Aeroecology of a solar eclipse

Abstract

Light cues elicit strong responses from nearly all forms of life, perhaps most notably as circadian rhythms entrained by periods of daylight and darkness. Atypical periods of darkness, like solar eclipses, provide rare opportunities to study biological responses to light cues. By using a continental scale radar network, we investigated responses of flying animals to the total solar eclipse of 21 August 2017. We quantified the number of biological targets in the atmosphere at 143 weather radar stations across the continental United States to investigate whether the decrease in light and temperature at an atypical time would initiate a response like that observed at sunset, when activity in the atmosphere usually increases. Overall, biological activity decreased in the period leading to totality, followed by a short low-altitude spike of biological activity during totality in some radars. This pattern suggests that cues associated with the eclipse were insufficient to initiate nocturnal activity comparable to that occurring at sunset but sufficient to suppress diurnal activity.

Keywords: light cues; nocturnal migration; radar aeroecology; solar eclipse.

Figure 1.

(a) The path of eclipse totality through the network of weather surveillance radars in the continental USA, 21 August 2017. All 143 sites included in this study, coloured by the maximum amount of obscuration. The path of totality, where obscuration is 100%, is shown in grey, and the eight sites located within the path of totality are outlined in black. (b–g) Patterns of biological activity in the atmosphere as sampled by NEXRAD. The smoothed mean (generalized additive model) amount of vertically integrated reflectivity (VIR) at all sites, grouped after amount of maximal obscuration: (b) during the eclipse (less than 80: n = 54, 80–95: n = 58, 95–100: n = 26), (c) at sunset the day of the eclipse (less than 80: n = 52, 80–95: n = 59, 95–100: n = 26); and at the time of day of the eclipse on: (d) 19 August (less than 80: n = 54, 80–95: n = 58, 95–100: n = 27), (e) 20 August (less than 80: n = 54, 80–95: n = 59, 95–100: n = 27), (f) 22 August (less than 80: n = 55, 80–95: n = 59, 95–100: n = 27), and (g) 23 August (less than 80: n = 55, 80–95: n = 60, 95–100: n = 27). Note different scale on y-axis in (c).

Figure 2.

Reflectivity from biological targets for each of the eight sites in the path of totality over time. Heatmap shows the amount of reflectivity at different altitudes (left y-axis), black line shows the sum of reflectivity integrated over altitude (VIR, right y-axis). Altitude is in metres above sea level (m.a.s.l.; data start at ground level) and time is local time at each site. Black vertical lines mark start and end of eclipse, dashed vertical line marks time of maximum sun obscuration (totality). Reflectivity, especially at higher altitudes, decreased during the eclipse. Note that at four sites (KHPX, KLSX, KOHX and KPAH) an increase in reflectivity (darker colour) is seen at low altitudes at the timestamp closest to totality, also seen as peaks in the VIR, and especially visible in animations of the lowest elevation scan (electronic supplementary material, movie S1). Grey areas are data gaps.