Breeding territory size affects fitness: an experimental study on competition at the individual level

1. Descriptive studies have shown that the annual mean fecundity and survival in bird populations decline as density increases. Experimental studies in which breeding density has been manipulated show that density causally affects reproduction in some but not other species. 2. In a 3-year study on great tits Parus major we manipulated density by removing about one-third of territorial great tit pairs from half the study area each year. The removal resulted in an increase in territory size of the remaining birds in the experimental sub-area. 3. Fecundity and survival did not differ between the experimental low-density and the control sub-areas. The experiment thus did not show a causal relationship between breeding density and fecundity and survival at the level of the population. 4. The data were analysed further with respect to individual territory size, the level at which competition is assumed to operate. Pairs in the experimental sub-area enlarged their territories. We argue that variation in territory enlargement is not caused by individual quality. 5. Both pre-manipulation territory size and territory enlargement positively affected the probability that a territorial pair nested, the growth rate of their chicks, the number of fledglings that recruited into the breeding population and the survival of the territorial adults. No effect of territory enlargement was observed on clutch size, nor on the probability that pairs started a second brood. 6. The positive effect of the experimental territory enlargement argues for a causal effect of territory size on these reproductive parameters, independent of individual quality. 7. An effect of the density reduction on fecundity may not be detectable at the population level, either due to the large variation in territory sizes within the experimental treatments or to the negative effect of the experiment per sc, which counter balances the positive effect of territory enlargement.