Although many hummingbird fanciers are convinced that heavy rainfall can dilute the concentration of sugar in a feeder, I do not know of any studies that corroborate this claim. However, enough people believe this to be the case that some manufacturers of hummingbird feeders offer feeders that are less susceptible to rain flowing into the food reservoirs on their feeders. In an effort to hinder rain draining through feeding portals, some hummingbird fans place a plastic dome over their feeders. Others simply shroud their feeders with plastic plates. Others address the problem by purchasing feeders featuring very small feeding portals. If water pouring into a feeder is a problem, it makes sense to use feeders equipped with small feeding ports.