Nest predation has been suggested to be the most important environmental factor determining the evolution of incubation and nestling periods in birds. This paradigm is based on a number of comparative analyses that have shown correlations between rates of nest predation and the relative duration of the nestling period. A second important natural selection pressure on reproduction is caused by parasitism. Here, I investigated the independent effects of daily nest predation rates and daily nest mortality rates due to parasites to assess the relative importance of these two environmental factors as determinants of the relative duration of incubation and nestling periods in birds, using the 43 species for which information on parasite-induced nestling mortality was available. The relative duration of the incubation period was negatively related to daily mortality rate due to parasitism, while nest predation had no significant effect. The relative duration of the nestling period was not significantly related to daily rate of mortality due to parasitism or nest predation. Daily nest predation rate was significantly negatively correlated with daily mortality rate due to parasitism in the subsample of species, for which some mortality due to parasitism was recorded. These findings are mainly based on hole nesters and colonially breeding species, because estimates of parasite-induced mortality generally is unavailable from open nesting and solitarily breeding species, precluding any generalizations to these species. However, the conclusions did not change after statistically controlling for nest site and sociality. These results suggest that developmental rates may be more influenced by the effects of parasites rather than nest predators, and that parasites constitute a neglected selective force affecting developmental rates.