A review of avian experimental translocations that measure movement through human-modified landscapes

Understanding how landscape characteristics such as matrix composition and configuration affect bird movement is essential for conservation in human-disturbed habitats. An important tool for such studies is experimental translocation, in which birds are removed from their territories, and observed, often using radio telemetry, as to whether they return (“home”). Here we reviewed 28 translocation studies to investigate (1) in what regions and habitats, and with what species the technique has been used, (2) what proportion of birds fail to home, a measure of “bird safety”, (3) the findings the studies have uncovered about bird movement, and (4) how the studies have dealt with confounding variables. A phylogenetically controlled meta-analysis complemented the qualitative results, testing the effects of the species’ habitat usage and migration ecology on the difference between birds’ homing success when moving through contiguous forest as compared to other land-use types. Of the total sample, 25 studies were conducted in the Western Hemisphere, and 17 in locations where pasturelands were the dominant land-use. The technique has been mostly used with small, forest-preferring passerine species, and males. Regarding safety, the proportion of individuals that failed to home was 27.1 ± 21.8 (SD)%. Translocation distance was found to be an important predictor of homing time or success in seven studies. The meta-analysis confirmed that on average birds move better through contiguous forests than through other land-use types, with habitat generalists and species with seasonal migration having less trouble moving through disturbed land-use types. The qualitative analyses also demonstrated the importance of habitat usage, with forest specialists especially affected by forest cover and connectivity. In considering the utility of translocations, it is important to remember that they are not measuring juvenile dispersal, and the birds’ homing mechanisms might influence their movements. However, long-distance translocations can avoid homing and hence simulate breeding (adult) dispersal. The confounding effects of variation among capture and release sites, or among individuals in their physiological condition, must be carefully considered. Overall, experimental translocation is a mature technique for which possible pitfalls have been identified. Strengths of the approach are that bird motivations to move can be standardized, and fine-scale movement patterns can be described with telemetry. However, we encourage consideration of bird safety, and meticulous reporting of birds that fail to home, or fail to find suitable habitat in long-distance translocation designs.