336