Analysis of mixtures of birds and insects in weather radar profile data

ABSTRACT Weather radars are increasingly used to study the spatial-temporal dynamics of airborne birds and insects. These two taxa often co-occur and separating their contributions is crucial for reliable interpretation of their movement patterns. Most studies have restricted analyses to locations, seasons, and periods in which one or the other taxa dominates. In this study, we describe an analytical method to estimate the proportion of birds and insects in cases where both taxa share the same airspace. Our approach partitions vertical profiles of biological reflectivity into bird and insect components, using assumptions of downwind heading selection by insects and information on expected airspeeds for birds and insects. We evaluated our method in regions, where existing approaches of studying bird migration with weather radars can be particularly challenging due to high airborne insect density: the tropics (Colombia) and the southern temperate zone (Southeast Australia). We found that bird and insect signals routinely reached similar magnitudes in these regions. Retrieved patterns of bird and insect occurrence across daily and annual cycles reflected expected biological patterns that are indicative of migratory and non-migratory movements in both climates and migration systems, particularly broad-front migration in birds. Contrary to fixed airspeed thresholding, we were able to partition birds along the full range of bird–insect proportions, retaining more spatial-temporal complexity that was crucial to revealing the aerial habitat use of both taxa. Our analytical procedure readily extends existing vertical profiling approaches, empowering ecologists to explore complex aerial ecosystems across a diverse range of climates, as well as potential diurnal movements of birds and insects that remain heavily understudied.