You may or may not consider the 40's balmy, but it's still good and above freezing, so I think a baggie feeder would work fine. As long as it's not getting much below 32 at night. Even then, the sugar in the syrup will lower the freezing point, and the bees clustering around will keep the temp up, so it'd have to get darn cold before a baggie feeder wouldn't work in the situation you're describing.