Abstract
Most insect migrants fly at considerable altitudes (hundreds of meters above the ground) where they utilize fast-flowing winds to achieve rapid and comparatively long-distance transport. The nocturnal aerial migrant fauna has been well studied with entomological radars, and many studies have demonstrated that flight orientations are frequently grouped around a common direction in a range of nocturnal insect migrants. Common orientation typically occurs close to the downwind direction (thus ensuring that a large component of the insects’ self-powered speed is directed downstream), and in nocturnal insects at least, the downwind headings are seemingly maintained by direct detection of wind-related turbulent cues. Despite being far more abundant and speciose, the day-flying windborne migrant fauna has been much less studied by radar; thus the frequency of wind-related common orientation patterns and the sensory mechanisms involved in their formation remain to be established. Here, we analyze a large dataset of >600,000 radar-detected “medium-sized” windborne insect migrants (body mass from 10 to 70 mg), flying hundreds of meters above southern UK, during the afternoon, in the period around sunset, and in the middle of the night. We found that wind-related common orientation was almost ubiquitous during the day (present in 97% of all “migration events” analyzed), and was also frequent at sunset (85%) and at night (81%). Headings were systematically offset to the right of the flow at night-time (as predicted from the use of turbulence cues for flow assessment), but there was no directional bias in the offsets during the day or at sunset. Orientation “performance” significantly increased with increasing flight altitude throughout the day and night. We conclude by discussing sensory mechanisms which most likely play a role in the selection and maintenance of wind-related flight headings.
Highlights
From virtually the first moments that special-purpose entomological scanning radars were deployed in the late1960s (Schaefer, 1976), observers were astonished to note that moderate and large-sized nocturnal insect migrants, flying independently of each other at high altitudes, often showed flight headings which were tightly grouped about some common direction, and that the common orientation may remain constant for periods of several hours
What is the nature of the environmental cue that is responsible for huge numbers of high-flying insects taking up and maintaining the same flight heading over very large spatial scales?
We reveal for the first time the frequency of common orientation of flight headings in these medium-sized day-flying insects, and the association of their heading distributions with the downwind direction, and discuss the implications of these findings for the likely sensory modality responsible for the patterns observed
Summary
From virtually the first moments that special-purpose entomological scanning radars were deployed in the late1960s (Schaefer, 1976), observers were astonished to note that moderate and large-sized nocturnal insect migrants, flying independently of each other at high altitudes, often showed flight headings (i.e., body orientations, not displacement directions) which were tightly grouped about some common direction, and that the common orientation may remain constant for periods of several hours. While there is good evidence that nocturnal insects migrating over the UK routinely demonstrate a high degree of common orientation closely aligned with the downwind direction, but typically offset to the right of the flow (Aralimarad et al, 2011; Chapman et al, 2015a) as expected from the theory (Reynolds et al, 2010), patterns of wind-related orientation in the considerably more diverse and abundant diurnal aerial insect fauna have not been studied at all.
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