Ardeola 48(1), 2001, 1-10

NOCTURNAL AUTUMN MIGRATION OF WATERBIRDS ( AND CHARADRIIFORMES) IN NORTH-EASTERN BULGARIA

Pavel ZEHTINDJIEV*

SUMMARY.—Autumn nocturnal migration of waterbirds (Anseriformes and Charadriiformes) in North- Eastern Bulgaria. Results of a moon-watch study on flight direction and density of nocturnal migration of wa- ders and waterfowl during 23 nights in autumn in NE Bulgaria are presented. Specific composition of possi- ble nocturnal migrants is analysed. The seasonal changes in the mean flight direction of nocturnal migration were caused by differential passage of species with different wintering areas. The average density of noctur- nal migration of waterbirds for the period of observations is presented. The moon-watch observations in au- tumn in the central part of the Lower Danube Plain revealed that nocturnal passage of waterfowl and goes along the general migration direction to the winter quarters, and not along the Danube river. Key words: Autumn, Balkan Peninsula, flight direction, migration density, migration traffic rate, moon- watch method, NE Bulgaria, seasonal changes.

RESUMEN.—Migración nocturna otoñal de aves acuáticas (Anseriformes y Charadriiformes) en el Noreste de Bulgaria. Se presentan los resultados de un estudio basado en observación sobre el disco lunar sobre la di- rección del vuelo y la densidad de la migración nocturna de patos y limícolas durante veintitrés noches de oto- ño en el NE de Bulgaria. Además de la posible composición específica de los migrantes nocturnos, se anali- zan los cambios estacionales en la dirección de vuelo media de la migración nocturna, que fueron probablemente debidos al paso diferencial de especies con diferentes áreas de invernada. Las observaciones sobre el disco lunar en otoño en la parte central de la llanura del bajo Danubio revelaron que el paso noctur- no de patos y limícolas va en la dirección general de la migración hacia los cuarteles de invierno, no siendo desviada por el río Danubio. Palabras clave: Cambios estacionales, densidad de migración, dirección de vuelo, NE de Bulgaria, ob- servación sobre el disco lunar, otoño, península de los Balcanes, tasa de movimiento migratorio.

INTRODUCTION diterranean are based on the data from the wes- tern part of the region (Bruderer & Liechti, The modern migration theory suggests that 1999), although probably many more are most birds migrate mainly at night (Bruderer, taking the eastern route to Africa. 1997; Fortin et al., 1999). So any research on Ringing recoveries suggest that an impor- migration that is claiming to be complete tant number of and most of the waders should concern not only the diurnal move- leave Europe to winter in Africa (Curry-Lin- ments, but also the nocturnal phase of the pro- dahl, 1975; Cramp & Simmons, 1983). There is cess. This paper is based on moon-watch ob- also a description of the networks of key sites servations carried out in NE Bulgaria during used by the in the African/Eurasian the study of nocturnal passage along the East- region (Scott & Rose, 1996). European migratory flyway (Bolshakov et al., The direct moon-watch observations were 1998). Nocturnal migration of waterbirds has conducted in order to answer the following not been discussed in the publications concer- questions: 1) Do waders and ducks fly on a broad ning the Mediterranean region (Bourne, 1960; front or along the rivers? 2) What is the migra- Bateson & Nisbet, 1961; Nisbet et al., 1961; tion traffic rate of waterbirds during nights in Adams, 1962; Casement, 1966). All the theo- autumn? 3) Is the main direction of migrating ries on sequential vector navigation in the Me- waterbirds constant during the autumn?

* Institute of Zoology. Bulgarian Academy of Sciences. Blvd. Tsar Osvoboditel 1. Sofia 1000, Bulgaria. 2 ZEHTINDJIEV, P.

STUDY SITE, MATERIAL AND METHODS pect to the size of crater Plato, D is the diame- ter of crater Plato (90 km) and α° is the height Nocturnal migration was observed by the te- of moon above the horizon. Some errors might lescope-moon-watch method during the autumn occur due to different sizes of birds and due to of 1995 at a point located in the central part of some scatter in estimating the proportion in re- the Lower Danube plain (44°00″N; 26°26″E; lation to the crater. Mean size for the ducks 60 m a.s.l) near the town of Tutrakan, NE Bul- and waders for every observation period was garia (Fig. 1). Observations were conducted at calculated according to species composition in the Biological Station «Kalimok» of the Bul- the study region. garian Academy of Sciences. The region was Observations were carried out with a 35x te- chosen in order to avoid the effects of the sea- lescope, for 30 min in each hour of darkness coast and mountains on migrating birds. A mo- starting 30 min after sunset. Only observations dified version of the moon-watch method was conducted when the moon had risen over 20° applied (Bolshakov, 1981; 1985). The flight and over 50% of the face of the moon was visi- heights of registered birds were calculated ble were used for quantitative estimates. using Wolf’s formula (Wolf, 1967): Only birds identified as Anseriformes or Charadriiformes were included in the analysis. H = E*L*sin α°/(F*D), For each flight direction, approximate alti- tude compared to the ground level up to 3000 m where E is the distance to the moon (384000 where the identification of birds if possible km), L is the size of the bird (in km), F is the (Liechti et al., 1995), and individual migration proportion of the size of silhouettes with res- traffic rate (MTR) were computed. Overall

FIG. 1.—Map of the Balkan Peninsula (inset) and the Lower Danube Plain. [Mapa de la península de los Balcanes y de la llanura del bajo Danubio.] NOCTURNAL AUTUMN MIGRATION OF WATERBIRDS IN NORTH-EASTERN BULGARIA 3

MTR was obtained by adding individual MTRs tumns of 1994-1995. The itinerary of 6 km in- (Bolshakov, 1981; 1985). The corrections for cluded 3.8 ha fishponds near the Danube river. the changing size and orientation of the visible The composition of species during au- part of the face of the moon (for every hour of tumn in Bulgaria was obtained from the availa- observation) and the approximate flight altitude ble literature (Dontschev, 1984; Nankinov et (for every bird) greatly improved the MTR es- al., 1998). The speed and direction of winds timates that had been previously available (Lo- were provided by the Institute of Meteorology wery, 1951; Nisbet, 1959; Richardson, 1982). and Hydrology of the Bulgarian Academy of A similar improved moon-watch method, Sciences. which gives the distance between the observer and the bird, was recently used for simultane- ous observations in the Alps and in the low- RESULTS lands of Switzerland (Liechti et al., 1996). Direction of migration was calculated for During 23 clear nights, 153 silhouettes of sectors from 0° to 360° by adding the indivi- birds belonging to from Anseriformes and Cha- dual MTRs in each sector. Distribution of flight radriiformes were registered. directions was computed for each observation session. As a measurement of the concentra- tion of individual directions, the length of the 1. Anseriformes mean vector (r) was used. The mean direction and the length of the mean vector for samples Autumn migration of waterfowl lasts at least of birds were calculated by individual vector three months (August, September and Octo- addition. To test whether the directions of a ber). The registered silhouettes from the Anati- sample differ significantly from randomness, dae family were recognised as ducks (96.7%) the Rayleigh test was used. For the statistical and geese (3.3%). All the geese and 35.5% of treatment, circular statistics were used (Bats- the ducks were flying in flocks; hence more chelet, 1981; Zar, 1984). Moon-watching data than half of the registered ducks (61.2%) were are not suited for usual statistical treatment flying separately. using the number of recorded birds. The distri- bution can be presented only as MTRs in each sector. The problem is that when using MTR, 1.1. Migration traffic rate and altitudinal sample size always reaches ∞. The coefficient distribution C (Bolshakov et al., 1998), which reflects both MTR and the number of observed birds, was MTR was estimated in bird × hour–1 × km–1 used throughout this study: as an average density for the nights with more than three hours of observations. It was found n MTRi that MTR reached its maximum in September N = ∑ , (05-06/09/95) when 114.3 ducks per hour flew i=1 C in a front of 1 km above the observation point. The average MTR in September was also the where the circle is divided in n sectors. MTRi is highest (71.16) compared to those in August the density of migration in each sector and N is and in October (24.37 and 24.18, respectively). the number of recorded birds. Two variables The MTR of ducks increased gradually in the were used as characteristics of the density of middle of August, between 5th and 8th Sep- migration: average MTR (bird × hour–1 × km–1) tember and between 3rd and 5th October (Ta- for all observation periods over the night and ble 1). At the point of observation, nocturnal average MTR (bird × hour–1 × km–1) for the 4- migration of waterfowl occurred from 400 m 5th hours after sunset. To analyse the signifi- up to 3000 m a.g.l. The silhouettes registered cance of the Danube valley for direction of mi- above 3000 m a.g.l may be due to the accu- gration we included birds flying at all altitudes. racy limitations of the moon-watch method The composition of species from the Anati- (Liechti et al., 1995; 1996). Most of the birds dae family was derived from regular counts (82.3%) flew between 1200 m and 3000 m conducted in the study region during the au- a.g.l. (Fig. 2). 4 ZEHTINDJIEV, P. endicular to the flight di- edia.] [Dirección (°)/velocidad )] –1 –1 km km × × –1 Tasa de migración (número aves que atra- –1 h h × × MTR: the whole night of wind (m/s) (bird 1 ABLE T Ducks Waders Ducks Waders 3020 m a.s.l 1460 m a.s.l. [Patos] [Limícolas] [Patos] [Limícolas] [m s.n.m.] [m s.n.m.] [Número de aves avistadas] [Duración de las por noche] (aves [Número de observaciones (min)] [MTR medio para del viento (m/s)] for the night observations observations (min) observaciones toda la noche Date Number of Duration of Number of recorded birds Average MTR for Direction (°)/speed [Fecha] 06/07.08.95*07/08.08.9508/09.08.9512/12.08.9513/14.08.95 314/15.08.95 503/04.09.95 504/05.09.95* 605/06.09.95 706/07.09.95* 507/08.09.95 408/09.09.95 1 8009/10.09.95* 150 411/12.09.95 1 15012/13.09.95 165 913/14.09.95 210 8 102/03.10.95 15003/04.10.95 9 12004/05.10.95 8 3005/06.10.95 5 2 11006/07.10.95 4 20 407/08.10.95 3 255 508/09.10.95 2 240 5 1 30 6 0 4 255 8 2 225 9 3 3 150 8 1 3 9 120 1 1 140 1 20 38.7 150 13 3 22.8 180 0 16.8 1 240 20 3 19.8 270 0 0 12 0 20.4 215 1.9 11 46.2 0 54.2 30.7 55.5 1 2 12 5 14.7 0 114.3 13.4 133/2.1 129/2.3 5 0 105.3 132/2.5 7 44.5 0 0 135/3.6 3 67.6 64.5 1 7 70/1.3 82/1.7 105.3 86/1.0 87.1 0 3 4 72/2.0 0 1 — 110/1.0 79/2.1 69.1 0 5 0 87.1 4 38/1.0 97/4.0 10 0 120/3.5 109/1.0 0 4 5.5 95/4.5 47.8 95/2.3 — 42.8 87/2.5 120/1.4 0 122/2.8 93/3.5 51.2 0 16 10.9 93/4.1 18.2 34.3 91/1.5 98/3.1 117/2.1 9.4 71/2.0 25.1 61.7 25.5 63/1.0 58.3 137/4.4 138/6.0 103/2.7 34.1 — 132/1.5 140/1.6 13/1.0 137/2.7 139/4.0 10/2.2 12/3.4 83/1.0 15/1.5 — — 15/2.8 16/3.1 — Results of the moon-watch observations in autumn 1995. MTR: migration traffic rate (number birds crossing a line 1km perp rection in one hour). *: nights not used for estimating the average density. [Resultados de observaciones sobre el disco lunar en otoño 1995 la llanura baja del Danubio. viesan una línea de 1 km perpendicular a la dirección del vuelo en hora. *: Noches no utilizadas para estimar densidad m NOCTURNAL AUTUMN MIGRATION OF WATERBIRDS IN NORTH-EASTERN BULGARIA 5

FIG. 2.—Altitudinal distribution of waterfowl migrating over the Danube Plain, expressed as the MTR (mi- gration traffic rates; see text for details) for 200–m intervals of altitude. [Distribución altitudinal de la tasa de migración (MTR; véase el texto para una descripción completa) de pa- tos y gansos sobre la llanura del bajo Danubio en intervalos de altitud de 200 m.]

1.2. Direction of migration 2. Charadriiformes

To estimate the night-to-night variation of Nocturnal migration of waders occurred du- migration direction in this group of birds was ring the whole period of observations (Table impossible, as the sample size was small. The 1). During 18 nights, 76 silhouettes of waders analysis of the MTR distribution by sectors of were registered. No birds were observed flying 22.5° revealed the mean vectors of migration in flocks. for every month (Fig. 3). In August the mean direction was 195° (r = 0.68, n = 16). In Sep- tember most of the waterfowl flew in a SSE 2.1. Migration traffic rate and altitudinal direction with a mean vector of 167° (r = 0.84, distribution n = 49). In October the mean flight direction was 191° (r = 0.77, n = 33). The distribution of The analysis of MTR for each night with MTR by 16 sectors is presented at Fig. 2. In more than three hours of observations showed August and October the distributions of direc- that most waders migrate in September. The tions were significantly different (Watson’s U2 maximum MTR for Charadriiformes was regis- test, Table 2), the mean azimuth being about tered in the night of 07-08/09/95 when 87.1 190° in both cases (Fig. 2, Table 2). At night on waders passed above the observation point in 03-04/10/95 a flock of six geese was observed one hour. The average MTRs for August, Sep- flying at 660 m a.g.l in SW direction (azimuth tember and October were 26.68, 64.82 and 229°). During the whole period of observations, 31.89, respectively. Night-to-night variations only five waterfowl silhouettes (6%; MTR = were as high as 2-5 fold. The density of migra- 276) were registered flying along the Danube tion increased on 8th August, from 3rd to 8th river. Four of these might have been taking off September and from 2nd to 5th October. In the or landing birds as they were registered during study region, nocturnal migration of waders oc- the first or the last hour of the night. curred from 560 m up to 4600 m a.g.l. Most of 6 ZEHTINDJIEV, P.

FIG. 3.—Distributions of flight directions of waterfowl migrating over the Danube Plain in sectors of 22.5°, expressed as proportions of the highest MTR recorded. A°: mean directions, r: length of the mean vector, n: number of registered silhouettes. All distributions are non-random (Rayleigh test, August 0.1 < P < 0.5, Sep- tember 0.001 < P < 0.5, October 0.002 < P < 0.5). [Distribuciones de las direcciones de vuelo de los patos y gansos migradores sobre la llanura del bajo Da- nubio en sectores de 22.5°, expresadas como proporciones de la tasa máxima de migración. A°: direcciones medias, r: longitud del vector medio, n: número de siluetas registradas. Todas las distribuciones difirieron sig- nificativamente de una distribución aleatoria (test de Rayleigh, agosto 0.1 < P < 0.5, septiembre 0.001 < P < 0.5, octubre 0.002 < P < 0.5).]

TABLE 2

Pairwise comparisons of the distributions of flight directions by 58 sectors for each cycle of moonwatching. [Comparación por pares de la distribución de direcciones de vuelo en 58 sectores para cada ciclo de ob- servación lunar.]

Watson’s U2 test [Test de la U2 de Watson]

Anseriformes Charadriiformes

U2 = 1.4249 U2 = 0.7913

August n1 = 16 n1 = 10 [Agosto] n2 = 49 n2 = 37 P > 0.5 P > 0.5

U2 = 0.6223 U2 = 1.1946

September n1 = 16 n1 = 10 [Septiembre] n2 = 33 n2 = 28 P > 0.5 P > 0.5

U2 = 1.0475 U2 = 0.6989

October n1 = 149 n1 = 37 [Octubre] n2 = 33 n2 = 28 P > 0.5 P > 0.5 NOCTURNAL AUTUMN MIGRATION OF WATERBIRDS IN NORTH-EASTERN BULGARIA 7

FIG. 4.—Altitudinal distribution of waders migrating over the Danube Plain, expressed as the MTR (migration traffic rates; see text for details) for 200-m intervals of altitude. [Distribución altitudinal de la tasa de migración (MTR; véase el texto para una descripción completa) de li- mícolas sobre la llanura del bajo Danubio en intervalos de altitud de 200 m.] them flew between 600 m and 1400 m a.g.l of the data from August and September revealed (Fig. 4). no significant relationship between distributions of directions as well as between mean vectors, depending on the flight altitude or wind condi- 2.2. Direction of migration tions. In October the mean vector of wader mi- gration changed (Fig. 5). The distribution of To estimate the night-to-night variation of flight directions was unimodal, with maximum migration direction in this group of birds was density in the SW sector and a mean vector of also impossible, as the sample size was small. In 194° (r = 0.83, n = 28). August the distribution of migration by 16 sec- tors showed a tendency to bimodality. Birds at altitudes over 1000 m a.g.l flew with a mean DISCUSSION vector of 140° (r = 0.81, n = 8) and two si- lhouettes of low-flying birds formed the W di- The moon-watch observations in autumn in rection. In September migrating waders flew the central part of the Lower Danube Plain re- from SSE to SSW directions with a mean vector vealed that nocturnal passage of waterfowl and of 171° (r = 0.64, n = 37). Birds at altitudes waders follows the general migration direction over 1000 m a.g.l flew almost in the same sec- to the winter quarters, not the Danube river. tor, from E to SSW with a mean azimuth of There were only 6% of waterfowl and no wa- 158° (r = 0.60, n = 31). The directional analysis ders (over 1000 m a.g.l) flying along the Danu- by sectors of 5° revealed statistically significant be. In this case it is possible to estimate the differences in the distributions of the MTR for large-scale spatial distribution of migrating wa- every month (Table 2). A reversed migration terfowl and waders at several sites situated from W to NNW sectors was observed during across the mean direction of migration (Dol- nights on 07/08 and 08/09 September under con- nik & Bolshakov, 1985; Dolnik, 1990). The ditions of opposite winds (Table 1). The analysis mean azimuth of migration changes from SSE 8 ZEHTINDJIEV, P.

FIG. 5.— Distributions of flight directions of waders migrating over the Danube Plain in sectors of 22.5°, ex- pressed as proportions of the highest MTR recorded. A°: mean directions, r: length of the mean vector, n: number of registered silhouettes. Mean direction (A°) is not given for the August data since its distribution showed a tendency to bimodality. All distributions are non-random (Rayleigh test, August 0.1 < P < 0.5, Sep- tember 0.001 < P < 0.5, October 0.002 < P < 0.5). [Distribuciones de las direcciones de vuelo de los limícolas migradores sobre la llanura del bajo Danubio en sectores de 22.5°, expresadas como proporciones de la tasa máxima de migración. A°: direcciones medias, r: longitud del vector medio, n: número de siluetas registradas. La dirección media (A°) para el mes de Agos- to no se calculó pues la distribución de los datos fue marcadamente bimodal. Todas las distribuciones difi- rieron significativamente de una distribución aleatoria (test de Rayleigh, agosto 0.1 < P < 0.5, septiembre 0.001 < P < 0.5, octubre 0.002

in August and September to SSW in October Garganey Anas querquedula in August and the for waders and remains almost southern (SSE- Common Teal Anas crecca in September, with SE to SSW-SW) for waterfowl. Assuming that azimuths of 197o and 167o, respectively. The the transit nocturnal migration (over 1000 m species composition of waterfowl migrants is a.g.l; weak winds) probably reflects the axis presented in Table 3. between breeding sites and winter quarters, the According to the regular counts conducted mean vector in autumn should indicate the lo- during the study, five species are the most nu- cation of these places. All observations were merous in the study region. The Mallard Anas carried out during nights with weak winds at platyrhynchos and the Pochard Aythya ferina the level of the flight altitudes of birds. For all are partially migratory while the Common Teal, nights, the wind speed varied from 1 to 3 m/s Garganey and Ferruginous Aythya nyroca at 1460 m a.s.l. Only on 02-03/10/95 was it are highly migratory (Scott & Rose, 1996). The 4.0 m/s (Table 1). The influence of these winds quantitative analysis reveals a prevalence of on the flight direction was ignored because the the Mallard and Garganey in August. The Po- birds probably completely compensated for the chard and the Ferruginous Duck also possibly drift (Alerstam, 1979a; Alerstam, 1979b; Ri- migrate in August. In September, the most nu- chardson, 1982), but the full compensation can merous species besides the Mallard is the Teal. only be assumed. It is also possible that in Au- The Ferruginous Duck is also present. In Octo- gust the detected waterfowl species migrate to- ber the situation changes, and only the Mallard wards appropriate moult sites. The distribution is abundant in the region. The proportion of of flight direction over 1000 m in August pro- species during autumn is almost constant, only bably reflects the specific and subspecific dif- the Garganey and the Teal change places in ferences in the general migratory direction of August and September. waterfowl migrants. The only species which The mean azimuth of migrating waders chan- can be identified as abundant waterfowl is the ges from SSE in August to SSW in October. A NOCTURNAL AUTUMN MIGRATION OF WATERBIRDS IN NORTH-EASTERN BULGARIA 9

TABLE 3

Numbers of the most common waterfowl species recorded during the autumn of 1995 at the point of moon- watch observations (NE Bulgaria). Maximum numbers for every species are presented. [Abundancia de las aves acuáticas más comunes en migración sobre la llanura baja del Danubio durante el otoño de 1995, obtenida mediante conteos en charcas próximas al punto de observación. Se presenta el nú- mero máximo obtenido para cada especie.]

Species 10.08.95 20.08.95 31.08.95 10.09.95 20.09.95 30.09.95 10.10.95 20.10.95 31.10.95 [Especies]

Anas crecca 0 0 0 0 220 250 250 100 50 A. plathyrhynchos 30 500 100 50 20 50 40 100 300 A. querquedula 500500000 0000 Aythya ferina 1051000000 A. nyroca 30302010109600 similar situation has been registered for noctur- (Cramp & Simmons, 1983). At least some po- nal passerine migrants at the same observation pulations of all these species spend the winter in point (Bolshakov et al., 1998). In August and Africa (Curry-Lindahl, 1975). September the flow of nocturnal wader migrants The data about the species composition are consists almost entirely of trans-equatorial mi- approximate, as there is no systematic research grants. Nocturnal migration of birds that spend on wader species during migration in Bulga- the winter in Northern Africa begins in October. ria. There is no literature about the phenology The species composition of waders during au- of wader migration through this area. tumn migration is not clear. According to the It has been reported that the migration den- available literature, 54 species of Charadriifor- sity of waterbirds increases under favourable mes migrate following the Bulgarian Black Sea wind conditions and stops during head winds coast from July to October (Dontschev, 1984). (Bulyuk, 1982). The analysis of directions of Diurnal movements of waders have been regis- migration in both Anseriformes and Charadrii- tered in September, October and December formes reveals changes in density and flight around Sofia. The quantity of birds in Septem- direction despite the almost constant winds du- ber and October has been almost equal (Nanki- ring autumn. Night-to-night variation of migra- nov et al., 1998). According to the same aut- tion density at the point of observation was as hors, the most numerous species in August is high as 5-10 fold. the Wood Sandpiper Tringa glareola. This spe- cies winters mainly in tropical and subtropical ACKNOWLEDGEMENTS.—Part of the research re- latitudes in Africa. Its passage is across Europe ported in this paper was conducted in conjunction and the Middle East on a broad front with a with Ms. Alexandra Sinelschikova from the biologi- peak in the second half of August (Cramp & cal station «Rybachy». Dr Viktor N. Bulyuk gave me helpful ideas that led to significant improvements Simmons, 1983). It seems to be one of the main of the original paper. I am most grateful to Ms. Maria migrants in the study region (according to re- Bogdanova and Ms. Jane M. Reid for translating and gistered calls at night during the observations in improving the English in this paper. August and September). Other abundant spe- cies are the Little Ringed Plover Charadrius dubius with a peak in August, and the Ruff Phi- BIBLIOGRAPHY lomachus pugnax, wintering in tropical Africa and migrating during August-September ADAMS, D. W. H. 1962. Radar observation of bird Ibis, (Cramp & Simmons, 1983). In October migrate migration in Cyprus. 104: 133-146. ALERSTAM, T. 1979a. Optimal use of wind by migra- the Snipe Gallinago gallinago and the Lapwing ting birds: combined drift and overcompensation. Vanellus vanellus. Observations of Snipes in Journal of theoretical Biology, 79: 341-353. oases and the Sahel indicate a broad front cros- ALERSTAM, T. 1979b. Wind as a selective agent in sing of the Sahara in September-October . Ornis Scandinavica, 10: 76-93. 10 ZEHTINDJIEV, P.

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