ORNITOLOGIA NEOTROPICAL 23: 555–567, 2012 © The Neotropical Ornithological Society REPRODUCTIVE BIOLOGY AND PAIR BEHAVIOR DURING INCUBATION OF THE BLACK-NECKED SWAN (CYGNUS MELANOCORYPHUS) Carmen Paz Silva1, Roberto Pablo Schlatter2 & Mauricio Soto-Gamboa1 1Instituto de Ciencias Ambientales y Evolutivas, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile. E-mail: [email protected] 2Instituto de Ciencias Marinas y Limnológicas, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile. Resumen. – Biología reproductiva y comportamiento del Cisne de Cuello Negro (Cygnus melano- coryphus). – El cisne de cuello negro (Cygnus melanocoryphus) es la única especie del género Cygnus que habita en Sudamérica. Si bien, el resto de las especies del género han sido extensamente estudia- das, para C. melanocoryphus aún no se ha descrito la mayor parte de su biología reproductiva. En este estudio evaluamos el comienzo y extensión de la temporada reproductiva, y el tamaño de nidada utili- zando datos recolectados en el Santuario Carlos Anwandter (SCA), Valdivia, Chile; correspondientes a 18 temporadas reproductivas. Por otro lado, estudiamos el comportamiento de las parejas durante el periodo de incubación, durante una temporada reproductiva. Se estimó el presupuesto de tiempo y dis- tancia con respecto al nido en seis parejas nidificantes. El periodo de nidificación se puede extender desde junio a enero y está influenciado por el tamaño de la población reproductiva y las condiciones cli- máticas. El tamaño de nidada se mantiene constante con una moda de tres huevos (promedio 3.13 ± 0.017; n = 5897). Ambos miembros de la pareja desarrollan actividades de vigilancia y mantención del nido, sin embargo, la incubación es una tarea exclusiva de la hembra, mientras que la defensa activa del nido es realizada por el macho. De esta forma, se propone que tanto la extensión del periodo reproduc- tivo y el número de parejas nidificantes podrían estar regulados por procesos denso-dependientes, el tamaño de nidada parece ser altamente conservativo. Finalmente, se observó una clara división de tareas por parte de cada sexo durante del periodo de incubación. Abstract. – The Black-necked Swan (Cygnus melanocoryphus) is the only species of the genus inhabit- ing South America. Although most of the members have been extensively studied, the reproductive biol- ogy of C. melanocoryphus, has not been described yet. In this study, we evaluate the starting and extension of the breeding season and clutch size number, using data collected at Carlos Anwandter San- tuary (CAS), Valdivia, Chile, corresponding to 18 reproductive seasons. We also studied male and female behavior during incubation period. We monitored time budget and the distance relative to the nest of 6 breeding pairs, during one reproductive season. Nesting period can extend from June to January and is influenced by the population number and weather conditions. Clutch size is constant across sea- sons with a mode of three eggs (mean of 3.13 ± 0.017 eggs; n = 5897). Both pair members cooperate in duties, such as vigilance and nest maintenance; however, incubation is an exclusive activity of females while the active defense of the nest is performed by males. Therefore, we propose that both extension of breeding season and number of breeding pairs are associated with density-dependent processes while clutch size seems to be highly conservative. Finally, we observed a clear separation in parental tasks for each sex during incubation period. Accepted 14 February 2013. Key words: Black-necked Swan, Cygnus melanocoryphus, breeding biology, breeding season, clutch size, nesting behavior, incubation, Chile. 555 SILVA ET AL. INTRODUCTION (Corti 1996), nesting only in areas where the emergent vegetation is abundant and the level The seven living species of swans of the of water is stable (Corti & Schlatter 2002). genus Cygnus are the biggest and most con- In this work, we aimed to determine two spicuous among the waterfowl. Swans form basic breeding parameters, extent of the life-long pairs, and both sexes remain with the breeding period and clutch size, using field young for extended periods of time (Kear data collected during 18 breeding seasons. We 1972, Ming & Dai 2000). Males have an im- also evaluated pair behavior and space utiliza- portant role during the reproductive period, tion during incubation period, using data col- and in some species they take part in the incu- lected during one breeding season. bation, as in the case of C. olor, C. bewickii, and C. atratus (Braithwaite 1981, Brugger & Ta- METHODS borsky 1994, Kondratiev 1991). Swans usually build large semi-floating nests, using vegeta- Study area. Field work was conducted at Carlos tion material of the environment although Anwandter Sanctuary (CAS), located at sometimes they may nest on dry land, but Cruces River (39º49’S, 73º15’W), Los Ríos always in the proximity of water (Braithwaite Region, Chile. CAS surface area covers about 1982, Owen & Black 1990, Weller 1999, Ming 4877 ha, 25 km long, and is 2 km wide on & Dai 2000). Incubation period and clutch average (Salazar, 1988). This extensive wet- size vary according to the species ranging land is part of an estuarine complex which from 1 to 12 eggs (Lack 1968, Scott 1972, includes marsh areas, calm waters, and a bot- Braithwaite 1977, Birkhead et al. 1983, Bart et tom substrate of lime and sand (Steubing et al. al. 1991, Mineyev 1991, Wlodarczyc & 1980, Ramírez et al. 1991). The weather type Wojciechowski 2001). The incubation period in this area is Csb3 according to Köppen clas- varies between 31 to 48 days (Scott 1972, sification system. The mean annual tempera- Braithwaite 1981, Birkhead & Perrins 1986, ture is 12.1°C and the mean annual rainfall is Hawkins 1986). The cygnets stay with their 2415 mm (Huber 1970, 1975). There are no parents at least six months (Scott 1977). dry months, but the highest rainfall occurs Swans are highly territorial during egg laying during winter (Di Castri & Hajek 1976). The and incubation period, only for C. atratus flora comprises 90 species, with more than there are reports of natural breeding colonies 30% of alochtonuous elements. Amongst the (Braithwaite 1982) although solitary nesting is submerged and flooded plant species Egeria also common (Miers & Murray 1969). densa, Elodea canadensis, Myriophyllum aquaticum, Most of Black-necked Swan (Cygnus mela- and Ludwigia peploides are dominant. Amongst nocoryphus) basic reproductive biology in the the marshland plants Scirpus californicus and wild is, unlike in the other species of swans, Juncus procerus are widely dominant (Ramírez et unknown (Canevary & Narosky 1995). The al. 1991, San Martín et al. 2000). Black-necked Swan is the only representative species of the genus in the Neotropics, its dis- Breeding parameters. Two breeding parameters tribution comprises southern Brazil, Paraguay, were evaluated, extent of the breeding period coastal Uruguay, almost all Argentina, and and clutch size. We analyzed data collected by Central and southern Chile (Schlatter et al. CONAF (Corporación Nacional Forestal) 1991a, Rodríguez et al. 2006). The Black- park rangers at CAS, on a monthly basis sur- necked Swan (BNS) is an efficient grazer, veys during 18 BNS breeding seasons (1986– consuming large amount of aquatic plants 2004), following the established protocol 556 BREEDING BIOLOGY OF BLACK-NECKED SWANS FIG. 1: Population estimators for 18 Black-necked Swan breeding seasons. Data collected from 1987– 2004 at Carlos Anwandter Santuary by CONAF park rangers; shown are duration of breeding seasons (bars) and number of nests (line) found during each season. described in Schlatter et al. (1991a) and Schlat- pair members as separate individuals, but ter & Mansilla (1998). simultaneously. The sex was determined based on the corporal size, coloration, and Breeding behavior. The data corresponding to size of the caruncle (Scott 1972, Nascimento breeding behavior were collected during the et al. 2001, MSG unpubl.). The individuals breeding season from September 2003 to Jan- were identified based on the pattern of the uary 2004. Black-necked Swans inhabit this eye lines (Seijas 1996). The observations were site during the whole year; however, the popu- made during the daylight hours, divided into lation size is fluctuating with a maximum in three periods: 09:00–12:00 h; 13:00–16:00 h; spring (September–December) and a mini- and 17:00–19:00 h. Each period was divided mum in winter (June–August). Six breeding in 30 intervals of 10 min, and recordings pairs were selected considering the visibility were made for each minute. The individual’s of their nests from inland. One nest was in positions was registered with regard to the the laying phase and the other five nests were nest distance: i) distance 0, individual posi- in the incubation stage when found. A kayak tioned on the nest; ii) distance 1, individual was used to visit the nests on a weekly basis positioned between 0.5 and 1 m in relation to after beginning the observations. Pairs were the nest; iii) distance 2, individual positioned observed through binoculars (10x42 mm) and between 1.0 and 10 m in relation to the nest; spotting scope (15–60x60mm) at distances of and iv) distance 3, individual positioned 100–300 m in order to keep disturbance levels between 10 m and beyond in relation to the at minimum. Instantaneous focal animal sam- nest. The activity or predominant state was pling (Altmann 1974) was used to record the also registered: i) incubation, individual stay- behavior of the male and female of each pair. ing on the nest, ii) resting, individual remain- All registrations were made considering the ing with the neck folded and resting on the 557 SILVA ET AL. FIG. 2. A) Relation between number of Black-necked Swan nests and breeding season duration at Carlos Anwandter Sanctuary. Linear regression shows a significant relation with r2 = 0.55. B) Relation between number of nest and the beginning of breeding season.