Precipitation scrubbing of aerial plankton: inferences from bird behavior.

Abstract

I conducted direct visual observations of aerial insect-eating birds concurrently with remote radar observations of aerial plankton before, during, and after the passage of an intense thunderstorm gust front in east-central Florida. Clear skies and convective conditions predominated in the area prior to local passage of the gust front. Shortly after passage, weather conditions shifted to a stratiform regime, with continuous low cloud cover and intermittent drizzly rain. Insectivorous birds appeared in the area briefly at the time of gust front passage, and became common following the onset of the drizzly rain. These birds remained in the area after the cessation of precipitation, feeding actively and very low, until mid-afternoon the following day, when the transition from stratiform back to convective conditions finally occurred. Almost immediately after this transition, the altitude of bird flight began increasing rapidly, and the birds quickly dispersed. I interpreted these observations together with radar data as indicating that (a) large quantities of aerial plankton were entrained by the gust front, "leaked" into the storm outflow, and were subsequently "scrubbed" out of the atmospheric boundary layer by precipitation; (b) after the rain ended, the insect species involved were prevented from reascending by stratiform sky conditions that inhibited thermal convection, and (c) the insects rapidly reascended at the first possible opportunity (i.e., almost immediately following the transition from stratiform back to convective conditions). This detailed case study showed that an individual convergence event in the atmospheric boundary layer caused a spatial redistribution of the aerial plankton which subjected the insects involved to locally intensified predation by avian consumers. However, despite the dramatic short-term effects, the apparently rapid reascent of the aerial plankton suggests that this particular scrubbing event probably had little impact on the local population dynamics of the insect species affected. Data on track directions of a large sample of summer gust fronts in east-central Florida suggest that the potential for net directional displacements of insect populations over seasonal time scales via the cumulative effects of individual convergence events and subsequent scrubbing is probably low.