Bearding at the entrances of Warré hives
A Warré hive was monitored with 6 temperature sensors from late winter 2017 onwards. For details see here. On two unusually hot days in May it exhibited bearding. The chart for the relevant period is displayed below. On both days the temperature exceeded 36C.
Below is a photo taken of the hive about two hours after sliding the hive forward on the floor to create a 20 mm wide full width entrance. The beard diminished a little during the two hours. The ambient temperature by then was about 25C.
29 June 2009 the temperature in the shade reached 29 degrees C. Two Warré hives developed bearding despite both having empty boxes at the bottom.
Hive W2, populated in 2007 with an artificial swarm from a National hive, is exposed to direct sun for a few hours round the sun's zenith. The dark colour of the wood stain probably worsens heat absorption from the sun.
Hive W9, populated in 2008 from a Warré hive swarm is in dappled shade most of the day. An empty (5th) box was added on 27 June. This beard was present at 14.30 hours but had gone by 17.00 hours.
More extensive bearding on a Warré is illustrated by Uli Schläpfer's photo (unfortunately not available at higher resolution):
Heaf's bee index