Consistency of spatio‐temporal patterns of avian migration across the Swiss lowlands

Abstract

Each year, billions of birds migrate across the continents by day and night through airspaces increasingly altered by human activity, resulting in the deaths of millions of birds every year through collisions with man‐made structures. To reduce these negative impacts on wildlife, forecasts of high migration intensities are needed to apply mitigation actions. While existing weather radar networks offer a unique possibility to monitor and forecast bird migration at large spatial scales, forecasts at the fine spatial scale within a complex terrain, such as the mountainous Swiss landscape, require a small‐scale network of ornithological radars. Before attempting to build such a network, it is crucial to first investigate the consistency of the migratory flow across space and time. In this study, we simultaneously operated three ornithological radar systems across the Swiss lowlands to assess the spatio‐temporal consistency of diurnal and nocturnal bird movements during the spring and autumn migration season. The relative temporal course of migration intensities was generally consistent between sites during peak migration, in particular for nocturnal movements in autumn, but absolute intensities differed greatly between sites. Outside peak migration, bird movement patterns were much less consistent and, unexpectedly, some presumably non‐migratory bird activity achieved intensities close to peak migration intensities, but without spatial correlations. Only nocturnal migration intensity in autumn could be predicted with consistently high accuracy, but including parameters of atmospheric conditions in the model improved predictability of diurnal movements considerably. Predictions for spring were less reliable, probably because we missed an important part of the migration season. Our results show that reliable forecasts of bird movements within a complex terrain call for a network of year‐round bird monitoring systems, whereas accurate information of atmospheric conditions can help to limit the number of measurement points.

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

“…However, the temporal pattern in the proportions and the mean flight directions, as presented in Figure 3, allowed us to determine periods of migratory movements (season-specific directional preferences). In birds, the migration trends coincide well with the main migratory periods documented for this region [15,32,[40][41][42]. In spring, diurnal and nocturnal migration cover the same period, whereas in autumn, diurnal migration is more concentrated and later in the season than nocturnal migration.…”

“…The Rayleigh test is sensitive to the chosen bin width and the sample size. Therefore, we simulated circular distributions for a range of sample sizes (10, 50, 100, 150, 300, 500, and 1000; number of echo per class and day/night in Table S1) and different proportions of directional (assuming a normal distribution centered around a mean, e.g., 270 • , and 40 • standard deviation, as observed for peak migration periods in Tschanz et al [32]) and non-directional movements (assuming a uniform distribution between 0 • and 360 • ) to test our method for different bin widths. We achieved the best results by applying the iterative Rayleigh test with a bin width of five degrees, whereas with low sample sizes, the proportion of directional movements was underestimated at low migration intensity and overestimated at high migration intensity (Figure 2).…”

Comparison of bird migration in a radar wind profiler and a dedicated bird radar

“…Quantification in both radar types is based on single targets, though on different MTR calculations (Schmid et al, 2019;Weisshaupt et al, 2017). As put forward in previous studies (e.g.…”

“…MTRs of the bird classes in the BirdScan MR1 data were computed using the R package 'birdscanR' (Haest et al, 2022), taking into account the size-specific (theoretical) monitored volume for each detected bird (Schmid et al, 2019). For the calculation of the October 2019 MTRs, we restricted the computation to the echoes detected using short-pulse to between 50 and 800 m a.g.l.…”

Crossing artificial obstacles during migration: The relative global ecological risks and interdependencies illustrated by the migration of common quail Coturnix coturnix

“…Changes in environmental factors can be used as indicators of the probability that a site will disappear from a migration network (Xu et al, 2021). Billions of birds migrate across the continents by night through airspaces increasingly altered by human activity, resulting in the deaths of millions of birds every year through collisions with man-made structures (Tschanz et al, 2020). In the US, it has been estimated that 500 million individual birds migrate each night during peak passage, and many collide with buildings, airplanes, and wind turbines (Van Doren and Horton, 2018).…”