Analysis of mixtures of birds and insects in weather radar 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 remains a major analytical challenge. Most studies have restricted analyses to locations, seasons, and periods when one or the other taxa dominates. In this study, we describe an analytical method to estimate the proportion of birds and insects from vertical profiles of biological reflectivities, using a minimal number of assumptions on the airspeeds of birds and insects. We evaluated our method on understudied regions where airborne insect density is too high for existing approaches of studying bird migration with weather radars: the tropics (Colombia) and the southern temperate zone (Southeast Australia). Our method estimates that bird and insect signals routinely reach similar magnitudes in these regions. Retrieved patterns across daily and annual cycles reflected expected biological patterns that are indicative of migratory and non-migratory movements in both climates and migration systems. Compared to fixed airspeed thresholding, we obtain finer separation and retain more spatial-temporal complexity that is crucial to revealing aerial habitat use of both taxa. Our analytical procedure is readily implemented into existing software, empowering ecologists to explore aerial ecosystems outside the northern temperate zone, as well as diurnal migration of birds and insects that remains heavily understudied.