Commencement of Nocturnal Restlessness in the European Robin Erithacus Rubecula During Migration

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). On the other hand, birds that land during migratory pauses, may stay for several days. This difference in stopover length may be caused by a difference in motivation for proceeding with migration which is itself caused by a complex interplay of external and internal factors (Dolnik 1975, Alerstam 1990, Berthold 1996). To estimate the difference in the time of onset of nocturnal restlessness in Robins trapped on peak and pause days (high and low levels of passage, respectively), we analysed the conditions on the day of capture. We used the data of daily captures of Robins in mist-nests carried out at the study site simultaneously with our experiment in the framework of a joint project undertaken by Biological Station Rybachy and Vogelwarte Radolfzell. Days with ringing totals of over 150 in autumn and over 250 in spring were considered to be the peaks, with smaller totals indicated the pauses. Difference in distributions of the time of beginning nocturnal restlessness was tested by the Kolmogorov-Smimov test. Mean body masses and condition index values were compared by t-test while differences in proportion were compared by the contingency table.

3. Results

3.1. Nocturnal restlessness in freshly trapped birds in autumn In autumn 18.1% of freshly trapped Robins displayed restlessness (52 out of 287). In September and October proportions of active Robins were not significantly different: 15.7% (n=125) and 22.6% (n=152), respectively (χ2 =1.74; p>0.05). In November seven out of 10 birds were active. The proportion of active birds was significantly higher in fatter birds and on days with a high number of stopover migrants than on quiet days (Tab. 1). Further, the proportion of active birds was 2.3 times larger under clear skies or limited cloud cover at dusk, than under overcast conditions. This difference was however not significant (Tab. 1). Both in the first and in the second half of the autumn migratory season, Robins that displayed activity were heavier than inactive birds. This difference was significant only in October and November (Tab. 2). In autumn, the time of the onset of nocturnal restlessness in different birds varied between the second and ninth hour after sunset. One-half of the birds did not become active until 3 hours after sunset (median 180 min) (Fig. la). In September nocturnal restlessness in Robins commenced on average 40 min earlier in respect to sunset than in October: medians 155 min (n=17) and 195 min (n°·28) after sunset. This difference was however not significant (λ=0.6; p>0.05) (Fig. lc). In November, in four out of seven Robins activity started 5 hours after sunset.

Figure 1. Frequency distribution of the onset of nocturnal restlessness in freshly trapped Robins, a, b - in different seasons, c, d - at different periods of the season. Arrows indicate the time of sunrise at the beginning and the end of the study period.

3 Table 1. Percentage of Robins with nocturnal activity in relation to fat score, cloud score during dusk, and number of stopover birds.

Season Fat score Significance Cloud cower * Significance Number of stopover Significance migrants** 0-1 2-3 4-5 overcast limited low high

Autumn 7.5 19.4 61.9 χ2=35.27 6.6 15.4 χ2=3.27 16.0 36.8 χ2=4.02 (n=106) (n=160) (n=21) ρ<0.001 (n=106) (n=104) p>0.05 (n=268) (n=19) p<0.05

Spring 7.4 (n=27) 19.4 17.2 χ2=2.18 21.3 13.8 χ2 = 1.2l; 10.6 23.4 χ2=5.34 (n=163) (n=29) ρ>0.05 (n=61) (n=123) ρ>0.05 (n=106) (n=113) p<0.05

Notes: *scored during the tests; "see text.

Table 2. Mean body mass and condition index of active and inactive Robins during autumn and spring.

Period Mean mass ± SD (n), gSignificance Mean condition index ± SD Significance active birds inactive birds active birds inactive birds September 16.4±1.17(17) 15.9±1.82(108) t=1.09;p>0.05 0.43±0.031 0.42±0.028 t=1.39; p>0.05

October & November 17.0±0.92 (35) 16.5±1.15(127) t=2.32; p<0.05 0.44+0.026 0.43±0.029 t=2.67; p<0.01

April 17.5±1.19 (22) 17.1±1.18(110) t=1.46; p>0.05 0.45±0.027 0.44±0.029 t=2.11;p<0.05

May 16.2±0.80 (16) 16.6±1.08(71) t=1.40; p>0.05 0.43±0.022 0.44±0.028 t=1.58; p>0.05

During the whole season of autumn migration there was a trend towards an earlier onset of nocturnal activity in respect to sunset (on average 54-67 min) in birds with high nutrient stores. The difference was however not significant (Tab. 3). No significant difference in distributions of time of beginning nocturnal activity in Robins trapped during waves and pauses was recorded either (λ=0.82; ρ>0.05). When motivation was low, Robins started their activity on average 20 min later in respect to sunset (medians 169 and 188 min, respectively).

Table 3. Medians and differences of distributions of the time of beginning nocturnal activity in Robins with high and low condition index values.

Season Median, min after sunset Significance high condition index low condition index September 114 168 λ=0.50; p>0.05 October 173 240 λ=0.55; p>0.05 Whole autumn 156 204 λ=1.03; p>0.05 April 138 158 λ=0.42; p>0.05 May 150 150 λ=0.36; p>0.05 Whole spring 143 156 λ=0.40; p>0.05

3.2. Nocturnal restlessness in freshly trapped birds in spring During spring migration, 17.3% of freshly trapped Robins showed nocturnal restlessness (38 out of 219). Proportions of active Robins were nearly similar in April and in May, 16.7% (n = 132) and 18.4% (n=87), respectively (χ2 =0.03; p>0.05). As in autumn, the proportion of active birds was higher on days with peak catches than during halts in passage, higher in fat birds than in lean ones. In the

4 latter case the difference was however not significant (Tab. 1). The proportion of birds that showed nocturnal activity, was higher under overcast skies at dusk than under clear skies or limited cloud cover, the difference not being significant (Tab. 1). Mean body mass of active and inactive birds was not significantly different in spring. However in April the mean condition index of active birds was higher than in inactive conspecifics (Tab. 2). The time of onset of nocturnal restlessness varied in different birds between the second and sixth hour after sunset. More than half the birds did not commence their activity until 2.5 hours after sunset (median 150 min) (Fig. Ib). No significant difference was found between the time of onset of nocturnal activity in late April (median 147 min) and in early May (median 153 min); λ=0.4; p>0.05 (Fig. Id). No difference was revealed between Robins with high and low condition index values, either (Tab. 3). Birds trapped during a wave of migration became restless on average 42 min earlier in respect to sunset than those trapped on days with low trapping figures (medians 120 and 162 min, respectively). This trends was however not significant (λ=1.12; ρ>0.05).

3.3. Nocturnal restlessness in birds held in captivity In contrast with freshly captured birds, in Robins held in captivity for a long time nocturnal activity was recorded in 65.8% (n=38) of cases in autumn and in 60% of cases (n=35) in spring. In autumn, the median time of beginning restlessness in captive birds (152 min; n=25) was 28 min earlier in relation to sunset than in freshly trapped individuals (180 min; n=52) (Fig. 2). Frequency distributions of the onset of activity in captive and freshly trapped birds differed significantly (λ=1.5; p<0.05). In spring frequency distributions of the onset of activity in captive (median 97 min; n=21) and freshly trapped birds (median 150 min; n=38) were also significantly different (λ=2.01; p<0.001) (Fig. 2).

Figure 2. Frequency distribution of the onset of nocturnal restlessness in Robins held in captivity. Symbols as in Fig. 1.

4. Discussion Although nocturnal migratory restlessness differs from the migratory flight by the type of locomotion, its onset should probably be strongly related to the time of take-off from the ground. Tested Robins were taken from the wild at the very end of the day, practically after finishing daytime foraging, and put into individual cages that allowed then to watch sunset, the stars, to perceive weather factors: air temperature, pressure, humidity. The natural behaviour was strongly limited only in the respect of choosing roosting places and corresponding behaviour. In tests with freshly captured birds we found nocturnal restlessness in a small fraction - 18.1% of bird in autumn and 17.3% in spring. Leaving possible handling stress on one side, the lack of nocturnal restlessness could have been caused by low endogenous motivation for nocturnal flight. The reason for this could be that many Robins were captured after completing migratory flights with low fuel stores. Especially numerous were such birds among those tested in autumn. Motivation to proceed with

5 migration is known to be positively related to fat stores (Dolnik 1975, Yong & Moore 1993, Berthold 1996). This was found also in our experiments with freshly trapped Robins: the proportion of active birds was 2.3-8.3 times higher among fat Robins (fat scores 4 and 5) than in their lean conspecifics (fat scores 0 and 1) (Tab. 1). The amount of stored fuel reserves had a pronounced influence on the decision to take-off on migratory flights in Robins during release experiments when studying orientation of free-flying birds (Sandberg et al. 1991). Both in autumn and in spring Robins captured during waves of migration, i.e. with high motivation for further flights (Titov & Chernetsov 1999), displayed nocturnal restlessness much more frequently than birds trapped during pauses of migration. Apart from endogenous factors, the decision to initiate migration can be influenced in passerine nocturnal migrants by the access to astronomical orientation cues at sunset and at nightfall (Moore 1987). In our experiments with freshly trapped Robins in spring more birds were night-active under overcast conditions in the evening, compared with limited or full access to celestial orientation cues. In autumn, on the contrary, the proportion of night-active birds was higher when skies were clear or with limited cloud cover in the evening, than when overcast (Tab. 1). In general, a low fraction of active birds in both seasons under varying cloud cover shows that nocturnal restlessness in tested Robins was governed by other factors with higher priority. This could have been caused by the fact that in spring birds tend to migrate within a wann sector of a cyclone, not infrequently near the frontal divide. In such cases mass migration inevitably occurs under skies that are considerably overcast. In autumn, peak migration tends to occur in the western part of a low, the eastern or central part of a high. Such areas generally have clearing or clear skies (Richardson 1978, Alerstam 1990). Our visual observations suggest that, between sunset and sunrise, Robins, though obviously preferring to migrate under clear skies or limited cloud cover, make their departure under overcast skies during the wave of passage, when the motivation to fly is high (Bolshakov & Bulyuk 1999). Our experiments showed that freshly captured Robins started to be active between 90 and 510 min after sunset in autumn and between 90 and 330 min after sunset in spring (Fig. la, b). Due to methodological reasons we did not record activity until 90 min after sunset, so at least in some birds the onset of nocturnal activity could have started before the middle of the second hour after sunset. In the wild, as suggested by visual observations of departures between sunset and sunrise, Robins in spring may depart as early as 29 min after sunset (Bolshakov & Bulyuk 1999), and in autumn 56 min after sunset (Bolshakov & Bulyuk in press). In some cases nocturnal spring migration in this species may begin as early as 6 min before sunset (Bolshakov & Rezvyi 1998). A comparison of the timing of beginning activity in freshly trapped birds in cages and the timing of nocturnal departures of the Robins in the wild during spring migration (Bolshakov & Bulyuk 1999) show their close coincidence. During the same season (the second half of April and the first half of May) observations showed that the overall duration of the departure period was from the first to the seventh hour. In our experiments the birds also started nocturnal activity until the seventh hour of the night (Fig. 1b). The median times of departures and of beginning nocturnal activity were 142 and 150 min, respectively. The proportion of Robins that took-off and started restlessness after the end of astronomical twilight (Sun elevation lower than 18°) were 23.3% and 26%, respectively. The results of both experimental and field studies suggest that in spring birds with high motivation to proceed with migration (during waves of passage) tend to depart and, respectively, start nocturnal restlessness earlier in respect to sunset, than birds with lower motivation to fly during the migratory pauses. The comparison of our data concerning the timing of onset of nocturnal activity in freshly trapped Robins and field data on departures during autumn migration (Bolshakov & Bulyuk in press) shows that: 1. The duration of the period of beginning nocturnal restlessness in caged Robins and take-off activity in free-living conspecifics are nearly identical. In the cages birds started restlessness between the second and the ninth hour after sunset. In the wild in autumn Robins departed between the first and the tenth hour after sunset. 2. In caged birds nocturnal activity started on average 1.5 hours earlier, than the take-off activity in wild Robins (medians 180 and 283 min after sunset, respectively). 3. Differences in the timing of the onset of nocturnal activity between caged birds and departures of wild conspecifics were due to a high proportion of birds commencing restlessness during first two

6 hours after sunset in the cages. In the experiment 25% of birds started restlessness in the second hour after sunset (Fig. la), whereas in the wild in the first and the second hour we recorded only 4.6% of all departing Robins (Bolshakov & Bulyuk in press). 4. Compared with caged birds, free-living Robins tended to depart later in respect to sunset when take-off activity was low, only in the first half of autumn migratory season. Interestingly, such a timing shift during low take-off activity was also recorded in long-distance migrants and Song Thrushes as the season progressed (Bolshakov & Bulyuk in press). The difference in the proportion of active birds at the beginning of the night in cages and in the wild can be explained by the peculiarities of nocturnal restlessness in a fraction of caged birds with high migratory motivation. In such birds the inactive phase before complete darkness, which is typical for the natural situation, was not infrequently lacking. As birds were active for one-two hours after sunset, they were considered to start nocturnal restlessness in the second hour. In the wild, departure activity in such birds apparently occurs later. Throughout the autumn season, birds with higher fuel stores were prone to start nocturnal activity earlier (on average 54-67 min). In spring such a trend was not recorded (Tab. 1). Relationships between the timing of beginning migration and fat stores differ in spring and in autumn. Besides, during the second part of the spring migratory season, migration takes place during relatively short nights, thus differences in fuel stores of birds departing early and late could be less pronounced. An alternative explanation of seasonal difference in the timing of nocturnal activity in fat and lean Robins is possible. In spring, the migration of Robins and many other short-distance migrants occurs under much more adverse and unpredictable weather conditions, than in autumn. During homeward migration birds are known to make migratory hops with considerable fuel stores, to have a safety margin in case of bad conditions at stopover and on the breeding grounds (Berthold 1996). In spring fat stores in short-distance migrants may control the readiness to depart for a migratory flight via endogenous factors, rather than govern the timing of departure. In contrast with the Robin, in freshly captured passerines on long-distance nocturnal migration tested in spring during the shorter nights, fat individuals started nocturnal activity much earlier than leaner ones (Bulyuk & Mukliin in press). Some long-distance migrants, e.g. Reed Warblers, may arrive on the breeding grounds with small fat stores (Chemetsov 1999). One can suggest that the detected difference was due to testing fat birds from populations in transit, and lean birds approaching the goal of their migration. The time of beginning nocturnal activity could be controlled by the distance from the migratory goal (Bulyuk & Mukhin in press). Long-distance migrants breeding in areas with adverse and/or unpredictable weather and feeding conditions in spring are known to arrive in spring with considerable fat stores (Moore & Kerlinger 1991, Sandberg 1996, Fransson & Jakobsson 1998). It is a possibility that these birds, like Robins, would not exhibit fat-related differences at the time of onset of nocturnal restlessness. Birds held in captivity showed a much greater proportion of active individuals, compared with freshly trapped Robins. Further, the time of beginning activity in this group was shifted towards an earlier period in respect to sunset. The difference was most pronounced in spring (on average 53 min). A high proportion of active birds among the Robins held in captivity could be explained by their higher fat scores. The fat deposits of Robins from this group were scored 2 to 5, in autumn on average 2.9, in spring 3.8. In spring, the considerably earlier time of beginning nocturnal activity in Robins held in captivity compared with those freshly trapped, could be caused by the delayed migration in the former. Observations of take-off behaviour of Robins on the southern coast of Gulf of Finland over several years showed, that in spring when migration is arrested by stable headwinds and overall migration speed is low, departure time in some birds was shifted considerably. Some individuals departed even before sunset (Bolshakov & Rezvyi 1998). By earlier departure birds probably compensate for delays by longer nocturnal migratory flights. In young birds such a compensation mechanism is possible only during spring migration when migration is controlled not only by an endogenous time programme, but by certain external factors as well (for review see Berthold 1996). Thus both field and experimental studies show, that in the Robin the time of beginning nocturnal activity in the cages and take-off behaviour in the wild is highly variable and may depend upon many

7 factors. Two points that were already discussed elsewhere (Bolshakov & Bulyuk in press) are in our opinion crucial: Firstly, a considerable variation in the time of beginning take-off activity among individual birds, even during a single night, shows that departure time is governed by individual endogenous programmes. Secondly, variation in the behaviour of the birds taking-off for a nocturnal migratory flight, may be connected with the control over the time of landing. Landing at dawn is optimal. In this case birds avoid collisions with different objects in the darkness. They may choose an optimal stopover site and at the final stages of migration they are able to find their goal avoiding searching in the darkness or in the morning (Bolshakov & Bulyuk 1999, in press). The study of the temporal schedule of flight in nocturnal migrant passerines should include studies of the timing of both departures and arrivals.

Acknowledgements We are grateful to Dr. C.V. Bolshakov for suggesting the experiments and for his helpful criticism of an earlier draft. John Walder and Rab Morton kindly improved the English.

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