We examined the effect of body and egg mass, clutch size and incubation period on the occurrence of intraspecific nest parasitism (INP) in Anseriformes and Galliformes, taking into account the phylogenetic relationships of the species concerned. INP has been reported for 46.0% of Anseriformes but only 11.3% of Galliformes, but these rates seem to be underestimates. In both orders INP appears to have multiple origins. Clutch size is significantly larger among INP species and among cavity nesters (all of which are Anseriformes). The length of the incubation period does not differ between INP and non-INP species. After controlling for body mass, we found that the Anseriformes have significantly larger clutches, longer incubation and larger eggs than the Galliformes. We suggest that the large difference in the rate of INP between the two orders is related to the significantly larger clutches of Anseriformes, which increase the period during which clutches are susceptible to parasitism. Furthermore, the larger eggs of the Anseriformes mean that their chicks are more precocial than those of the Galliformes, so that parasitism is less costly in terms of parental care, and less effort is made by nesting females to drive away potential parasites. Lack of coloniality among the Galliformes also contributes to a lower rate of parasitism in this order, because nests are not concentrated in a small space. Lastly, lack of cavity nesting among Galliformes means that competition for nesting sites is lower in this group.