During winter, honey bees will eat honey they stored earlier in the year. Typically, bees can make much more honey than they will need to eat during the winter. But, under certain circumstances, bees can run out of honey to eat during the winter. Most commonly, it is the beekeeper who over harvested the honey the bees made, either on purpose or by accident. Another possibility is the weather was so poor during the nectar flow, bees were unable to make extra honey.
Either way, it is sometimes necessary to feed your bees during the winter. The rest of the year while it's warm, you can feed bees sugar syrup, but during the cold winter, feeding sugar syrup isn't a good idea.
What to feed... Some beekeepers feed only honey and others feed mostly sugar syrup. There is no right and wrong, because it depends on what race of bee is being considered and their suitability for the conditions that they are being wintered in.
"Normal" nectar is mainly sucrose and water and as it is concentrated by evaporation, the bees add enzymes that invert the sucrose into mainly levulose and dextrose, with a small amount of sucrose left uninverted. The concentration of the resulting honey is so strong that it does not support mould growth.
When bees pack away sugar syrup it is already fairly concentrated and their opportunity to add enzymes is limited, but a small amount of inversion does actually take place.
The difference between stored honey and stored syrup is the ratios of the sugars in the mixture and that stored syrup contains much lower numbers of pollen grains and a smaller amount of the other minor constituents of honey.
The change in temperatures between night and day can cause syrup containers to drip cold syrup onto the bees.
Syrup has more water in it than honey and bees will burn excess energy trying to remove the additional water.
Sugar syrup will be very cold during the winter, and if the syrup is too cold, the bees will be unable to drink it.
Syrup can mold easily if the bees do not consume it quickly, rendering the syrup undrinkable for the bees.
Cold weather may keep the bees from getting to the syrup, leaving an open window for other insects to eat the syrup.