Could be mounting height and/or heat. Heat actually raises the RF noise floor in 2.4 and 5GHz, the AP might be rated to operate at higher temps, but no AP can operate well at high temps - it turns into an RF physics problem. The long dipole type antennas commonly used on outdoor APs typically squish the signal into a flatter pancake than most. Outdoor APs are also commonly placed too high for their attached antennas. Its possible that your AP may be high up enough that your missing the main coverage lobe from the antenna, giving the birds great signal but not the users down below.
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