Testing the use of temperature data loggers to assess breeding success of burrow-nesting seabirds

Breeding success can be challenging and labour-intensive to estimate, especially for birds that nest in subterranean burrows such as many smaller Procellariiformes. In burrowing seabirds, breeding success is often assessed using a burrowscope to inspect nests early in incubation and again shortly before fledging. Miniature temperature loggers that measure nest temperature throughout the nesting period may offer an alternative or supplementary method, allowing breeding success to be inferred without a time-sensitive visit just before fledging. Temperature loggers have given reliable estimates of breeding success in birds that nest above ground; however, their efficacy in large, subterranean burrows is uncertain. We tested whether temperature loggers (iButtons) deployed at early incubation stage in the nesting chambers of a burrow-nesting seabird (tītī or sooty shearwater; Ardenna grisea) could be used to assess if a nesting attempt was ultimately successful (i.e., fledged a chick). We monitored the nest chamber temperatures of 96 known-fate nesting attempts, of which 73 (76%) successfully fledged a chick. Using functional data analysis, we show that the probability of correctly assessing a nest attempt’s fate at a new site based on its nest chamber temperature data alone was 0.86 (95% CI: 0.79 to 0.93). Temperature loggers may therefore offer an alternative method of assessing nest fate in burrowing seabirds compared to periodic burrowscoping. However, the thermal buffering effects of the burrows suggest that temperature data may not be able to determine fine-scale nesting behaviour such as nest stage transitions.