A radar study of emigratory flight and layer formation by insects at dawn over southern Britain

Reynolds, D.R., Smith, A.D. and Chapman, J.W. (2008) A radar study of emigratory flight and layer formation by insects at dawn over southern Britain. Bulletin of Entomological Research, 98 (1). pp. 35-52. ISSN ISSN: 0007-4853 EISSN: 1475-2670

Abstract

Radar observations have consistently shown that high-altitude migratory flight in insects generally occurs after mass take-off at dusk or after take-off over a more
extended period during the day (in association with the growth of atmospheric convection). In this paper, we focus on a less-studied third category of emigration –
the ‘dawn take-off’ – as recorded by insect-monitoring radars during the summer months in southern England. In particular, we describe occasions when dawn emigrants formed notable layer concentrations centred at altitudes ranging from ca. 240m to 700m above ground, very probably due to the insects responding to local temperature maxima in the atmosphere, such as the tops of inversions. After
persisting for several hours through the early morning, the layers eventually merged into the insect activity building up later in the morning (from 06.00–08.00 h
onwards) in conjunction with the development of daytime convection. The species forming the dawn layers have not been positively identified, but their masses lay
predominantly in the 16–32mg range, and they evidently formed a fauna quite distinct from that in flight during the previous night. The displacement and common
orientation (mutual alignment) characteristics of the migrants are described.

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