Burrow depth, carbon dioxide and reproductive success in Sand Martins <i>Riparia riparia</i>

<p><b>Capsule:</b> Carbon dioxide (CO₂) concentrations in the burrows of Sand Martins <i>Riparia riparia</i> increase with depth but have no detectable impact on fledging success.</p> <p><b>Aims:</b> To investigate whether burrow depth and CO₂ concentrations influence reproductive success in Sand Martins.</p> <p><b>Methods:</b> We monitored two Sand Martin colonies along the River Lune, Lancashire, UK, to investigate the effect of burrow depth on reproductive success. We also measured CO₂ levels in a sample of burrows to test whether burrow depth predicts CO₂ concentration, and to test for a relationship between CO₂ concentration and breeding success.</p> <p><b>Results:</b> Burrow depth was significantly correlated with fledging success, but the correlation was positive in first broods and negative in second broods. The highest CO₂ concentration recorded was 73 650 ppm and the mean concentration across burrows was 31 757 ppm. However, while CO₂ concentrations were positively correlated with burrow depth after controlling for the number and age of nestlings, they were not correlated with reproductive success.</p> <p><b>Conclusion:</b> There are reproductive costs associated with deeper burrows in second broods, but these could not be attributed to CO₂ concentrations despite the exceptionally high levels recorded. This study highlights the need for further investigation into gas exchange and the potential impacts of, or adaptations to, CO₂ accumulation in avian burrows.