EXPERIMENTAL-EVIDENCE FOR DENSITY-DEPENDENCE OF REPRODUCTION IN GREATTITS

1. Density dependence of avian reproduction has often been analysed us ing correlations between annual mean reproductive output and populatio n density. Experiments are necessary to prove that density is the caus e of the observed patterns, but so far, three out of four experimental studies do not support a direct causal effect of density on reproduct ion. 2. This paper presents experimental evidence that reproductive de cisions in great tits, Parus major L., are causally affected by breedi ng density. The breeding density of great tits was manipulated by prov iding nest-boxes at different densities in an ecologically homogeneous area. 3. Within years the densities in the high and low density plots differed approximately 8-fold. During the 11 years of the experiment, clutch size, nestling mass and the proportion of birds starting a sec ond brood were all lower in the high density plot. In 5 years with equ al breeding densities in both parts, clutch size did not differ betwee n the plots. The patterns found were consistent with the density effec ts as predicted from the non-experimental data.