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
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
P. Sackl: Form and function of aerial courtship displays in Black Storks Ciconia nigra
ate as a long-distance signal for conspecifics announc-ing occupation of the nest-site and/or breeding colony (Schüz 1944).
Nest-sites of Black Storks Ciconia nigra are normal-ly hidden within the canopy of closed woodlands. The behaviour patterns and social displays of the soli-tary nesting species, which is distributed throughout the closed and more open woodlands of the warmer boreal and warm temperate climatic zones of Europe and northern Asia with smaller and more isolated populations on the Iberian Peninsula and in southern Africa, are described in more detail by Siewert (1932) and Kahl (1972a). Their investigations done almost exclusively by observing from hides at occupied nestsites failed to confirm the existence and social signifi-cance of display flights during pair formation hither-to noticed just incidentally in Rohweder (1905), Bauer & Glutz von Blotzheim (1966) as well as Schröder & Burmeister (1974). In this paper field observations of aerial courtship displays for the most part published by Sackl (1993) and their significance for pair formation and courtship behaviour in Black Storks will be investigated.
2. Methods and materials
Between 1979 and 1991, aerial displays were obser-ved incidentally on 18 occasions during nesting and population surveys of Black Storks in northern and southeastern Austria. On the average, breeding densi-ty of Black Storks in their continuous Austrian breed-ing area is about 0.4 breeding pairs (bp)/100 km2 reaching up to 0.8 and 2.9 bp/100 km2 in the main study areas in the Waldviertel (Lower Austria) and the prealpine lowlands of southeastern Styria, respec-tively (Sackl 1985, unpubl. data). In a 380 km2 heav-ily wooded study area in the district of Fürstenfeld in eastern Styria nearest neighbour distances of simulta-neously occupied nest-sites (cf. Clark & Evans 1954) vary from 1.3–11.1 km (x = 3.6 km, s = 2.8, n = 11). To collect data on nest occupation and breeding suc-cess, 6–13 nests and the home ranges of additional 5–15 breeding pairs were visited annually at least twice during the breeding season from the end of March till August. Additional surveys were done occasionally by observing the birds from elevated landmarks within the home ranges of selected breed-ing pairs during the courtship and laying period from late March till early May. In 1979–1982, three pairs were investigated more closely by visiting nest-sites on a weekly routine. So far as possible under field conditions birds were aged and sexed according to Bauer & Glutz von Blotzheim (1966) and Cramp 224
(1977). For this paper, materials of the author were complemented by occasional observations of aerial displays made by Franz Samwald and Otto Samwald in southeastern Austria (n = 5) as well as by personal reports from M. Forsberg, J. Pried-nieks and M. Strazds (n = 3) recorded in Latvia between 1990–1993.
3. Results and discussion
3.1. Seasonal occurrence and social context
With a frequency of 63.7% of all locomotion flights observed in Austria 1979–1991 (n = 171) thermal soaring of one or both partners, relying on thermal upcurrents of air gaining altitude and then gliding for 3 to >6 kilometres, slowly losing height, is like in other ciconiiforms the most frequent method of loco-motion of nesting Black Storks between nest and feeding sites. In early stages of the breeding cycle from April to May and again in August, after the young have fledged, single birds as well as groups of two to up to 8 individuals were seen more frequently soaring in heights estimated from 200 – approx. 1.500 metres above nest-sites and home ranges than during the feeding period in June and July (cf. Fig. 1).
Although during the hot months of summer from late June to mid-August nesting Black Storks were seen using thermal soaring for early morning as well as late afternoon feeding trips, according to its dependence from the formation of upcurrents of warm air during late morning, thermal soaring is used most frequently for the locomotion between nest and feeding sites around midday from 10 a.m. to 4 p.m. (Fig. 2).
Ceremonial flights, characterized by elaborated and highly stereotyped behavioural patterns described below, were performed on three occasions (11.5%) by single birds and in 23 or 88.5% of all 26 observa-tion bouts available by two birds according to sex and high synchronization of behaviour regarded as breed-ing partners. 84.6% of all ceremonial flights of single birds as well as mates were performed shortly after their arrival at the nest-site in spring (first observa-tions at the nest-site 15.3.–2.4., median date = 28.3., n = 10, SE Austria 1980–1990) between early April and mid-May (Fig. 1). On three occasions aerial displays were seen in June, July, and in early August, respectively. According to the participation of birds with immature plumages initially paired birds or those who had bred unsuccessfully were involved in two of these cases (M. Strazds, in lit.). Once a pair was also encountered performing ceremonial flights
Acrocephalus 21 (102-103): 223 – 229, 2000
13    15   17 MAR
19   21    23 APR
25    27    29 MAY
37    39    41 JUL
43   45   47 AUG
49    51    53 SEP
31    33   35 JUN Time of season (pentodes) / letni èas (pentode)
¦ Courtship flights / dvorjenje v zraku (n = 26)     — Thermal soaring / dviganje na term, vzgorniku (n = 130)
Figure 1: Seasonal occurrence of thermal soaring and courtship flights in Black Storks Ciconia nigra in northern and eastern Austria between 1979-1991 (three occasional records of courtship flights from Latvia 1990-1993 included)
Slika 1: Sezonsko termalno poletavanje visoko proti nebu in snubitveni leti ~rne {torklje Ciconia nigra v severni in vzhodni Avstriji v letih 1979-1991 (skupaj s tremi naklju~nimi zapisi snubitvenih letov iz Latvije v letih 1990-1993)
i
M
¦s
o o
L
40 35 30 25 20 15 10 5


6-7     8-9    10-11  12-13 14-15 16-17  18-19 20-21
Time of day / èas dneva
D Thermal soaring / dviganje na term. vzgorniku (n = 109) ¦ Courtship flights / dvorjenje v zraku (n = 23)
Figure 2: Diurnal frequencies (two-hour intervals) of thermal soaring and courtship flights in nesting Black Storks Ciconia nigra in northern and eastern Austria during 1979-1991
Slika 2: Dnevne frekvence (dveurni intervali) termalnega poletavanja proti nebu in snubitvenih letov gnezde~ih ~rnih {torkelj Ciconia nigra v severni in vzhodni Avstriji v obdobju 1979-1991
within its home range in early September (cf. Fig. 1). Furthermore, 68.2% of all courtship flights by pairs for which actual nesting sites were known were per-formed above tree tops in the immediate vicinity of the nest-site, on all other occasions in other parts within the home range 3–5 kilometres from the nest. On at least three occasions ceremonial flights were finished by both partners disappearing close to each other in the canopy very close to the nest-site. This was followed on one occasion by an Up-down display and simultaneous preening behaviour of both partners in the nest. In another case of obviously initially paired mates landing in late April in a dry fish pond away from the nest the adult male tried for several times to copulate with its subadult mate.
3.2. Behaviour patterns of courtship flights
Courtship flights of Black Storks as illustrated in Fig. 3 are characterized by four clearly distinguish-able behaviour patterns:
1. Parallel Soaring (Synchronkreisen): Soaring of both mates in a highly synchronized manner, flying tight together one after and/or closely above the other, generally 5–30 metres above tree tops – but sometimes also much higher – is the basic character
225
P. Sackl: Form and function of aerial courtship displays in Black Storks Ciconia nigra
(1) Parallel Soaring
(4) Whiffling & Darting Flights
(2) Displaying Undertail-Coverts
(3) Flight Calls
Figure 3: Schematic synopsis of the behavioural elements of courtship flights in Black Storks Ciconia nigra
Slika 3: Shemati~ni pregled vedenjskih elementov snubitvenih letov ~rne {torklje Ciconia nigra: (1) vzporedno dviganje, (2) razkazovanje podrepnih krovcev, (3) ogla{anje v zraku in (4) sunkovito pihanje in pikiranje
of all courtship flights. Although Parallel Soaring was performed at times 3–5 kilometres from the nest, birds were usually seen circling in this manner 5–20 minutes in an area <500 metres around the nest-site. According to its dependence on thermal upcurrents of air for Parallel Soaring courtship flights were, like ther-mal soaring, performed generally between early morn-ing and late afternoon feeding bouts from the late morning hours onwards to mid-afternoon (Fig. 2).
2. Displaying the Undertail-Coverts (Flaggen): In the course of 53.9% of all courtship flights observed by one of the birds involved the white undertail coverts were widely spread protruding conspicuously out the laterally compressed black tail feathers (Fig. 4). On all occasions, when position and/or sex were noticed (for both categories n = 5), only the leading
226
bird and/or the male was seen spreading its undertail-coverts. Displaying the undertail-coverts is also a com-ponent of the Head-Shaking Crouch and Up-down display of C. nigra and other “typical” storks (Kahl 1972a). There is a strong resemblance of this behav-iour to the displaying of the white undertail-coverts during aerial displays in some raptors like the highly syntopic Goshawk Accipiter gentilis and Sparrowhawks A. nisus.
3. Flight Calls (Flugrufe): Sometimes birds were heard to utter soft, melodious “fii”-, “diuu”- or “hi”-calls differing from calls given during Up-down and threat displays at the nest. These calls seem to be equivalent to the rarely heard flight calls mentioned in Bauer & Glutz von Blotzheim (1966). At least on one occasion calls were given beyond any doubt by
Acrocephalus 21 (102-103): 223 – 229, 2000
both birds lifting up their heads and necks during     4. Conclusion
soaring as well as simultaneously lowering both legs
when calling. In some other cases soaring birds bend
down their necks and shake the head horizontally like
in Head-shaking Crouches and Up-down displays (cf.
Bauer & Glutz von Blotzheim 1966, Kahl 1972a,
Cramp 1977).
4. Whiffling and Darting Flights (Schaukel- und Wuchtelflüge): On rare occasions (in 5 or 19.2% of all observation bouts) soaring birds were seen whiffling simultaneously (Schaukelflug) or perform-ing sudden simultaneous darting flights diving steeply 5–10 metres from their route and rolling dur-ing the dive from one side to the other (Wuchtelflug), resembling the better known song-flights of Lapwings Vanellus vanellus. Thereafter Parallel Soaring was con-tinued. Whiffling and in particular darting flights may possibly have evolved from hostile pursuit flights occurring in many storks during initial stages of pair formation and occupation of nest-sites.
Display flights of Black Storks clearly differ from other forms of locomotion flights as thermal soaring by (1) the regular participation of both breeding partners, (2) their regular performance around nesting sites and/or within the home range, (3) their largely restricted occurrence during early stages of the breed-ing cycle in April and May as well as (4) by their specific pattern of highly stereotyped and elaborated behavioural elements (Parallel Soaring, Displaying the Undertail-Coverts, Flight Calls, Whiffling and Darting Flights). According to these characteristics they generally may function as aerial courtship displays by pointing out nest-sites to potential mates and neighbours, stimulating pair formation and syn-chronization of mates.
In many solitary nesting species of storks birds return to the same nest year after year and remate with the same partner. In the case of such rematings many courtship displays are subdued or even lacking
Figure 4: Adult Black Stork Ciconia nigra displaying the white Undertail-Coverts while soaring close to its nest-site, April 30th 1999, Steyrtal, Upper Austria (photo: Norbert Pühringer)
Slika 4: Odrasla ~rna {torklja Ciconia nigra razkazuje svoje bele podrepne krovce med poletom visoko proti nebu v bli`ini svojega gnezdi{~a; 30.4.1999, Steyrtal, Gornja Avstrija (foto: Norbert Pühringer)
227
P. Sackl: Form and function of aerial courtship displays in Black Storks Ciconia nigra
(Kahl 1972a). Without regard to general difficulties in observing Black Storks aside nest-sites in their heavily vegetated nesting habitats, this may be the reason why up to now aerial courtship displays have been overlooked by most researches and are noticed just incidentally throughout literature. Like courtship and other social displays in many other storks, courtship flights in Black Storks are most frequent during early stages of the breeding cycle and gradual-ly wane during the breeding and nestling period. In palearctic White Storks and other species the resur-rection of courtship behaviour like greeting displays, nest construction and copulating when young have fledged in late summer is a well known phenomenon. Accordingly, the occasional reappearance of courtship flights in Black Storks near the end of the breeding sea-son (or after the loss of nests earlier during the breed-ing cycle) is not contradictory to their importance for the occupation of nest-sites and pair formation in spring (Schüz 1943ab, Kahl 1978, Creutz 1988).
Although some other forms of aerial displays are known to exist in other storks, the highly elaborated courtship flights of Black Storks seem to be unique within the “typical” storks of the tribe Ciconiini and coincide with the solitary nesting habits of the species in heavily wooded areas, differing from other colo-nial and semi-colonial storks nesting in more open wetland and savannah habitats (Kahl 1971, 1972ab). Flying in the energetically most economical form by using thermal upcurrents of air for soaring high above nest-sites (cf. Rüppell 1975, Alexander 1982), which are normally hidden within the canopy of their forest nesting habitats, and performing conspicuous dart-ing flights as well as displaying simultaneously maxi-mally visible optical signals like their white undertail-coverts are important components of the courtship behaviour of many large forest-living birds. In partic-ular many solitary nesting raptors point out nest-sites in this way to potential mates as well as to neighbours and intruders (Newton 1979). Thus, analogous to the aerial displays of raptors courtship flights of Black Stork, may also assist in spacing by discouraging other birds from settling close. In the same way Buxton et al. (1978) explained the regular soaring of both sexes above nest-sites in solitary nesting Shoebills or Whale-headed Storks Balaeniceps rex as a form of territorial advertisement. For practical use during population surveys, systematically collected recordings from elevated landmarks and the mapping of courtship flights in spring from early April till mid-May may produce valuable information on the loca-tion of nest-sites, home ranges and breeding densities (Schneider-Jacoby 1999). 228
Evolutionarily courtship flights of Black Storks may be derived from locomotion flights as thermal soaring or hostile pursuit flights occurring during initial stages of nest occupation and pair formation. Courtship flights of Black Storks very probably are homologous to some other forms of more slightly ritualised and little known aerial displays in other storks, like the Flying Around display observed by Kahl (1972c) in all living species of wood-storks of the genus Mycteria. In White Storks similiar flights around the nest-site following the intrusion of opponents (“Demonstrationsflug”) were described recently by Lakeberg (1993). According to Kahl (1972a), the unique Mock Fighting displays in Abdim’s Stork Ciconia abdimii sometimes end by performing erratic bat-like darting flights. However, the only other “typical” stork in which elaborated aerial courtship displays may exist is the little studied, solitary nesting Woolly-necked Stork C. episcopus of the tropical wetlands and open woodlands of Africa and southern Asia (Mackworth-Praed & Grant 1973, Brown et al. 1982, Hancock et al. 1992). So far as the evolution of highly ritualized courtship flights in storks really coincides with their solitary nesting habits within the canopy of heavily vegetated areas, the existence of similar behaviour pat-terns in other forest-living storks may be expected, like in the recently discovered and little known Storm’s Stork C. stormi of peninsular tropical Asia (cf. Hancock et al. 1992, Danielsen et al. 1997).
5. Povzetek
Avtor obravnava do danes neznano obliko dvorjenja ~rnih {torkelj Ciconia ciconia v zraku, ve~inoma zabele`eno med popisovanjem pti~jih populacij v severni in vzhodni Avstriji med letoma 1979 in 1991. Dvorjenja v zraku so bila opazovana predvsem v zgodnjih fazah gnezditvenega obdobja, in sicer od aprila do sredine maja (sl. 1). Glavna zna~ilnost dvorjenja ~rne {torklje je naglo in nadvse sinhrono dviganje para proti nebu tesno skupaj nad gne-zdi{~em ali drugod v njunem doma~em okoli{u (vzporedno dviganje). Oba partnerja hkrati spu{~ata melodi~ne glasove in razgaljata podrepne krovce. Tu in tam so proti nebu dvigajo~i se pari sunkovito pihali ali pa so simultano pikirali proti tlom (sl. 3). Glede na (1) redno sodelovanje obeh gnezde~ih partnerjev, (2) njihovo stalno nastopanje nad gnezdi{~em in/ali doma~im okoli{em, (3) njihovo v veliki meri zmanj{ano pojavljanje v zgodnjih fazah njihovega gnezditvenega obdobja in (4) tudi glede na zna~ilni vzorec njihovih stereotipnih in dovr{enih vedenjskih vzorcev (vzporedno dviganje, razkazovanje podrepnih
Acrocephalus 21 (102-103): 223 – 229, 2000
krovcev, ogla{anje v zraku, sunkovito pihanje in pikiranje) se zdi, da so obredni poleti ~rnih {torkelj visoko ritualizirani snubitveni leti. Analogno z razkazovanjem drugih velikih gozdnih ptic v zraku – na primer mnogih plenilcev – morebiti pomagajo pri sporo~anju morebitnim partnerjem, da je tu gne-zdi{~e, stimulirajo oblikovanje parov in pomagajo pri razporejanju parov z odganjanjem tekmecev. Izredno dovr{eni snubitveni leti ~rnih {torkelj se zdijo enkratni med “tipi~nimi” {torkljami iz plemena Ciconiini in so povsem v skladu s solitarnimi gnezditvenimi navadami te pti~je vrste znotraj s kro{njami zaprtih in gostih gozdnatih obmo~ij.
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Arrived / Prispelo: 26.11.1999 Accepted / Sprejeto: 12.12.2000
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