Jim Kellam, biology, presented "Predation Risk, Starvation Risk, and Radio Transmitters in Downy Woodpeckers" to the Association of Field Ornithologists and the Wilson Ornithological Society, Cornell University, April 2004.
Attaching radio transmitters to small birds in winter could increase both starvation risk (through greater energy demand) and predation risk (through higher wing-loading) unless individuals adjust body mass to compensate.
In my study of downy woodpeckers (Picoides pubescens; 28g), I predicted that relatively lightweight individuals, for whom starvation risk might be greatest, would gain weight after several weeks of wearing a 1.1g radio transmitter, while relatively heavy individuals, for whom predation risk might be greatest, would lose weight.
These predictions were confirmed after statistically accounting for temporal patterns in the body mass data. However, the relationship was mostly due to a few unusually light and heavy individuals that gained and lost significant amounts of weight, respectively. In contrast, most downy woodpeckers maintained about the same body weight while wearing radio transmitters compared to their pre-transmitter weight. This resulted in higher wing-loading and presumably higher predation risk during the period.
Overall, it appears that minimizing starvation risk was the most common strategy of woodpeckers wearing radio transmitters, even when predation risk was potentially elevated.
Contributed by Nancy Pierce