Interspecific Competition and Niche Shifts in Tits and the Goldcrest: An Experiment

Abstract

(1) In coniferous forests in Central Sweden willow, (Parus montanus) and crested tits (P. cristatus) forage in the inner canopy while coal tits (P. ater) and goldcrests (Regulus regulus) forage on the outer, needled tree parts. (2) To test if interspecific competition affects foraging site selection of the two latter species, we reduced the numbers of willow and crested tits by more than half in three 40 ha study plots in the beginning of winter. (3) Later in winter, coal tits and goldcrests increased their foraging in inner canopy in the experimental plots in comparison with three control plots. (4) An experiment is powerful in excluding alternative explanations only if it is based on a sufficient number of repetitions. Observations within a plot may be dependent because they are derived from animals inhabiting the same environment. The simplest way around the problems is to use each plot as a single observation in test. Both coal tits and goldcrests exploited inner tree parts more in all of the three experimental plots than any of the three control plots, which alone is a significant difference. (5) The experiment indicates that interspecific competition affects foraging site selection of sympatric tits and goldcrests, as has been previously suggested by many nonexperimental studies.