Contextual generality of roosttime restlessness in captive American robins (Turdus migratorius)

Abstract

A series of experiments with captive American robins examined factors affecting roosttime restlessness, that is, the burst of locomotor activity that appears at dusk and is thought to be a correlate of the communal roosting habit of this species. First, the restlessness occurred earlier on cloudy days than on sunny ones, suggesting the importance of illumination level in initiating the activity burst. Second, in studies on social effects, the restlessness was greater when robins were totally isolated than when they had visual-auditory access to conspecifics; indeed, when a small flock of robins was housed together, restlessness at dusk was suppressed completely. Third, the birds were more restless when they could see a human observer than when they could not, implying an interaction of fearfulness with the roosting tendency. Finally, restlessness occurred in a large flight cage as well as in small cages, as long as the birds were not maintained in a flock. In sum, roosttime restlessness appears to be II robust phenomenon that occurs under a wide range of laboratory conditions, but which also can be substantially influenced by numerous environmental variables. Further experimental examination of these variables may advance present understanding of the survival value of communal roosting in this species.The daily activity pattern of captive American robins iTurdus migratorius) under conditions of natural illumination typically consists of moderateto-high activity in the early daylight hours, a gradual decline in activity as the day progresses, and finally a brief period of high activity at dusk just before the birds settle down for the night (Eiserer, 1979; Palmer, 1(70). Several lines of evidence suggest that the high a.tivity at dusk is a behavioral correlate of the robin's tendency to fly to a communal roost each evening and sleep there with conspecifics.First, free-living robins show a characteristic restlessness around dusk preliminary to their flights to the roost (Brewster, 1890). A comparable display of restlessness by captive robins might be expected, much as many songbirds show "migratory restlessness" on a seasonal basis, whether free-living or in captivity (Eyster, 1954). Second, the high activity of captive robins at dusk is often accompanied by an increase in the occurrence of wing-flapping which may represent flight-intention movements or an "urge to fly" (Eiserer, 1979). Third, the activity peak at dusk appears every month of the year (Eiserer, 1979; 198Oc), a finding that is in agreement with field reports that wild robins travel to a communal roost during all four seasons

Unlike American Robins (Turdus migratorius), European Starlings (Sturnus vulgaris) do not display roost-time restlessness

“…Previous research with robins has demonstrated that the visible presence of a quiet observer does not change the overall pattern of circadian activity in that species (Eiserer, 1981).…”

“…This phenomenon is reflected also in the daily activity pattern of captive robins under conditions of natural illumination (Eiserer, 1979(Eiserer, , 1981. In particular, moderately high activity levels in the early morning gradually decline as the day progresses, but a brief period of high activity invariably occurs at dusk, before the birds settle down for the night (Eiserer, 1979;Palmer, 1970).…”