Anthropogenic noise levels are steadily increasing worldwide and may potentially affect many species. Short-term experimental noise exposure under field and lab conditions has revealed that noise can affect the behaviour and physiology of birds. However, few studies have been able to link these short-term effects to longer-term consequences. Here we report on two long-term noise exposure field experiments to assess the direct and indirect effects of noise on avian reproductive success. In one experiment we provided two types of nesting sites in our study area, a nest box with traffic noise broadcast inside, as well as a control nest box. We found great tits to avoid breeding in noisy nest boxes, in particular when they had both nest box types available in their territory. Interestingly, we found blue tits to breed more in the noisy nest boxes compared to the control nest boxes. In another experiment we randomly assigned nest box treatments to breeding great tits after settlement and started noise exposure prior to egg laying or hatching. We found no significant effect of noise treatment on clutch size, number of fledglings, or any other life history trait we measured. Our results show that great tits avoid breeding in noisy locations. Birds that are however forced to breed in noise, either through experimental manipulation, or potentially through competitive exclusion, do not suffer from reduced reproductive success. Thus, anthropogenic noise can affect settlement behaviour, but breeding inside a noisy nest box has no reproductive consequences, at least not for great tits.
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