www.ornitologia.org
Òscar Gordo
The abnormal behaviour of a group of Barn Swallows roosting in a maize field is described in detail. Approximately 700 individuals forming a spherical flock performed spectacular manoeuvres over a maize field for several minutes.
After sunset, individuals plunged vertically in small groups into the maize field in very fast flight. Observed behaviour agreed largely with previously reports, although some peculiarities were recorded such as an attempt to roost on sparsely grassed ground, attacks by Yellow Wagtails Motacilla flava on landing swallows and loud twittering during all the aerial manoeuvres. The bad weather conditions are the most likely cause for the roosting of these migrating individuals. Key words: Barn Swallow, Hirundo rustica, roosting, behaviour, maize field, autumn migration, NE Spain Current address: Òscar Gordo, Dept. Ecología Evolutiva, Museo Nacional de Ciencias Naturales (CSIC), c/José Gutiérrez Abascal, 2, 28600 Madrid. E-mail: ogordo@mncn.csic.es Received: 17.04.06; Accepted: 03.04.07 / Edited by L. Brotons
Communal roosts are typical in the Barn Swallow Hirundo rustica outside the breeding season. In African wintering quarters, large numbers of Barn Swallows are found in a small number of roosting sites, in which thousands or even millions of birds congregate every evening (Vrijdagh 1951, de Bant 1962, Curry-Lindahl 1963, van den Brink et al. 1998, 2000, Deuzeman et al. 2004). These huge aggregations occur typically in reed-beds close to rivers and lakes (Verheyen 1952, de Bant 1962, Curry-Lindahl 1963, van den Brink et al. 2000, Nutall 2000; but see also Cramp 1988, Fry et al. 1988, Nutall 1998), where birds can forage nearby. This gregariousness also benefits individuals as a means of reducing predation risk (Bijlsma & van den Brink 2005). Roosts have been also reported in European breeding grounds, above all in late summer and autumn (Cramp 1988). In these cases, Barn Swallows have been recorded in different types of habitats (e.g. bracken, James 1995; reed-beds, Kose 1993) and also locations such as maize fields on several occasions (Ford & Elphick 1993, Lukac 1994, Spence 1995). These postbreeding roosts are thought to be associated with increased predation risk and/or bad weather conditions (Stagg 2005). Here, the unusual behaviour of a roosting flock of Barn Swallows observed on 29 September 2005 is described in detail. The observation took place in an area of open ground of 40 ha named Pla de Can Morató, between the villages of Cardedeu and Sant Antoni de Vilamajor (Vallès Oriental; UTM 447-4610) at 210 m a.s.l. in NE Spain. The area is covered by a mosaic of crops, pastures and small patches of Pinus pinea woodland, all of which are surrounded by builtup areas and mixed forest. Most of the area was lying fallow at the time of observation and only a maize field of ca. 2 ha was still unharvested. These plants were approximately 1-1.5 m high. The observation took place in the evening between 18:50 h and 20:10 h in bad weather conditions, with a dense cover of dark clouds, intermittent gusts of wind and some rain between 18:55 and 19:05 h.
At 18:50 h, approximately 100 Barn Swallows appeared in the area and apparently began to forage near the ground (maximum 10 m high). When it started raining five minutes later, more individuals arrived from nearby built-up areas, increasing the number of individuals to ca. 200. When it stopped raining, all of the birds suddenly disappeared northwards. A few minutes later (19:10 h) a large number of birds (about 300) re-appeared. However, on this occasion, all of the birds were flying together in a compact flock that then proceeded to move northwards and southwards, performing spectacular manoeuvres over the fields of Pla de Can Morató. At 19:20 h most of the Barns Swallows began to land on open ground. A number of Yellow Wagtails Motacilla flava, which are regular visitors in small groups of 10-20 individuals to this area during both during spring and autumn passages, attacked the swallows that were attempting to land. Five minutes later all of the swallows took off and formed a compact flock that once again began to perform spectacular aerial movements over the fields. A single Sand Martin Riparia riparia was detected among swallows. At 19:35 h a strong wind from the north (4-5 on the Beaufort scale) dispersed the swallows, which disappeared for ten minutes. The sun set at around 19:40 h, but the swallows soon returned (19:45 h), forming a compact spherical flock, in which birds whirled very fast about 10 m high above the maize field. Unlike the flocks described previously, this group was very noisy. For a few minutes, many individuals arrived from different directions and joined the flock. Eventually, the flock totalled approximately 700 individuals and continued its perfectly synchronised high-speed spiral flight. At 19:55 h small groups of birds (about 20-50 individuals) began to plunge vertically into the maize field at great speed. By 20:00 h all birds were perched in the maize, concentrated in just a few plants in an area of around 30 m 2 in the middle of the field. An intense twittering continued for a few minutes. Some aggressive interaction between individuals was observed, which resulted in some individuals flying from one plant to another. After perching, all individuals began to preen. At 20:10 h the twittering began to fade and I left the area. Similar incidents of birds on autumn migration roosting communally in a maize field have never been observed in the same place either before or subsequently: it was, thus, an isolated and exceptional incident, probably caused by adverse weather conditions (Stagg 2005). This fact could explain the differences between the observed behaviour, that is, the attempts to roost on open ground, the aggressive response by Yellow Wagtails and the aerial manoeuvres with much twittering, and the descriptions of roosting behaviour in Cramp (1988). However, the type of habitat used finally to roost in (Ford & Elphick 1993, Lukac 1994, Spence 1995) and the presence of Sand Martins (Cramp 1988) both agree with previous descriptions. Resum Comportament inusual d’un grup d’orenetes Hirundo rustica durant la migració de tardor abans d’ajocar-se Es fa una descripció detallada del comportament d’un grup d’unes 700 orenetes vulgars abans d’ajocar-se en un camp de blat de moro. Van formar un estol de forma esfèrica i durant varis minuts van fer tota mena de vols sincronitzats força espectaculars per sobre de l’àrea on finalment van aturar-se. Després de la posta de sol, alguns individus van anar baixant en grups petits amb vols molt ràpids per anar a parar-se sobre les plantes de blat de moro. Els comportaments observats es corresponen en general amb el que ja es coneixia, tot i que es van observar alguns fets inèdits, com ara: un intent d’aturar-se al terra, l’atac de les cueretes grogues Motacilla flava durant aquest intent i el continu soroll dels individus durant tot el procés. Aquests individus en pas van haver d’aturar-se probablement a causa de les condicions meteorològiques adverses. Resumen Comportamiento inusual de un grupo de golondrinas comunes Hirundo rustica durante la migración de otoño antes de acceder al dormidero Se hace una descripción detallada del comportamiento de un grupo de unas 700 golondrinas comunes antes de acceder a un dormidero situado en un campo de maíz. Formaron un bando esférico que durante varios minutos realizó todo tipo de vuelos sincronizados muy espectaculares por encima del área en la que finalmente se posaron. Después de la puesta de sol, algunos individuos fueron bajando en peque ños grupos mediante vuelos muy rápidos a posarse en las plantas del campo de maíz. Las observaciones corresponden en general con lo anteriormente descrito, pese a que se observaron algunos hechos inéditos: el intento de posarse en tierra, el ataque de lavanderas boyeras Motacilla flava durante dicho intento y el ruido continuo de los individuos durante todo el proceso. Las condiciones meteorológicas adversas probablemente fueron la causa de que estos individuos en paso se pararan. References Bijlsma, R. G. & van den Brink, B. 2005. A Barn Swallow Hirundo rustica roost under attack: timing and risks in the presence of African Hobbies Falco cuvieri. Ardea 93: 37–48. De Bont, A. F. 1962. Composition des bandes d’Hirondelles de chemineé, Hirundo rustica rustica L., hivernant au Katanga et analyse de la mue des rémiges primaries. Gerfaut 52: 298–343. Cramp, S. 1988. The birds of the Western Palearctic. Vol. V. Oxford: Oxford University Press. Curry-Lindahl, K. 1963. Roosts of Swallows (Hirundo rustica) and House Martins (Delichon urbica) during the migration in Tropical Africa. Ostrich 34: 99–101. Deuzeman, S. B., van der Have. T. M., de Nobel, W. T & van den Brink, B. 2004. European Swallows Hirundo rustica and other songbirds of wetlands in Ghana, December 1997. WIWO report 80: 1–59. Ford, A. A. & Elphick, D. 1993. Barn Swallows roosting in maize. British Birds 86: 95-96. Keith, S. & Urban, E. K. & Fry, C. H.1988. The Birds of Africa. Vol IV. London: Academic Press. James, R. M. R. 1995. Barn swallows roosting away from water, low down in bracken. British Birds 88: 226–227. Kose, M. 1993. Swallows roost in reedbeds. Eesti Loodus 8: 270–272. Lukac, G. 1994. Kultura kukuruza, Zea mays, nociliste lastavice pokucarke, Hirundo rustica. Troglodytes 7: 72. Nuttall, R. J. 1998. European swallows roosting in suburban Bloemfontein. Mirafra 15: 37–38. Nuttall, R. J. 2000. European swallow roosts in the Memel District, Free State, South Africa. Mirafra 17: 57–58. Spence, I. M. 1995. Swallows roosting in maize. Welsh Birds 1: 35. Stagg, A. 2005. Hirundine cold-weather behaviour. British Birds 98: 264. van den Brink, B., Bijlsma, R. G. & van der Have, T. M. 1998. European songbirds and Barn Swallows Hirundo rustica in Ghana: a quest for Constant Effort Sites and swallow roosts in December/January 1996/97. WIWO report 58: 1–55. van den Brink B., Bijlsma R. G. & van der Have T. M. 2000. European Swallows Hirundo rustica in Botswana during three non-breeding seasons: the effects of rainfall on moult. Ostrich 71: 198–204. Verheyen, R. 1952. Nos hirondelles (Riparia riparia, Delichon urbica, Hirundo rustica) dans leus quartiers d’hiver. Gerfaut 42: 92–124. Vrijdagh, J. M. 1951. Comportament des Hirondelles de chimenée dans leur quartier d’hiver, au Nord du Congo Belge. Gerfaut 41:177–195.
