Experiments on competition between Great and Blue Tit: Effects on Blue Titreproductive success and population processes

Great Tits Parus major and Blue Tits Parus caeruleus compete for food and f or cavities. By providing nest boxes with entrance holes small enough to ex clude Great Tits, but allowing access to Blue Tits, either by themselves or in combination with standard nest boxes we manipulated the breeding densit ies of Blue and Great Tits independently. We compared populations differing in experimental densities over five-year periods. Components of Blue Tit r eproductive success are adversely affected when either Blue Tit or Great Ti t densities are experimentally increased. At high Blue Tit density adult su rvival does not change. When small-holed nest boxes are provided and Blue T it density is increased, the number of local born males that recruit into t he population increases considerably, so that the proportion of local born males is much higher than at low density. Similarly the local recruitment r ate (the number of local recruits per adult breeding in the previous year) increases strongly for males, but not for females. The presence of small-ho led nest boxes increases habitat quality, especially in winter when Blue Ti ts use these boxes for roosting, so that juveniles disperse less, and breed ing density increases.