Beekeepers keep feed close to the spring broodnest, so that the bees are encouraged to use it. Broodnest temperature must be maintained at 92-94 F. Although feeders are at the top or sides of the broodnest and at therefore at lower temperatures, the temperature is still raised enough to speed fermentation. 2:1 ferments slowly if at all, but 1:1 doesn’t seem to last a week at hivetop temperatures.
If the bees are consuming 1:1 fast enough, fermentation is not an issue. However, bees are very sensitive to mold or other contaminants in their feed. Glass jar hivetop feeders allow beekeepers to see the start of fermentation. Picking up a jar and shaking the liquid feed will show mold filaments floating before fermentation would be otherwise detectable. That is a reason for preferring them to black in-frame feeders. If such feeders are used, it is recommended to have a spare feeder scrubbed out and/or bleached and rinsed that is swapped for the feeder used for the past week or so. Simply adding fresh feed to what may have started fermenting may be adding feed to a fermentation starter culture. If bees are draining the feed provided, all is probably well, but if they suddenly stop taking feed and they still need it, they may be turning their “noses” (antennae?) up at mold.
Advantage of clear feeders: Glass jars and lids cleaned in a home dishwasher is such a timesaver that it reduces the urge of a busy beekeeper to add fresh feed to what may already be fermenting in used jars.