Nocturnal Activity of Caged Resident and Migrant Silvereyes (Zosteropidae: Aves)

Night‐activity patterns of individually‐caged grey‐breasted silvereyes Zosterops lateralis of resident and migrant status were investigated. True migratory restlessness or ‘Zugunruhe’ was exhibited by some individuals, but there was also considerable ‘noise’ in the data caused by cageing and neighbouring disturbance. In general, ‘Zugunruhe’ took place from about 2 h before, to about 4 h after ‘sunrise’, and both the daily timing and annual duration of Zugunruhe were consistent with the migration pattern observed in the field. A total of 42 % of resident birds from Tasmania, 77 % of migrant birds from Tasmania and 90 % of migrant birds from the mainland were considered to have exhibited true ‘Zugunruhe’, while none of the resident birds from Brisbane were considered to have displayed it. A few individuals spontaneously developed ‘Zugunruhe’ in the absence of other birds, but many birds (both residents and migrants) displayed no elevated night unrest when placed in a room on their own, although displaying unrest when placed among birds exhibiting ‘Zugunruhe’. The migration of the silvereye probably has a genetic basis, but whether an individual migrates or not is also influenced by social factors which differ in magnitude between individuals.