Of course they can, as long as it’s not straight out of a solar water heater.
But don’t take my word for it when you can easily prove it to yourself. Leave your hummingbird feeder in full sun for a few hours on a hot summer day, then check the temperature of the sugar water.
Unless you’ve hung your feeder near a highly reflective surface and/or inside an enclosed greenhouse-like space, the liquid won’t be much warmer than the surrounding air. This is because a relatively small small volume of liquid in an uninsulated container loses heat to the surrounding air about as fast as it gains it.
Hummingbirds are not delicate, fragile creatures. If the feeder solution isn’t hot enough to damage human skin, it’s not going to burn their tongues.
Addendum, June 23, 2014: A complete answer to this question involves numbers, and I finally got around to getting some on a hot afternoon here in the high desert of southern Arizona:
- Air temperature in full sun: 95° F. (35° C.)*
- Feeder solution temperature in full sun: 105° F. (40.6° C.)*
- Hummingbird body temperature: ~104° F. (~40° C.)
- Highest air temperature ever recorded in the U.S.: 134° F (56.7° C).
- Optimum hot beverage temperature: 136° F (57.8° C).
- Water hot enough to cause third-degree burns to human skin in 5 seconds: 140° F. (60° C.)
- McDonald’s coffee, pre-lawsuit: 180–190° F. (82–88° C.)
* Air and sugar water temperature measured with a cooking thermometer in my yard at around 2 p.m.; the feeder is a fancy HummZinger with a translucent top, which probably adds a little greenhouse effect to the direct solar heating.
Brown F., and K.R. Diller. 2008. Calculating the optimum temperature for serving hot beverages. Burns Aug;34(5):648-54. Link (PubMed)