An Aerodynamic Basis for Selecting Transmitter Loads in Birds

An accepted practice in radio telemetry studies is to limit transmitter size to 5% of body mass irrespective of bird size. This approach is unsatisfactory because it (1) ignores aerodynamic relationships indicating that small birds can carry loads equaling a larger proportion of their body mass than large birds, and (2) fails to provide an estimate of energetic costs of transporting the transmitter. We developed a method to select transmitter mass based on estimates of power requirements for flight and total power available for flight. We provide a general method based only on body mass, but we also show how estimates can be improved for individual species by taking several simple measurements. A minimum value for body mass is selected considering factors affecting weight such as annual cycle. This is the base mass and is used to calculate the maximum sustainable rate of flight power (power available). Next, power requirements for flight are calculated. As power requirements vary with flight velocity, a single velocity must be selected: we use the most efficient velocity. To assure conservative estimates of a bird's ability to carry a transmitter, base mass is adjusted upwards by a percentage approximating the normal range in body mass. Adjusted mass is used to estimate the power required to fly at the most efficient velocity. The difference between power available for flight and the power required to fly at the most efficient velocity is surplus power. Adding a transmitter increases power requirements. We evaluate a bird's ability to carry a transmitter by calculating the reduction in surplus power caused by transmitters of various sizes. Power surplus is proportionately greater for small birds than large birds, so basing transmitter size on a fixed percentage of body mass results in conservative loadings for small birds and liberal loadings for large birds. Our method allows an investigator to select transmitter size according to the reduction in power surplus that is considered appropriate for the experimental conditions, and at the same time it provides an estimate of the energetic cost of transporting the added mass of the transmitter.