Fat level and temporal pattern of diurnal movements of Robins (Erithacus rubecula) at an autumn stopover site

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1 Avion Ecol. Behav. 2,1999: 89-99: Fat level and temporal pattern of diurnal movements of Robins (Erithacus rubecula) at an autumn stopover site Nikolay Titov Abstract: Titov, N. (1999): Fat level and temporal pattern of diurnal movements of Robins (Erithacus rubecula) at an autumn stopover site. Avian Ecol. Behav. 2: I compared level and temporal pattern of diurnal movements of Robins with different fat scores at a stopover site. Fat birds were less mobile than lean ones. Lean birds were active in the morning and before noon, whereas the activity pattern of fat Robins had two peaks at sunrise and at sunset. It is suggested that the evening activity peak refers to pre-take-off movements of birds ready for migratory flight. Key words: Robin, migration, behaviour, fat. Address: Biological Station Rybachy, Rybachy, Kaliningrad Region, Russia. titov@bioryb.koenig.su 1. Introduction The analysis of trapped nocturnal passerine migrants at stopovers is an important tool for the study of migratory behaviour (Dolnik 1975, Brensing 1989, Berthold 1993, 1996). The main aim of daytime stopovers is thought to be to accumulate fuel for migration and probably to solve orientation problems. For efficient fuelling birds not infrequently need to locate optimal habitats which costs time. Studies of caged nocturnal migrants showed that overall activity of fat birds is lower than observed in lean conspecifics (Dolnik 1975, Brensing 1989, Berthold 1993, 1996). Fat trans-sahara migrants not only interrupt flight at daytime, but are also practically motionless during stopovers (Biebach 1990). Such behaviour may also apply in the less extreme environment of the temperate zone. For a number of nocturnal migrants the records of trapped birds form two distinct peaks, in the morning before sunrise and in the evening before sunset. The latter peak is more typical for retrapped birds, i.e. for those staying at stopover sites for longer periods. This suggests that the evening activity is reduced with developing migratory state (Brensing 1989, Berthold 1993, 1996). According to experimental studies of caged migrants the evening peak is present at all stages of refuelling with a shift towards dark period in fatter birds. In free-living birds diurnal activity is divided from the nocturnal one by a gap of an hour or more (Dolnik 1974,1975). Apart from endogenous causes, the pattern of diurnal activity may also be influenced by distance between roosting and feeding sites, and by the need to orientate. In spring Robins try to move to open habitats or to tops of trees and bushes before sunset. Birds fly off (false take-offs) and return to the ground (Bolshakov & Rezvyi 1982). This behaviour also influences the trapping pattern and deserves special attention. The present paper aims to analyse the possible importance of endogenous causes (fat level, annual clock) and environmental stimuli (habitat quality) for the pattern of diurnal activity during autumn stopovers in a typical nocturnal migrant, Robin. Robins are believed to fly over land exclusively at night (Bolshakov & Rezvyi 1998), ground-level movements in vegetation at stopover sites refer to behaviour other than migratory flight. If so, this species should be very suitable for the study of different aspects of stopover ecology, also due to its high numbers and high capture probability in stationary mist nets. It is also possible to compare activity patterns in free-living and in caged birds.

2 2. Study site, methods and material The study is based on mist-netting and ringing data from Rybachy, on the Courish Spit of the Baltic Sea (Kaliningrad Region of Russia) obtained in a joint trapping project of Biological Station Rybachy and Vogelwarte Radolfzell, Germany. The trapping site is a partly isolated patch of vegetation, mainly reed (Phragmites australis), willow (Salix spp.) scrub, and trees (Sambucus nigra, Ainus sp. Betula sp., etc.). Its overall area is ca. 0.6 ha (Fig. 1). Birds were trapped in mist-nets following the guidelines of ESF programme (Bairlein et al. 1995) in In this study data collected during the autumn passage of Robins between August 28 and November 6 is considered. In some years Robins migrate through the area later than this date. Net rounds were started before sunrise and finished after darkness. During daylight nets were checked at least once an hour. Out of 73 standard 6 m long mist-nets, 31 were placed in open habitats (reed with willow admixed), and 42 in closed habitats (scrub and trees). Figure 1. Map of the study site. In this paper, time before sunrise and one hour after sunrise was defined as morning, with one hour before sunset until after dark as evening. All the rest was defined as daytime. I did not especially analyse the activity pattern of birds that left the study site on the day of arrival, either by migratory flight or by diurnal movements. The emphasis is on the trapping pattern of birds with minimum stopover length (MSL) of more than 1 day (but see Fig. 6). 3. Results 3.1. Fat level related differences in trapping patterns A total of 1283 Robins had MSL of two days or more. Assuming all of these birds were present at the stopover site the day after the first catch, the proportion of birds trapped on this day may be a measure of diurnal activity (low altitude movements in vegetation) at migratory stopovers. Significant differences in the activity level were recorded between lean and fat birds (Fig. 2). Over 40% of lean birds with MSL of 2 days were retrapped on the next day after arrival, compared with less than 10% of fat birds (fat score 3 or more after Kaiser [1993]), t = 7,9; p < 0,05. Robins with fat score 1 or 2 were intermediate (Fig. 2).

3 Figure 2. Proportion of birds with different fat score (MSL 2 days) retrapped on the next day after arrival. Vertical bars represent 95% confidence interval Patterns of diurnal activity in birds with different fat scores I divided the migratory period of the Robin into three parts, early, mid and late migration (Fig. 3). In all three periods lean birds were most active before noon. No evening peak was recorded in lean individuals, activity was sharply declining during the afternoon. Activity patterns of fat birds were more uniform, during early migration two distinct peaks were recorded, in the morning and in the evening. During late migration no distinct evening peak occurred (Fig. 3 С). During mid migration an intermediate pattern was recorded with slight peaks at sunrise and sunset. Trapping patterns of lean and fat birds over the whole migratory period are significantly different (Tab. 1). Activity patterns of Robins with fat scores 1 and 2 were close to those of fat birds (Fig. 3, Tab. 1). During the whole migratory period Robins were trapped mainly between sunrise and sunset. With light period getting shorter, beginning and end of activity shifted towards later (respectively earlier) times (Fig. 3). Table 1. Trapping pattern of Robins with different fat score. Fat score Proportion of traps morning midday evening Number of captures >2 0, t-test > Activity pattern of Robins on the day of arrival and on subsequent days Activity patterns of lean migrants appeared to be nearly identical on the day of arrival and on subsequent days (Fig. 4 A, Tab. 2). It is however noteworthy that on the day of arrival a small evening activity peak was recorded, which completely ceases on subsequent days. In fat and intermediate birds activity pattern was bimodal with maxima during sunrise and sunset periods. In fat birds sunset maximum was more distinct than in birds with intermediate fat deposits.

4 Table 2. Trapping pattern of Robins with similar fat scores in relation to the day of stopover. Fat score Proportion of traps morning midday evening Number of captures 0 first day subsequent days t-test >0,1 >0.1 > first day subsequent days t-test < < > >2 first day subsequent days t-test > Figure 3. Activity patterns of Robins with different fat score during early (A, 28/08-21/09), mid (В, 22/09-15/10) and late migration (C, 16/10-6/11). X axis represents local time. The lines connect points of sunrise and sunset for earliest and latest calendar dates. Y axis represents the proportion of catches in particular hour. For results of statistical treatment s. Tab. 1.

5 After the arrival day activity patterns of both groups were more uniform, though fat birds still showed the trend of bimodal pattern of low-altitude movements (Fig. 4, Tab. 2). On the arrival day the evening peak of activity was more distinct in fat Robins than in those with intermediate fat reserves. On the subsequent days no significant difference was found between these groups (Tab. 3). Table 3. Trapping pattern of Robins in relation to their fat score. Fat score Proportion of traps Number of captures morning midday evening 1-2 first day >2 first day t-test >0.1 > subsequent days >2 subsequent days t-test >0.l >0.l > Figure 4. Activity patterns on the day of first catch and on subsequent days between September 18 and October 2. Arrows show sunrise and sunset on respective day. Dashed lines show activity patterns on the day of the first catch, solid lines - of retraps on subsequent days. A - fat score 0; В - fat score 1 or 2, С - fat score 2 3). Further symbols as in Fig. 3. For results of statistical treatments. Tab. 2-3.

6 3.4. Activity patterns in open and closed habitats Activity pattern in lean Robins did not differ between open and closed habitats neither on the day of arrival nor later during the stopover (Fig. 5). Lean birds were most active at sunrise and before noon, with low activity in the evening. In closed habitats the morning activity was more prolonged comprising ca. 6 hours (Fig. 5 A). This suggests that lean Robins quickly leave reedbeds but actively move in scrub before noon. Activity pattern of moderate fat Robins is also similar across habitats but is clearly distinct from that of lean birds. These birds are trapped uniformly across the whole day in both stopover habitats. In open habitats a tendency of higher activity of moderate fat Robins during sunset is recorded (Fig. 5 В). Activity patterns of such birds probably refer only to foraging movements, probably within already defined home ranges (Titov, in prep.). It is also possible that a proportion of moderate fat Robins move to open habitats at sunrise and especially at sunset. Figure 5. Activity pattern of birds with different fat score between September 18 and October 2. Dashed lines show activity patterns in open habitats, mainly reed, solid line closed habitats (scrub and trees). Further symbols as in Fig. 4.

7 In fat Robins considerable differences were found between the habitats. Activity pattern in scrub is similar to that of the previous group. However, in open habitats the trapping pattern has two distinct maxima at sunrise and at sunset with very low activity during the daytime. A considerable fraction of fat Robins probably move to open habitats in the morning and in the evening (Fig. 5 С). Birds trapped in closed habitats are also more active at that time. Of fat Robins trapped in reedbed 26% are trapped in the morning and 33% in the evening. Of fat robins trapped in scrub 15% are in the morning and 12% in the evening (Tab. 4). Table 4. Trapping pattern of fat Robins (fat score >2) in relation to habitat. Habitat Proportion of traps morning midday evening reed scrub t-test > Number of captures 4. Discussion Our data show that among Robins making stopovers on the Courish Spit for two and more days, the level of movements is nearly 4-fold lower in fat birds than in lean individuals. Low daytime activity in fat nocturnal migrants has been demonstrated in cages (Dolnik 1975, Brensing 1989, Berthold 1993, 1996) and in the wild during trans-saharan migration (Biebach 1990). In fat birds motivation to forage is probably low, the dominant need is to preserve the available fuel for the next migratory flight. However, our data on retrapped Robins only partly agree with the suggestion that the evening activity peak is reduced in birds with developed migratory state (Brensing 1989). In autumn on the Courish Spit a distinctive evening activity peak in fat Robins during early and mid-migration season, including the main migratory period between September 18 and October 2 is recorded. Only during the last part of the migratory season when days are short (October 16 to November 6) is the evening peak lacking in fat birds. Lean birds were not active in evening during the whole autumn migratory season. Apart from a difference in the period of migration between the Courish Spit and SW Germany (and the difference in the length of day) the observed difference could be due to methodology. Brensing (1989) compared first trapped birds and retraps. First traps showed an uni-modal distribution with a morning peak, retraps were distributed bimodally with morning and evening peaks. Moreover, the author suggested that as the proportion of fat birds was lower among retrapped individuals, that these trapped birds are yielding information on the behaviour of birds not ready for migratory flights. Our data presented in a similar manner (Fig. 6) are in better accordance with Brensing's (1989) results, but no peaks are evident in the distribution of activity of retraps. Two facts are important to explain the difference between the two studies. Firstly, the morning peak of first traps is without doubt exaggerated. The bulk of Robins finishing the migratory flight are trapped in the morning, and a proportion of them leave the landing site within a few hours (Titov & Chernetsov 1999). Secondly, although fat Robins are a small proportion of those caught at stopover sites, these are the birds that form the evening peak of retraps, as evening activity of lean birds is very low (Fig. 3-5). The studies of caged birds (Dolnik 1975, Brensing 1989, Berthold 1993, 1996) has shown that the main locomotor activity of nocturnal migrants with developed migratory intention occurs during the night. This activity (Zugunruhe) which refers to nocturnal migratory flights (Berthold 1975, 1988, Dolnik 1975) is probabaly not related to the evening activity peak of fat birds recorded in this study. Movements of fat birds towards open habitats in the evening is probably a special form of migratory behaviour related to choosing the take-off site. In cages this activity may be poorly discerned due to the high level of nocturnal activity or may be completely suppressed or masked. It is noteworthy that on the day of first capture, which is usually the day of arrival (Titov & Chernetsov 1999), the evening activity peak in fat Robins is more pronounced than on subsequent

8 days. This holds true also for medium fat birds. A small activity peak during sunset is recorded even in lean birds. It could refer to a certain inertia of migratory rhythm on the first day after their nocturnal flight. The suppression of evening activity in fat Robins on the subsequent days of stopover requires special attention. Figure 6. Trapping pattern of first catches (solid line) and retraps (dashed line) between September 18 and October 2. Further symbols as in Fig. 4. References Bairlein, P., Jenni, L., Kaiser, A., Karlsson, L, Noordwijk, A., Peach, W., Pilastro, A., Spina, F. & G. Walinder European-African Songbird Migration Network: Manual of field methods. ESF. Wilhelmshaven. Berthold, P Migration: Control and Metabolic Physiology. In: Farner, D.S. & J.R. King (eds.) Avian Biology 5: Berthold, P Unruhe-Aktivitat bei Vogein: eine Ubersicht. Vogelwarte 34: Berthold, P Bird migration: a general survey. Oxford Univ. Press, Oxford. Berthold, P Control of Bird Migration. Chapman and Hall, London. Biebach, H Strategies of trans-sahara migrants. In: Gwinner, E. (ed.) Bird Migration: physiology and ecophysiology. Springer, Berlin - Heidelberg - New York: Bolshakov, C.V. & S.P. Rezvyi An analysis of pre-start and start activity of the Red-breast Robin (Erithacus rubecula L.) during night migration. In: Gavrilov, V.M. & R.L. Potapov (eds.) Ornithological studies in the USSR, 2: Moscow. Bolshakov, C.V. & S.P. Rezvyi Time of nocturnal flight initiation (take-off activity) in the European Robin (Erithacus rubecula) during spring migration: visual observations between sunset and darkness. Avian Ecol. Behav. 1: Brensing, D Ökologische Untersuchungen der Tagesperiodik von Kleinvogeln. Ökol. Vogel (Ecol. Birds) 11: Dolnik, V.R Daily rhythms of feeding and locomotor activity in migratory birds. Proc. Zool. Inst. 55: 3-13 (in Russian). Dolnik, V.R Migratory disposition in birds. Nauka Press, Moscow (in Russian). Kaiser, A A new multi-category classification of subcutaneous fat deposits of songbirds. J. Field Ornithol. 64: Titov N.V., Chernetsov N.S Stochastic models as a new method for estimating length of migratory stopovers in birds. Uspekhi sovremennoi biologii 119: (in Russian with English summary).