Continental efforts: migration speed in spring ... - Wiley Online Library

J. Avian Biol. 39: 13
18, 2008 doi: 10.1111/j.2007.0908-8857.04300.x # 2008 The Authors. J. Compilation # 2008 J. Avian Biol. Received 25 June 2007, accepted 30 September 2007

Continental efforts: migration speed in spring and autumn in an inner-Asian migrant Michael Raess M. Raess, Department of Biological Rhythms and Behaviour, Max Planck Institute for Ornithology, Von-der-Tann-Str. 7, 82346 Andechs, Germany. E-mail: raess@orn.mpg.de

Migration speed in passerines is generally assumed to be higher in spring than in autumn. So far this has been only shown for the western Palaearctic-Afrotropic migration system. I compiled published records of the movements of Siberian stonechats Saxicola torquata maura in Central and northern Asia to reconstruct their spatiotemporal movement patterns in this region and to estimate migration speed in spring and autumn. My estimate of spring migration speed in the Siberian stonechats does not differ from that in autumn and is lower than the reported spring migration speeds in European passerines. Northward progression of Siberian stonechats seems to be constrained by the prevailing environmental conditions, as indicated by low temperatures and vegetation indices. Low food availability at stopover and the obstruction of the migration route by steep environmental gradients may apply also to other migratory species in the area.

Despite our growing knowledge about many aspects of avian migration, identifying and understanding the basic movement patterns of bird populations remains a major challenge. Keeping track of the recent changes in migratory phenology which come about with the current climatic changes (e.g. Thorup et al. 2007) makes knowledge about the current status-quo of migration patterns a pressing issue. Unfortunately, however, most studies on bird movements are based in Europe and North America. As a result, the western Palaearctic-Afrotropic and Nearctic-Neotropic migration systems are much better studied than other migration systems. This geographical bias is exemplified in the widespread assumption that migration speeds in passerines are higher in spring than in autumn. This idea is supported by two studies on species that migrate within Europe, or between Europe and Africa (Fransson 1995, Yohannes 2004). The explanations of these results
selection for early arrival in spring, due to a negative relationship between reproductive timing and reproductive success (Kokko 1999), and/ or an effect of the different day-lengths experienced during spring and autumn migration (Bauchinger and Klaassen 2005)
should be valid also in other geographical regions. Empirical support from other migration systems is however missing, because for many parts of the world detailed data on avian movement patterns is not available. The stonechat Saxicola torquata is a small insectivorous passerine that inhabits open landscapes in an area stretching from the eastern Palaearctic to the southern Afrotropis

(Urquhart 2002, Collar 2005). For the European stonechat populations there are plenty of ringing recoveries, allowing detailed analyses of their movement patterns (van Hecke 1965, Zink 1973, Helm et al. 2006). For other migratory populations, such as the stonechats of the central and eastern Palaearctic, no comparable data exists. Therefore, I assembled available information on the spatiotemporal patterns of the migrations of Siberian stonechats S. t. maura within northern and Central Asia. Siberian stonechats breed in the Asian part of Russia, western and central Siberia, and parts of Central Asia, Mongolia and China. The wintering areas are located in southern Iran, Afghanistan, Pakistan and northern India. An unknown proportion of birds overwinters in the Middle East, parts of the Arabian Peninsula and north-east Africa (Urquhart 2002). Because ringing recoveries do not exist, I had to rely on published records of stonechat movements (sources listed in materials and methods). With this data I try to create a comprehensive picture of the spatiotemporal migration patterns of Siberian stonechats within Central and northwestern Asia. To get a better picture of the conditions prevailing during the migratory periods, I complement the spatiotemperal data with information on local climatic parameters (temperature, vegetation indeces). The spatiotemporal movement patterns are used to estimate migration speed of Siberian stonechats in the two migratory seasons. The results are compared with the data on European passerines and discussed in the light of the climatic conditions in Central and northern Asia.

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Materials and methods This paper is only concerned with the western Siberian stonechat Saxicola torquata maura or Saxicola maura maura, and it will be referred to as Siberian stonechat throughout the text. The eastern Siberian stonechat S. t./ S. m. steijnegeri is not treated (see Urquhart 2002, Irwin and Irwin 2005). I conducted a literature survey to build up a data base on migratory movements of Siberian stonechats within Central Asia and western Siberia (approximately from 408 N to 668 N and from 508 E to 908 E). The treated area includes Kazakhstan and parts of Russia, Mongolia, China, Kyrgyztan, Tadzhikistan, Uzbekistan, and Turkmenistan as well as the Caspian Sea, Lake Aral, the Ural Mountains, Altai Mountains and Tian Shan Mountains (cf. Fig. 1A). Information on stonechat migrations was compiled from English and Russian handbooks on Palearctic birds (Shnitnikov 1949, Dementiev and Gladkov 1954, Kuzmina 1970, Cramp 1988, Glutz von Blotzheim and Bauer 1988, Rogacheva 1992, Collar 2005), a monograph on stonechats (Urquhart 2002), and original publications in Russian language (Toropova and Eremchenko 1979, Chernyshov

1982, Egorov 1985, Erokhov and Gavrilov 1993, Karyakin et al. 1999, Tsybulin 1999, Stakheev 2000). Altogether, I was able to obtain information on Siberian stonechat migratory movements in 42 different sites in Central Asia and western Siberia. All observation sites are located north of the wintering range of the Siberian stonechat. Observations range from the year 1897 to the year 2000. Not all sources state the exact year for each observation. In these cases, I assumed that observations were made 10 years before publication (review articles) or 5 years before publication (original articles). This led to 12 records from 1897 to 1925, 12 records from 1926 to 1950, 54 records from 1951 to 1975, and 30 records from 1976 to 2000. I used only records that could be attributed to one of the following categories: (1) first birds (to be seen in the area), n (spring/autumn) 32/14, (2) main passage (often also referred to as peak migration, bulk of migrants passing, etc.), n 18/19, and (3) last birds (last observations of stonechats in the area), n 10/15. Thirty-eight out of the 42 observation sites are located within the breeding range of the Siberian stonechat. Therefore, in the ‘first birds’ and ‘last birds’ local breeders and birds on migration cannot be

Fig. 1. Spring movements of Siberian stonechat within Central and northern Asia. The left panels A-C show the spatiotemporal migration patterns within the area. The geographic features shown in these panels are the borders of Kazakhstan and (cf. numbers in panel A) the Ural Mountains (1), the rivers Irtysh (2), Ob (3), and Yenisey (4); the Caspian Sea (5), the Lake Aral (6), the Tian Shan Mountains (7), and the Altai Mountains (8). The right panels D
F show the migration distances plotted against the observation date. The regression lines (straight lines) that were used to estimate migration speed are indicated. The hatched line in panel F shows the regression line when one ‘outlier’ is excluded. A/D: first birds, B/E: main passage, and C/F: last birds.

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distinguished. The main passage group, however, contains only records of birds on passage (except for the northernmost obersvation sites, where the records indicate the peak arrival or the peak departure of local breeders). The accuracy of the temporal information differs between sources. Whereas some authors give exact dates and sometimes even mean dates over several years, others report only rough estimates such as ‘‘in the beginning of May’’. To account for these differences, all observations were assigned to one of three 10-day periods (or decades) of each month (1st decade 1
10 of a month, 2nd decade 11
20, and 3rd decade 21 to end). For each observation site climate data was obtained from the METART website of the Food and Agriculture Organisation of the United Nations (FAO): http://metart. fao.org/. The original monthly temperature data (monthly temperatures averaged from 1961 to 1990) were transformed into the decadal format of the observation data. This was done by assigning the original monthly mean to the 2nd decade of a month; the value of the 1st decade was obtained by weighting 2/3 of the given and 1/3 of the previous month’s temperature values; and the values of the 3rd decade were obtained by weighting 2/3 of the given and 1/3 of the next month’s temperature values. The METART website also provided decadal vegetation index data from the VEGETATION instrument of the SPOT-4 polar orbiting satellite with a resolution of about 1 km2 per pixel. For each observation site the vegetation indices (mean data from 1998 to 2006) in an encompassing area of approximately 50 km2 (77 pixel) were analysed. Using the colour code provided with the data an average vegetation index ranging from 1 to 12 (1 snow cover, 2
3 bare soil, 4
5 sparse vegetation, 6
7 light vegetation, 8
9 medium vegetation, 10
12 heavy vegetation) was calculated. Isolines for temperature and vegetation indices were created by fitting quadratic polymial functions to the data averaged for each 500 km interval of migration distance. The spatiotemporal movement patterns of Siberian stonechats are represented as plots of observation data on twodimensional grids with distance (in kilometers) from Tashkent in Uzbekistan (41.38 N, 69.28 E) as a scale. Tashkent was chosen as reference point because: (1) it is the southernmost point in the data set, (2) data for both migratory seasons and all the migratory phases is available, and (3) it is the first area where stonechats are observed in spring and the last area where stonechats are observed in autumn. The spatiotemporal patterns can be used to calculate a ‘population level’ speed of migration, comparable to calculations of migration speed based on passage data at ringing stations along a migratory route (Ellegren 1990, Yohannes 2004). As an estimate of migration speed I used the slope of the linear regression of migration distance from Tashkent against the observation decades. For all observations west of Tashkent I assumed that the birds fly straight north or south as their flight path is not obstructed by high mountain ranges. Therefore migration distance was calculated as the latitudinal distance from Tashkent. For all observations east of Tashkent I assumed that the birds fly a detour to avoid the Central Asian highlands east of Tashkent (Dolnik 1990, Bolshakov 2002, 2003, Irwin and Irwin 2005). Therefore migration distance was calculated as great-circle distance (Gudmundsson and Alerstam 1998)

from Tashkent. I did the calculations for each migratory phase separately. Thus, I assumed that first birds, main passage birds and last birds moved in distinct migratory waves. While this may not be true for each single individual
an early arrival in the south in spring may end up in the main passage group further north if it is migrating slowly
it may still apply to the majority of birds.

Results The spatiotemporal patterns of the stonechat migrations in Central and northern Asia are summarised in Fig. 1 and 2. In spring the first Siberian stonechats are observed in the area of Tashkent, which is situated in the plains to the west of the Tian Shan Mountains (Fig. 1A). In late April to early May the first birds have reached northern Kazakhstan and southern Russia. At the northernmost breeding sites some 2,300 km north of Tashkent the first stonechats are not observed before mid-May (Fig. 1A). Main passage follows a similar pattern, but is shifted by about 2
3 weeks (2 decades; Fig. 1B). The last birds pass southern Kazakhstan in early May, northern Kazakhstan in late May, and they reach southern Russia in early June (Fig. 1C). In autumn, the first birds set out on their southward migration in late July and early August. Migratory activity starts at about the same time in many different areas. Therefore, the first migrating birds can be seen at the same time in Tashkent, as in a location 2,000 km further north (Fig. 2A). An increase in bird numbers due to a peak in passage migration is observed in the northern parts of the treated area in late August and in the southern parts all over September (Fig. 2B). The last stonechats are observed in southern Russia and northern Kazakhstan in September. From mid-September until mid-October the last birds pass through southern Kazakhstan, and in late October the last birds leave Tashkent (Fig. 2C). My estimates of migration speed of Siberian stonechats in spring and autumn are summarised in Table 1. Observation date explains 53% to 77% (or even 80% without the outlier) of the variation in migration distance in the different migratory phases in spring. This suggests that the spring movements are rather coordinated within the population (Table 1, Fig. 1D
F). In autumn, migratory movements are less synchronous than in spring. In the first birds only 3% of the variation in the relative observation distances is explained by the observation date (Table 1, Fig. 2D). Accordingly, I could not obtain an estimate of migration speed in this migratory phase. In the main passage group and in the last birds observation date explaines 22% and 59% of the variation in migration distance (Table 1, Fig. 2D
F). Migration speed does not differ between the migratory phases within a season and between spring and autumn, even when the outlier is excluded from the analysis (GLM with migration distance as dependent variable, migratory phase as fixed factor, observation date as covariate; autumn first birds not included in the model: migratory phase observation date: df 4, F 1.802, P 0.371). In spring, first birds migrate at temperatures of 5.59 4.98 C (mean9SD), and vegetation indices (VI) of 4.091.1. Main passage occurs at of 8.594.08 C and

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Fig. 2. Autumn movements of Siberian stonechat within Central and northern Asia. The left panels A
C show the spatiotemporal migration patterns within the area. The geographic features shown in these panels are explained in Fig. 1. The right panels D
F show the migration distances plotted against the observation date. The regression lines that were used to estimate migration speed are indicated. A/D: first birds, B/E: main passage, and C/F: last birds.

5.591.9 VI. Last birds migrate at 14.695.78 C and 6.29 1.9 VI. In autumn, first birds migrate at 13.196.98 C and 6.892.1 VI, main passage occurs at of 13.095.48 C and 5.591.9 VI, last birds migrate at 9.193.68 C and 4.791.7 VI (see also Fig. 3).

Discussion The results of this study do not support the widespread assumption that migration speed is generally higher in

spring than in autumn: the speed of Siberian stonechat migrations within Central and northern Asia does not differ between the two migratory seasons (Table 1). This is in contrast to the two existing studies that compare spring and autumn migration speeds in European passerines: Fransson (1995) reported average (9SEM) migration speeds of 138.0915.5 km/d in spring and of 91.696.4 km/d in autumn for five Sylvia species migrating between northern Europe and the Mediterranean area. Yohannes (2004) estimated average (9SEM) migration speeds of 71.09 10.4 km/d in spring and 39.692.6 km/d in autumn for

Table 1. The speed of migration in Siberian stonechats during spring and autumn (means9SE) based on the slope of a linear regression of observation distance relative to Tashkent against observation date. Migratory period Spring

first birds main passage last birds

Autumn

first birds main passage last birds

Speed (km day 1)

df

R2

P

27.694.3 31.994.4 33.8911.3 (56.0910.7 6.9910.8 27.1912.5 36.598.4

31 17 9 8 13 18 14

0.58 0.77 0.53 0.80 0.03 0.22 0.59

B0.001 B0.001 B0.05 B0.05)* 0.54 B0.05 B0.005

* One value which may be considered as an outlier removed (cf. Fig. 1F).

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Fig. 3. Migration patterns of Siberian stonechats in relation to local environmental conditions. Similar coloured areas indicate similar temperature ranges (panel A) or vegetation indices (panel B) in relation to the migration distances of first birds and main passage birds and migration dates (means9SD).

twelve passerine species migrating between Europe and eastern Africa. There are no indications that selection for early breeding, which has been used as an explanation for higher migration speed in spring, is less pronounced in Siberian stonechats than in European passerine birds. As has been reported for many passerines breeding in Europe, reproductive output (measured as the number of fledglings produced per clutch) decreases with calendar date in Siberian stonechats breeding in northern Kazakhstan (Raess et al. unpubl. data). Furthermore, the effects of the photoperiod on migration speed are expected to be similar to those in the European birds (cf. Bauchinger and Klaassen 2005). Therefore, other factors seem to be responsible for the differences between European and Asian birds. I suggest migration speed of Siberian stonechats is constrained by the environmental conditions they encounter during spring. Passerines en route to the Central Palaearctic in spring migrate through the arid zones of Central Asia in order to avoid the snow-covered storm-ridden highlands further east (Dolnik 1990, Bolshakov 2002, 2003). Dolnik (1990) studied energy accumulation at stopover sites in Central Asia in several passerine species. He calculated on the basis of observed refuelling rates that the energy reserves obtained during one day of stopover in spring sustains only about one hour of migratory flight. Birds migrating in this area seem to be energy-constrained rather than time-constrained (Alerstam and Lindstro¨m 1990, Hedenstro¨m and Alerstam 1997). Wikelski et al. (2003) showed that in Catharus thrushes on spring migration in North America energy

expenditure during stopover is highly dependent on ambient temperature. Given the fact that migrating birds spend a much larger amount of time during stopover than during actual migration and spend about two thirds of the total energy during that time (Alerstam and Hedenstro¨m 1998, Wikelski et al. 2003), Siberian stonechats may simply not be able to acquire enough reserves for a faster flight during spring migration. However, the steep environmental gradients shown in Fig. 3 may indicate another constraint: Even if the Siberian stonechats were able to muster enough energy for faster flight, it would not pay off because the environmental conditions they would encounter further north are still too harsh. Low vegetation growth and ambient temperatures affect both abundance and activity of invertebrate prey. Particularly the first birds seem to migrate as early and as fast as the climatic conditions permit (Fig. 3). The birds, forced to wait for an amelioration of spring conditions, could instead allocate resources to other fitness relevant processes, such as the development of the reproductive system (Raess and Gwinner 2005, Bauchinger et al. 2007). Spring migrants moving from northern Africa to Europe experience less steep environmental gradients than inner-Asian migrants: the temperature difference between coastal Africa (Tunisia) and southern Sveden (approx. 2,600 km distance) in April is about 128 C. The temperature difference between Tashkent and Central Siberia (same distance) in April is about 218 C. The higher spring migration speeds found in the European migrants may thus reflect a greater potential for time-selected migratory strategies (Alerstam 2006) due to the less constraining environmental conditions in Europe. Relatively low migration speeds in combination with steep environmental gradients, on the other hand, may not only characterise spring migration in the Siberian stonechat, but rather be a general feature of the migrations within Central and northern Asia.

Acknowledgements
I would like to thank Barbara Helm and Mikhail Banik who brought most of the Russian publications on stonechats to my attention. The comments of Barbara Helm and two anonymous reviewers helped to improve earlier drafts of the manuscript considerably. The English excerpts and translations of the Russian sources by Evgeny Shergalin were of great help. Elizabeth Yohannes shared her insights on migration speed with me. This manuscript is dedicated to Ebo Gwinner.

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