AN EXPERIMENTAL TEST OF OPTIMAL CLUTCH SIZE OF THE FIELDFARE - WITH ADISCUSSION ON WHY BROOD PARASITES REMOVE EGGS WHEN THEY PARASITIZE A HOST SPECIES

The aim of the present study was to test the ''Incubation limit'' hypo thesis; this states that the probability of hatching decreases when th e number of eggs in a clutch increases above a certain limit. The resu lts of this study indicate that experimentally-enlarged Fieldfare Turd us pilaris clutches had a relatively lower hatching success than natur al-sized ones. The experimental clutches were 1.56 larger than natural clutches. This resulted in a 8% reduction in the relative hatching su ccess for this experimental group compared with natural-sized clutches . Increasing the natural clutch size seems to have a negative effect o n the relative hatching success of the Fieldfare in two ways: (1) eggs remaining unhatched, and (2) eggs becoming broken due to wear and tea r during the incubation period. The results of this study therefore su pport the ''Incubation limit'' hypothesis. This hypothesis is also dis cussed in relation to the question whether brood parasites remove one or more host eggs in order to increase the hatching probability of the parasite's own egg.