Density dependence of avian clutch size in resident and migrant species: is there a constraint on the predictability of competitor density?

The presence of density dependence of clutch size is tested in 57 long-term population studies of 10 passerine bird species. In about half of the stud ies of tit species Parus spp. density dependence of clutch size was found, while none was found in studies of two flycatcher species Ficedula spp. One hypothesis explaining this difference is that migrants are less able to pr edict the final competitor density, because new pairs are still settling wh en the first females start laying eggs. Such unpredictability is only a pro blem for early laying females. If this explanation is true, the commonly ob served negative correlation between clutch size and laying date should be s tronger in high-density years. I tested this prediction in three population s of Pied Flycatcher Ficedula hypoleuca, and compared the results with thre e populations of Great Tit Parus major. In none of the sis populations was there a significant correlation between the strength of the seasonal declin e in clutch size and population density. Thus the lack of density dependenc e of clutch size in Pied Flycatchers was not consistent with the idea that this is caused by the unpredictability of final density at the time of egg- laying of the earliest females in the population. Furthermore, density does not have any adverse effect on reproductive output of Pied Flycatchers, an d therefore they do not adjust clutch size to density.