Apparently this is called 'bearding'. In really hot and humid weather the bees can struggle to keep the hive from overheating. If the hive overheats it can kill the brood so they have a variety of tactics they uses: * set up a line of bees at the front door and use their wings to fan air through the hive
* collect water and deposit small droplets in the hive to absorb heat
If all that fails, then rather than add heat from their bodies they 'hang out' on the outside of the hive like this.
The common advise is to ensure they are on an open mesh floor, which is already the case in this hive so we will just have to hope they get through it.