Migratory restlessness or Zugunruhe in birds - a description based on video recordings under infrared illumination

Migratory restlessness - defined as the (mainly nocturnal) migratory activi ty of caged birds - has been known since at least the 18th Century and is r egularly used to study questions regarding bird migration. However, it had not been satisfactorily described until 1988, when we first became able to obtain and analyze video recordings of this behaviour made in infrared ligh t. Thanks to refined video and reproduction techniques, we are now able to present such recordings here, for the first time in printed form (Figs. 1 a nd 2). The birds involved are first year Blackcaps (Sylvia atricapilla) fro m southern Germany during their first migration period. According to our ex perience, the migratory restlessness they show is typical, i.e. not affecte d by the recording procedure. In the Blackcap - as shown previously for Gar den Warblers - by far the most prominent element in this behaviour is typic al "wing whirring": beating the wings while perched, with high frequency an d low amplitude. We interpret this "sedentary migration" as a kind of "flyi ng with the brakes on" in adaptation to cage conditions. Migratory restless ness, which in addition to whirring includes hopping, climbing, fluttering and flying, is exhibited in all parts of the cage, not only on the perches but also on the floor and walls and even while hanging from the ceiling. He nce theoretically it can be monitored in its entirety only by ultrasound, v ideo technology, bundles of light barriers or cages rigged for general vibr ation detection. When applied to many experimental animals, however, these methods require elaborate equipment and are susceptible to failure. In our experience the best results are obtained with recording cages having two mo vable perches mounted on microswitches. If the switch sensitivity is adjust ed to the birds' body weight, about 95 % of the migratory restlessness obse rved in video pictures will be recorded. Because of the large interindividu al variation in the parts of the cage where the relevant behaviour occurs a nd the variability in use of the recording perches, the best measure for qu antitative studies of migratory restlessness is not the number of individua l events ("hops") but larger-scale units such as half- or whole-hour period s with (or without) migratory activity. In our experience half-hour interva ls are quite suitable for many purposes, including quantitative genetic stu dies.