Although it is commonly believed that nest boxes yield artificially high estimates of nest success, few investigators have compared nesting success in nest boxes to tree cavities in the same locality during the same time period. I studied nesting success of Great Crested Flycatchers (Myiarchus crinitus) breeding in nest boxes and natural sites (i.e., old woodpecker cavities and natural tree hollows) on the same pine plantations in northern Florida. Mayfield estimates of nesting success were nearly identical between nest boxes (0.37 ± 0.05 SE, n = 32 nests) and tree cavities (0.38 ± 0.06 SE, n = 27 nests) during a 2-year period. However, nesting success was greater in nest boxes (0.53 ± 0.06 SE) than in cavities (0.33 ± 0.10 SE) during 1997 and lower in nest boxes (0.26 ± 0.07 SE) than in cavities (0.42 ± 0.09 SE) during 1998. Lower nest success in nest boxes during 1998 was due to increased predation during the incubation period. Nest predation accounted for ≥83% of all nest failures. Documented nest predators included the southern flying squirrel (Glaucomys volans) and corn snake (Elaphe guttata). Nest boxes and cavity nests did not differ significantly in any habitat variable that would influence nest concealment, nor did these variables differ significantly between years. Evidence suggests that nest predators may learn to exploit nest boxes as a prey resource, either through the development of search images or through long term spatial memory. This study demonstrates that nest boxes are not always safer sites than tree cavities and that static comparisons may give misleading results.