Response to: Green Light for Nocturnally Migrating Birds

Abstract

Ideally, field studies comparing the effects of different light characteristics would be carried out under uniform natural light and cloud conditions, i. e., cloud ceiling height above ground and percent of sky covered. In this study, designated “overcast” study periods apparently might have had anywhere from a high cloud ceiling, e.g., 2000 m, to one at ground level, e.g., fog. Cloud ceiling information is critical to consider as a variable in these experiments because in conditions of high ceiling, migrating birds can see distant light sources and the horizon quite well. In such cases they may not be as susceptible to effects from any particular light source on the ground compared with when they are migrating in the limited visibility under or within a very low cloud layer, e.g., 300 m, such lights could very likely be visible to night migrating birds, if not directly, then indirectly via reflection off the cloud ceiling. The authors’ conclusion that red light causes more disorientation for night migrating birds than green light appears to be based on the small sample that 13 of 24 groups or individual birds were noted to react in red light. Such a small sample, without involving the variable of cloud ceiling height during the different light study periods, does not support confidence in the authors’ conclusions.