1. Introduction Pair formation and courtship behaviour among storks Ciconiidae is quite uniform, consisting of highly stereotyped behavioural patterns that have evolved as social signals through the process of ritualization. Most of the social displays of colonial and semi-colo- nial nesting “typical” storks of the tribe Ciconiini more closely studied by Kahl (1972a) work as short range signals at the nest-site or its immediate vicinity, whereas most species appear to ignore each other away from the nest in the feeding areas (Kahl 1971, 1972b, King 1988, Hancock et al. 1992). As Kushlan (1977) has demonstrated by field experiments in the freshwater marshes of the Everglades in southern Florida, highly visible white plumage colorations in ciconiiform wading birds act as long-distance stimuli for the formation and local enhancement of feeding aggregations. In the same way in storks with white, maximally visible plumages nesting in open habitats, like the palearctic White Stork Ciconia ciconia, just standing around conspicuously at the nest may oper- 223 Acrocephalus 21 (102-103): 223 – 229, 2000 Form and function of aerial courtship displays in Black Storks Ciconia nigra Oblika in funkcija dvorjenja ~rnih {torkelj Ciconia nigra v zraku Peter Sackl Steiermärkisches Landesmuseum Joanneum, Forschungsstätte Furtnerteich, Raubergasse 10, A-8010 Graz, Austria e-mail: peter.sackl@stmk.gv.at Hitherto unknown aerial courtship displays of Black Storks Ciconia nigra recorded for the most part during population surveys in northern and eastern Austria between 1979-1991 are described. Aerial displays were seen mainly during early stages of the breeding cycle in April till mid-May (Fig. 1). They are characterized by mates soaring tight together in a highly synchronized manner above the nest-site or in other parts of the home range (Parallel Soaring). Additionally, melodious flight calls are given by both partners and the white undertail-coverts are widely spread. Occasionally soaring birds were seen whiffling or performing simultaneous darting flights (Fig. 3). According to (1) the regular participation of both breeding partners, (2) their regular performance around nest sites and/or within home ranges, (3) their largely restricted occurrence during early stages of the breeding cycle as well as (4) by their specific pattern of stereotyped and elaborated behavioural elements (Parallel Soaring, Displaying the Undertail-Coverts, Flight Calls, Whiffling and Darting Flights) ceremonial flights in Black Storks may generally operate as highly ritualised courtship flights. Thus, analogous to aerial displays in other large forest-living birds – like in many raptors – they may help in point- ing out nest-sites to potential mates, stimulate pair formation and assist in spacing by discouraging other birds from settling close. The highly elaborat- ed courtship flights in Black Storks seem to be unique within the “typical” storks of the tribe Ciconiini and coincide with the solitary nesting habit of the species within the closed canopies of heavily wooded areas. Key words: behaviour, Black Stork, Ciconia nigra, thermal soaring, aerial displays, courtship flight, pair formation, spacing Klju~ne besede: vedenje, ~rna {torklja, Ciconia nigra, dviganje na termi~nem vzgorniku, dvorjenje v zraku, snubitveni let, oblikovanje para, razporejanje