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

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 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.