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ORNITOLOGIA NEOTROPICAL 23: 529–544, 2012 © The Neotropical Ornithological Society MORPHOLOGICAL AND FORAGING BEHAVIORAL DIFFERENCES BETWEEN SEXES OF THE MAGELLANIC WOODPECKER (CAMPEPHILUS MAGELLANICUS) Laura Chazarreta1, Valeria Ojeda1, & Martjan Lammertink2,3 1Departamentos de Ecología y de Zoología, Universidad Nacional del Comahue-INIBIOMA, Quintral 1250, (8400) Bariloche, Argentina. E-mail: [email protected] 2CICyTTP-CONICET, Matteri y España s/n, (3105) Diamante, Entre Ríos, Argentina. 3Cornell Lab of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY 14850, USA. Resumen. – Diferencias morfológicas y de comportamiento alimentario entre sexos del Carpintero Gigante (Campephilus magellanicus). – La divergencia ecológica producto de diferencias morfológicas y comportamentales junto con la dominancia social, promueve la separación de nichos entre sexos en aves. La ausencia de competidores favorecería estas diferencias intersexuales. El Carpintero Gigante (Campephilus magellanicus) es el único pícido de gran tamaño en los bosques de Nothofagus de la Patagonia, sin competidores. Una investigación preliminar documentó diferencias mor- fológicas entre sexos y postuló la existencia de diferencias en el comportamiento alimentario, correla- cionadas con ese dimorfismo. Nuestros objetivos fueron analizar las diferencias morfológicas e investigar si existen diferencias intersexuales en el comportamiento alimentario. Obtuvimos datos mor- fológicos de colecciones de museos y registros comportamentales de poblaciones de Patagonia Argen- tina. Estimamos amplitud y solapamiento de nichos en variables de alimentación. Los machos fueron más grandes y tu-vieron picos 12,4% más largos que las hembras. Ambos sexos se alimentaron princi- palmente sobre árboles vivos, pero los machos usaron sustratos más grandes (troncos) que las hem- bras y alturas intermedias (5–10 m), mientras que las hembras usaron sustratos más pequeños (ramas) dentro de la copa, a mayor altura (> 15 m). La tasa de captura de presas fue similar entre sexos (0,28 presas/min), pero los machos capturaron presas más grandes (larvas xilófagas) que las hembras (pre- sas superficiales). Registramos dominancia social de los machos. Según el análisis de amplitud de nicho, las hembras fueron más generalistas que los machos en el uso de microhábitats y en la posición del cuerpo mientras se alimentaban. El dimorfismo sexual en tamaño corporal y largo del pico fue acorde a la especialización por sustratos en los sexos, lo cual fue probablemente reforzado por la dominancia de los machos. La falta de competencia interespecífica en este bioma contribuiría a la diferenciación observada entre los sexos. Abstract. – Ecological differentiation arising from morphological and behavioral differences, together with social dominance, is known to promote niche differentiation between sexes in birds. The absence of competing species would favor intersexual differences. The Magellanic Woodpecker (Campephilus magellanicus) is the only large woodpecker in the southern beech Nothofagus forests of Patagonia, with no competitors. Sexual divergence in morphology had been documented by preliminary research, and differences in foraging behavior were proposed as a correlate. Our aims are to analyze intersexual diver- gence in morphology and to investigate whether the behavioral differences exist. We obtained morpho- logical data from ornithological collections and foraging records from populations from Argentine Patagonia. We estimated foraging niche breadth and the overlap of foraging variables. Adult males were larger and had bills 12.4% longer than those of females. Both sexes foraged mostly on living trees, but males foraged on larger substrates (trunks) at intermediate heights (5–10 m), while females foraged 529 CHAZARRETA ET AL. higher within the crown (> 15 m) on smaller substrates (branches). The rate of captured prey was similar between sexes (0.28 prey/min), but males consumed larger prey (wood-boring larvae) than females (near-surface prey). Social dominance by males was recorded. Based on analysis of niche breadth, females were more generalist than males in microhabitat use and body posture when foraging. Sexual dimorphism in body and bill size seems to be in line with specialization in the use of different substrates between the sexes, which is probably reinforced by male dominance. Lack of interspecific competition in this biome probably contributed to the differentiation observed between sexes. Accepted 23 December 2012. Key words: Magellanic Woodpecker, Campephilus magellanicus, Austral Temperate Forest, insular biome, niche breadth, sexual dimorphism. INTRODUCTION Woodpeckers (Picidae) are often sexually dimorphic in plumage and morphology, and Sexual dimorphism in size and morphology thus, are an interesting group for studying dif- is widespread among animals (Hedrick & ferences in foraging behaviors between male Témeles 1989) and has traditionally been and female (Catry et al. 2005). In particular, related to sexual selection, especially in bird sexual differences in bill size have been species with a polygamous mating system related to differences in feeding behavior, (Amadon 1959, Selander 1972). Nevertheless, explored habitats, and consumed items the evolutionary origin of sexual dimorphism (Selander 1966, Aulén & Lundberg 1991). has multiple pathways (Hedrick & Témeles The southern beech (Nothofagus) temperate 1989), and there are many cases where differ- forests covering southern Chile and Argen- ent size and morphology between males and tina (35°S–56°S), comprise a narrow strip ca. females can be related to ecological causes 2000 km long and up to 120 km wide that (Slatkin 1984, Shine 1989), as in Green Wood- evolved in isolation from other South Ameri- hoopoes (Phoeniculus purpureus, Radford & can forests, constituting a habitat island with Plessis 2003), Darwin’s finches (Grant 1986), high levels of endemism (Vuilleumier 1985, and Varied Sittella (Daphoenositta chrysoptera) Armesto et al. 1996). In the interior of Nothof- and treecreepers (Climacteridae) (Noske agus forests, only two woodpecker species 1986). occur, the small Striped Woodpecker (Venilior- Segregation between sexes in morphologi- nis lignarius, ~ 16 cm, 35–38 g) and the very cal traits is thought to alleviate intersexual large Magellanic Woodpecker (Campephilus competition (Selander 1966, Ruckstuhl & magellanicus, ~ 40 cm, 276–363 g) (Short Clutton-Brock 2005). The absence of com- 1982). In this particular setting, the Magel- peting species would favor a larger niche lanic Woodpecker has no potential ecological width of a species, leading to greater struc- competitor and therefore would show a broad tural dimorphism and the subdivision woodpecker niche (Short 1970a). Pioneering between sexes of the foraging niche (Selander research conducted by Short (1970a) revealed 1966, Shine 1989). Social dominance, where a noticeable sexual dimorphism in bill size the dominant sex (usually male) excludes the (males larger than females), which he pro- subordinate from favored patches or sub- posed would correlate to differences in feed- strates, explains sexual niche segregation in ing behavior and explored habitat, but this several cases (Hogstad 1978, Morse 1980, remained untested. Our aims are to analyze Peters & Grubb 1983, Matthysen et al. 1991, sexual dimorphism in morphology of Magel- Osiejuk 1994, Ruckstuhl 2007). lanic Woodpeckers and to explore sex-specific 530 DIFFERENTIATION BETWEEN SEXES OF MAGELLANIC WOODPECKERS foraging behavior and feeding habitats of this ing Baldwin et al. (1931) guidelines. We only species. used adult specimens as determined from plumage patterns (Ojeda 2004), museum METHODS labels, and size (Short 1970a). We collected the following variables from each skin: wing Study area. Study sites (n = 8) were located and tail length, culmen cord (bill length), and within the Lanín (39°35’–41°19’S) and tarsus, toe (third), and claw (third) length. Nahuel Huapi National Parks (71°17’– Some skins were damaged so we were unable 71°36’W), north and south of Bariloche city, to collect all measurements from those speci- in northern Argentine Patagonia. The climate mens (n = 7). We took most measurements to in the region is characterized by a relatively the nearest 0.01 mm when using calipers, and long winter with rain and snow and by dry to the nearest 1 mm using a ruler for wings summers, with mean monthly winter (June– and tail. August) temperatures of 2°–3°C, and summer Nearly all specimen measurements (98%) (December–March) temperatures of 12°– were taken by one of us (LC) and the remain- 14°C (data from Bariloche Airport, 41°09’S, der was taken by museum curators instructed 71°10’W). by LC. Although skin dimensions vary with Subalpine forests between 1100–1700 m specimen age, preparation technique, and a.s.l. are composed of pure stands of the storage conditions (Bortolotti 1984), we con- deciduous lenga Nothofagus pumilio. Our field sidered this variation a random effect in our sites were old-growth lenga stands over 1000 data set. ha each, which were contiguous with other Nothofagus stands. Lenga forest was selected Foraging behavior. We conducted fieldwork dur- because of the great extent of this forest type ing three consecutive years from early spring in the study area and its mostly open under- to late summer (September–April, 2008– story that allows following woodpeckers for 2010). Most behavioral data (85% of observa- monitoring their behavior. tion hours) were obtained from a population
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