Heat stress increases risk taking in foraging shorebirds

Animals often face a trade-off between food acquisition and predation/disturbance avoidance. Yet, the extent to which this trade-off is affected by modulating factors such as thermal risk and foraging opportunities has been largely overlooked.Here, we examined the influence of temporal and environmental gradients on the flight initiation distance (FID, the distance at which animals flee from an approaching human-simulated predator) and escape mode (flying/low risk versus running/low cost) in 16 species of shorebirds foraging on tidal flats of the Bijagos Archipelago, Guinea-Bissau. We measured escape responses throughout the low tide period during wet and dry seasons and simultaneously recorded microclimate variables and occurrence of heat-reduction behaviour (ptiloerection). Furthermore, we measured corticosterone metabolites (CORTm) from droppings in red knots Calidris canutus to assess whether ptiloerection is associated to a physiological stress response to hot conditions.Overall, birds tolerated a closer approach at higher environmental temperatures and when showing ptiloerection. They also had shorter FIDs during the dry season and towards the start/end of the low tide period. FIDs also increased with body mass and decreased in areas with more human presence. In red knots, individuals showing ptiloerection had higher levels of CORTm, demonstrating a link between physiological and behavioural stress coping responses to heat events.Our results suggest that heat-stressed shorebirds take greater risks, supporting the idea of a thermoregulation-predation risk trade-off. They also indicate that shorebirds adjust risk taking to tidal and seasonal cycles, generally reducing FIDs when the energetic costs of escape are expected to be large. Finally, they suggest that shorebirds habituate to non-lethal human presence and respond to perceived predation risk in accordance with the predictions of optimal escape theory.These results are relevant to many animals that face a tight window for foraging activity while being exposed to predation/disturbance and heat during the day. We discuss management implications of our results in the context of global change.

Automated summary

Animals often face trade-offs between food acquisition and predation/disturbance avoidance. However, the impact of modulating factors such as thermal risk and foraging opportunities on this trade-off has been largely disregarded. In this study, we investigated the influence of temporal and environmental gradients on the flight initiation distance (FID) and escape mode in shorebirds, and found that heat-stressed shorebirds take greater risks and adjust risk-taking behavior according to tidal and seasonal cycles.