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Nest predation has been hypothesized as a major factor influencing clutch size in birds. Indirect and correlative evidence for clutch size reduction under high nest predation risk is accumulating, but such a reduction has never been shown experimentally. We experimentally tested this "nest predation hypothesis" by manipulating nest predation rate in the Collared Flycatcher, Ficedula albicollis. As predicted, clutch size decreased in the year following the experiment in "depredated" areas (in which 20-35% of the nests were totally depredated) compared to control ones (no predation). This could result from (1) phenotypic plasticity in clutch size, or (2) differential breeding habitat selection, with dominant individuals or individuals laying large clutches preferentially leaving depredated areas and settling in control ones. There was no evidence for differential dispersal from areas of the two treatments, nor differential competitive abilities of breeders in areas of the two treatments. Conversely, all individuals had a smaller clutch size in depredated areas, supporting the hypothesis of clutch size plasticity. These results provide experimental support for the nest predation hypothesis of clutch size variation in birds and suggest the importance of information available to individual birds on local nest predation risk and previous experience for determining clutch size. [ABSTRACT FROM AUTHOR]