Posted: 08/08/2015 at 18:45
It could well be honeybees .... and, if it is, then your chimney is now their new home! They will begin to create comb, fill it with some nectar and pollen, and try and build the colony up to a sufficient size to make it through the winter. So, they may still be there next spring and beyond.
If you start getting a sticky mess (and possibly bees) down the chimney and into your house it's a good idea to buy one of those chimney balloons that people use to stop drafts.
Of course when you light your fire they will be history ......