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

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Dhondt, A.A. & Adriaensen, F. 1999. Experiments on competition between Great and Blue Tit: Effects on Blue Tit reproductive success and population processes. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 70 (1): 39–48. Great Tits Parus major and Blue Tits Parus caeruleus compete for food and for cavities. By providing nest boxes with entrance holes small enough to exclude Great Tits, but allowing access to Blue Tits, either by themselves or in combination with standard nest boxes we manipulated the breeding densities of Blue and Great Tits independently. We compared populations differing in experimental densities over five-year periods. Components of Blue Tit reproductive success are adversely affected when either Blue Tit or Great Tit densities are experimentally increased. At high Blue Tit density adult survival does not change. When small-holed nest boxes are provided and Blue Tit density is increased, the number of local born males that recruit into the population increases considerably, so that the proportion of local born males is much higher than at low density. Similarly the local recruitment rate (the number of local recruits per adult breeding in the previous year) increases strongly for males, but not for females. The presence of small-holed nest boxes increases habitat quality, especially in winter when Blue Tits use these boxes for roosting, so that juveniles disperse less, and breeding density increases.