Externally attached biologgers cause compensatory body mass loss in birds

Animal-borne logging devices are now commonly used to record and monitor the movements, physiology and behaviours of free-living animals. It is imperative that the impacts these devices have on the animals themselves are minimised. One important consideration is the interaction between the body mass of the animal and the mass of the device. Using captive homing pigeons, we demonstrate that birds lose the equivalent amount of body mass compared to that of the logging device attached. With our experiments, we calculated that the compensatory mass loss because of the logging device equates to a total loss of 1,140 kJ of energy to the bird, over the 25-day period. This equates to 32% per day of their total daily energy budget. We suggest that practitioners of biologging give due consideration to the possibility of a device-induced decrease in body mass when making decisions regarding device size, and when considering the period of the time of the year at which devices are attached. It appears, based on the results of the present study, that device attachment is likely to be most disruptive during periods of regulated mass change, especially when periods of mass gain precede periods in which stored energy reserves are extensively utilised. These findings have significant consequences for anyone using biologging technology on both wild and captive volant animals. Further studies utilising captive birds are now needed to fully understand how context- and species-dependent physiological responses to externally attached devices are.