Ardea
Official journal of the Netherlands Ornithologists' Union

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Dhondt A.A. & Eyckerman R. (1980) Competition and the regulation of numbers in Great and Blue Tit. ARDEA 68 (1-4): 121-132
Using the data from the long term study on titmice at Ghent (Belgium) we investigated the importance of both intra- and interspecific competition on reproductive output in different types of habitat, and using a conceptual model tried to determine if this competition ultimately influences change in breeding numbers. We find that during the breeding season Great Tit reproductive rate is lowered both at higher numbers of Great and Blue Tits, and that competition is most pronounced in the most suitable habitats. The effect, both of intra- and interspecific competition, is much weaker on Blue Tit reproductive rate. Changes in breeding numbers are highly correlated to juvenile recruitment rate but not significantly to adult survival rate. Juvenile recruitment rate in its turn, is mainly influenced by changes in juvenile survival rate and less so by changes in reproductive rate. For the Blue Tit we think that therefore the impact on change in numbers of the effect of competition during the breeding season will be small. For the Great Tit, however, such an impact might be more important because juvenile survival rate is highly and positively correlated with reproductive rate, both being higher when food conditions in the breeding season are good. We argue that food conditions are affected by intra- and interspecific competition. We describe an experiment to test the hypothesis that outside the breeding season interspecific competition would affect Blue Tit numbers. After excluding Great Tits from the nest boxes in winter Blue Tit breeding density increased the following spring. We attribute this to a decreased Great Tit winter population that results in a competitive release of Blue Tits. We argue that during the winter Great and Blue Tits compete for roosting sites and for food. The increase in Blue Tit numbers was less caused by an increase in local survival than by an increase in immigration of both juveniles and adults. We conclude that Great Tit numbers are mainly set by Great Tit population size at an earlier point in time, whereas Blue Tit numbers are strongly influenced by interspecific competition with the Great Tit.


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