Avion Ecol. Behov. 3, 1999: 79-90 Commencement of nocturnal restlessness in the European Robin Erithacus rubecula during migration Victor N. Bulyuk & Andrey Mukhin Abstract: Bulyuk, V.N. & Mukhin, A. (1999): Commencement of nocturnal restlessness in the European Robin Erithacus rubecula during migration. Avian Ecol. Behav. 3: 79-90. We studied the time of onset of nocturnal migratory restlessness in the European Robin during spring and autumn migration. In 1996-1998 birds were specially trapped an hour or two before sunset in spring (10 April - 20 May; n = 219) and in autumn (7 September- 4 November; n = 287) and put in separate cages under the open sky with a view of sunset and the stars. Their behaviour was watched through infrared vision binoculars or recorded in automatic registration cages. Birds were released before sunset. In autumn, nocturnal restlessness was recorded in 18.1% of freshly captured Robins (15.7% and 22.6% in September and October, respectively). The proportion of Robins that displayed nocturnal activity was significantly positively related to their fat stores. Time of onset of nocturnal activity varied in different individuals between the second and the ninth hour after sunset (median 180 min). In September the median time was 155 min, in October 195 min. In November in four Robins out of seven nocturnal activity started five hours after sunset. In birds with large fat stores there was an observable trend to start nocturnal activity earlier in respect to sunset, than in leaner birds. In spring, nocturnal activity was recorded in 17.3% of freshly trapped Robins (16.7% and 18.4% in April and May, respectively). The time of onset of nocturnal restlessness varied between the second and the sixth hour after sunset (median 150 min). In late April the median was 147 min, in early May 153 min. Birds trapped during waves of passage (mass transit migration) tended to start nocturnal restlessness earlier in respect to sunset than conspecifics captured during migratory pauses. In birds held in captivity for some time, the proportion of active birds was much higher than in freshly trapped ones (65.8% in autumn, 60% in spring), the median being shifted towards the sunset: for 28 min in autumn and for 47 min in spring. The difference between freshly trapped and caged individuals are explained by higher fat scores of the latter, and by their delay on the migratory route. Variation in the timing of the onset of nocturnal activity in caged Robins is discussed, together with the relationship between behaviour in cages and migratory take-off activity in the wild. Key words: European Robin, Erithacus rubecula, migration, nocturnal restlessness, take-off activity, temporal distribution. Address: Biological Station Rybachy, 238535 Rybachy, Kaliningrad Region, Russia E-mail: [email protected] 1. Introduction The data of visual and searchlight observations over departures of Robins from stopovers showed that these birds can initiate migration at different times of night, both in spring and in autumn (Bolshakov & Rezvyi 1975, 1982, 1998, Bolshakov & Bulyuk 1999, in press). Up to now, broad variation in the timing of take-off activity during the night was found in a number of passerine nocturnal migrants, both short- and long-distance (Cochran et al. 1967, Cochran 1987, Bolshakov 1992, Åkesson et al. 1996, Moore & Abom 1996, Bolshakov & Bulyuk, in press). The available data suggest that m nocturnal passerine migrants the diel pattern of departures may vary not only between the species, but in a single species in different seasons, and on different nights within the season (Bolshakov & Bulyuk, in press). Thus, the concept of mass departure of nocturnal passerine migrants within a limited period of dusk (Kerlinger & Moore 1989, Alerstam 1990, Berthold 1993) is not supported. Timing of take-off in small passerines is supposed to be related to various factors; availability of celestial cues necessary for orientation at sunset and at twilight, variation in the atmospheric structure, the energy condition of the migrants (Moore 1987, Kerlinger & Moore 1989, Moore & Abom 1996). It may reflect the interplay of endogenous time programmes with LD period changes over the season along the migratory route, and also the position of birds in respect to their migration destination and their migratory speed under different winds (Bolshakov & Rezvyi 1998, Bolshakov & Bulyuk 1999). It has long been known that many passerines taken from the wild and put into cages, display nocturnal restlessness and seasonally-appropriate orientation during their migratory period. Nocturnal restlessness is a cage-adapted form of migratory flight, a kind of "migration in a sitting position" 1 (Berthold 1993). The onset of activity occurs when a free living bird takes-off and starts its migratory flight. If freshly captured birds are put in cages for the night and their activity recorded, it becomes possible to study the timing of departures experimentally, and to estimate the importance of some internal and external factors that influence nocturnal activity. This idea formed the basis of the present study. Our aims were: firstly, to estimate the proportion of trapped Robins showing nocturnal activity and whether this parameter is related to the season, fat score, availability of celestial cues (sunset, the sun) and conditions under which the birds were trapped (wave of passage or migratory pause). Secondly, to measure the variation in the time of onset of nocturnal restlessness in freshly trapped Robins and its relationship to their energy condition. Thirdly, to compare the time of onset of nocturnal activity in freshly trapped Robins with similar data from birds held in captivity for a considerable time. Fourthly, to compare the experimental results with the data from observations of take-off in the wild. 2. Material and methods Nocturnal activity was studied in two groups of Robins specially mist-netted on the Courish Spit of the Ealtic Sea during seasonal migrations in 1996-1998. Birds from the first groups were daily captured an hour or two before sunset in spring (10 April - 20 May; n=219) and in autumn (7 September - 4 November; n=287). After ageing, measuring body mass, wing-length and estimating fat according to the 9-score system of Kaiser (1993) birds were put in separate cages for the whole night (up to 12 birds every night). The cages were situated under the open sky, so the birds could see sunset and the stars. The cages had solid walls which prevented birds from seeing each other. In 1996-1997, their behaviour (n=184 in spring, n=210 in autumn) was watched by infra-red vision binoculars from 10 m. In 1998, nocturnal activity (n=35 in spring, n=77 in autumn) was recorded in automatic registration cages. Their activity was watched for 10 min in the middle of each hour of the night from 90 min after sunset until 90 min before sunrise. In automatic cages during the same time interval (from 90 min after sunset until 90 min before sunrise) for each hour all hops were counted. Birds were released before sunset. The bulk of birds from this group were in their first year in autumn (90.6%) or in their second year in spring (85.8%). Birds from the second group were trapped on 5-10 September 1996 (n=6) and 21 April - 6 May 1997 (n=9) and kept indoors in separate cages under natural photoperiod for one month. From 10 days after capture their nocturnal activity was recorded once in 4-7 days under the same conditions in the same cages as in freshly trapped conspecifics. All birds from this group were in their first year (second year in spring). In this report we analysed variation of only two parameters of nocturnal activity in Robins from both groups: (1) the presence or absence of nocturnal activity on the test night; (2) the hour in respect to sunset when activity commenced. During visual registration those birds were considered to be active which showed constant restlessness for at least two hours. Birds that were inactive the whole night or showed episodic activity at a low level, were considered not active. During automatic registration, birds with an average number of hops al0 per hour were considered active. We believed that the nocturnal activity commenced (1) in the second hour after sunset if a bird was constantly active during this and subsequent hours without visible interruption; (2) in the third hour if after the lack of activity between 90 and 120 min after sunset the birds started to hop in the first or second half of the third hour; (3) in the fourth hour if after the lack of measurable activity in the second and third hour a bird started activity in the fourth hour, etc. To study the relationship between the birds' energy condition and the onset of nocturnal restlessness, we used the condition index of the birds. This index was calculated as m-w0852 where m is body mass, and w is wing-length (Titov & Chemetsov, this volume). The mean value of the condition index calculated for the periods of the spring and autumn migratory seasons, were the border dividing birds into two groups, with condition index below the mean value and above it. This referred to high and low fuel stores in experimental birds. The condition index was applied, as the bulk of tested birds had fat scores of 2 or 3 (Tab. 1). The use of the condition index, with a correcting factor for individual size, in our opinion, yields more exact estimates of nutrient stores than visual estimates of fat between 2 two adjacent scores. Many Robins that make stopovers at the study site during peak numbers (waves of migration), proceed with their migration during the next night (Titov & Chemetsov 1999).