With the recent spell of very cold weather, colonies will have been using up their honey stores more rapidly than normal. As the outside temperature drops, the winter cluster becomes tighter and tighter in an attempt to conserve heat. If the temperature drops even lower honeybees start to consume more in order to keep warm and survive. This is what will have been happening over recent weeks.
Now that it is a little warmer, you should check that your colonies are not running short of stores. With experience it is possible to judge the quantity of stores remaining by ‘hefting’, or lifting one side of the hive and judging the weight. Another method is to carefully remove the roof and look through the feed hole in the crownboard. If the bees are clustered immediately below the hole, they could be short of stores whereas if they are much lower down on the combs, they are likely OK.
If you need to feed your bees during the winter you should use fondant icing, not liquid feeds. Liquid feeds can cause problems during the winter as the honeybees cannot deal with the excess moisture.
The best way to feed fondant is to put it in a plastic container on top of the crownboard with a hole in the base of the container.
Bees will come up to the fondant and slowly take it down into the cluster. You will need to add an empty super box or other spacer around the feed to allow the roof to be put back.