Seasonal Trends in Movement Patterns of Birds and Insects Aloft Simultaneously Recorded by Radar

Abstract

Airspace is a key but not well-understood habitat for many animal species. Enormous amounts of insects and birds use the airspace to forage, disperse, and migrate. Despite numerous studies on migration, the year-round flight activities of both birds and insects are still poorly studied. We used a 2 year dataset from a vertical-looking radar in Central Europe and developed an iterative hypothesis-testing algorithm to investigate the general temporal pattern of migratory and local movements. We estimated at least 3 million bird and 20 million insect passages over a 1 km transect annually. Most surprisingly, peak non-directional bird movement intensities during summer were of the same magnitude as seasonal directional movement peaks. Birds showed clear peaks in seasonally directional movements during day and night, coinciding well with the main migration period documented in this region. Directional insect movements occurred throughout the year, paralleling non-directional movements. In spring and summer, insect movements were non-directional; in autumn, their movements concentrated toward the southwest, similar to birds. Notably, the nocturnal movements of insects did not appear until April, while directional movements mainly occurred in autumn. This simple monitoring reveals how little we still know about the movement of biomass through airspace.

Favorable winds speed up bird migration in spring but not in autumn

“…The degree of directionality can be assessed with the value of the standard deviation of the radial velocity. We found a higher standard deviation of the radial velocity in spring than in autumn (see Figure ), which is in line with the results of a bird tracking radar study (Shi et al, 2021). Therefore, the slightly lower autumn airspeed estimated in this study could be explained by birds migrating in more diverse directions.…”

“…We can only speculate that this difference is at least partially linked to the larger scattering of flight directions, or to their treatment of insect contamination. Surprisingly, they excluded about the same amount of insect contamination in spring and autumn, although we would expect more insects in autumn (Larkin, 1991;Nussbaumer et al, 2021;Shi et al, 2021). In addition, with half of their radars located on the coast, one can also expect that the strong seasonally dependant coastal effect (Horton, van Doren, Stepanian, Hochachka, et al, 2016) might cause increased airspeed in autumn (e.g., more compensation required with wind blowing bird offshore).…”

Insect decline in forests depends on species’ traits and may be mitigated by management

“…Traps were placed at randomly selected site corners at ~1.5 m above the ground and operated from March to October (yearly sampled sites) or from March to July (remaining sites), with collection jars replaced in approximately monthly intervals. To harmonize sampling effort among yearly vs. triennially-sampled sites, all site-level analyses were based on three sampling rounds covering May, June and July, the main flight-activity period of insects in Central Europe 92 .…”

BATScan: A radar classification tool reveals large‐scale bat migration patterns

“…RCS is a radar measure related to target size (amount of reflected radiation) that ranges from actual object size to ~0 depending on various technical factors, (Supporting Information 3.1) but never exceeds actual size, so we only used an upper threshold of RCS values. Finally, we restricted altitude on a technical basis to above 50 m for avoiding ground clutter, and below 800 m, which is the upper detection threshold of the MR1 for the smallest insectivorous bat in the region P. pipistrellus (Shi et al, 2021).…”