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

Migratory restlessness - defined as the (mainly nocturnal) migratory activity of caged birds - has been known since at least the 18th Century and is regularly 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 light. 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 and 2). The birds involved are first year Blackcaps (Sylvia atricapilla) from southern Germany during their first migration period. According to our experience, the migratory restlessness they show is typical, i.e. not affected by the recording procedure. In the Blackcap - as shown previously for Garden Warblers - by far the most prominent element in this behaviour is typical "wing whirring": beating the wings while perched, with high frequency and low amplitude. We interpret this "sedentary migration" as a kind of "flying with the brakes on" in adaptation to cage conditions. Migratory restlessness, 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. Hence theoretically it can be monitored in its entirety only by ultrasound, video technology, bundles of light barriers or cages rigged for general vibration 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 movable perches mounted on microswitches. If the switch sensitivity is adjusted to the birds' body weight, about 95 % of the migratory restlessness observed in video pictures will be recorded. Because of the large interindividual variation in the parts of the cage where the relevant behaviour occurs and the variability in use of the recording perches, the best measure for quantitative studies of migratory restlessness is not the number of individual events ("hops") but larger-scale units such as half- or whole-hour periods with (or without) migratory activity. In our experience half-hour intervals are quite suitable for many purposes, including quantitative genetic studies.