Unshaded coffee imposes a heavier load on thermoregulation than shaded coffee for birds in a tropical mountainous region

Distribution shifts and local extinctions in tropical montane birds are expected to result from intolerance to elevated temperatures driven by both climate warming and changes in land use which affect local, microclimatic temperatures (Tamic). However, the physiological basis of this phenomenon is yet insufficiently explored. Here, we experimentally measured the response to heat of four wild-caught bird species and predicted how heat exposure would increase the demand for evaporative cooling in a tropical montane region dominated by coffee farming. For this, we first measured Tamic inside shaded and unshaded coffee farms from March to July 2020. Subsequently, we exposed the birds to a temperature gradient in a controlled laboratory environment and measured body temperature and the rates of resting metabolism and evaporative water loss. Using segmented and linear mixed regression models, we determined the temperatures in which these parameters sharply increased (i.e. inflection points). We then used these values to predict the daily amount of time in which Tamic would demand a cooling response and compared it between farm types with generalised linear regression models. Finally, we estimated the water lost to cooling as a percentage of the species-specific body mass. Maximum Tamic in unshaded coffee was significantly higher than in shaded coffee, while Tamic in shaded farms was similar to Tamic inside forest patches used as a reference. The four tested bird species exhibited different rates of change and inflection points of the metabolic parameters. For all of them, shifting from a shaded to an unshaded coffee farm increased the number of hours of exposure by 2-3 times. Interspecific differences were found in the amount of water predicted to be lost to cooling inside the farms. However, these daily losses, even under the most extreme scenario, represented only a small proportion of body mass lost in the form of water. Taken together, we demonstrate that these four bird species are able to endure the current thermal environment of a tropical montane agricultural landscape, without the risks of chronic or lethal dehydration. Nonetheless, we found that unshaded coffee farms increase the demand for water necessary to maintain water balance over consecutive days, which might become more critical if current scenarios of global warming in the tropics are realised in the future.

Automated summary

This study explores the heat tolerance of tropical montane birds and suggests that future global warming may pose a threat to their water balance and survival. The results indicate that unshaded coffee farms increase the demand for water to maintain stability, which could have negative effects on their health and survival.