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Territory Size and Stability in a Sedentary Neotropical Passerine: Is Resource Partitioning a Necessary Condition?

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J. Field Ornithol. 76(4):395±401, 2005 Territory size and stability in a sedentary neotropical passerine: is resource partitioning a necessary condition? Janet V. Gorrell,1 Gary Ritchison,1,3 and Eugene S. Morton2 1 Department of Biological Sciences, Eastern Kentucky University, Richmond, Kentucky 40475 USA 2 Hemlock Hill Field Station, 22318 Teepleville Flats Road, Cambridge Springs, Pennsylvania 16403 USA Received 9 April 2004; accepted 11 April 2005 ABSTRACT. Long-term pair bonds and defense of territories year-round are common among tropical passer- ines. The boundaries of these territories tend to be stable, perhaps re¯ecting the need to defend an area that, regardless of conditions, provides suf®cient food resources. If, however, these stable territories are not, even tem- porarily, suf®ciently large, then intra-pair competition for available food may result, particularly in species with no sexual size dimorphism. With such competition, sex-speci®c differences in foraging behavior may result. Male and female Dusky Antbirds (Cercomacra tyrannina) are not size dimorphic, and pairs jointly defend territories throughout the year. Our objective was to determine if paired Dusky Antbirds exhibited sex-speci®c differences in foraging behavior. Foraging antbirds were observed in central Panama from February±July 2002 to determine if pairs par- titioned food resources. Males and females exhibited no differences in foraging behavior, with individuals of both sexes foraging at similar heights and using the same foraging maneuvers (glean, probe, and sally) and substrates (leaves, rolled leaves, and woody surfaces). These results suggest that Dusky Antbirds do not partition resources and that territory switching, rather than resource partitioning, may be the means by which they gain access to additional food resources. SINOPSIS. TamanÄo del territorio y estabilidad en un paserino neotropical sedentario: es una con- dicioÂn necesaria la reparticioÂn de recursos? El mantener la pareja por mucho timepo y la defensa del territoria a traveÂs de todo el anÄo es comuÂn en los paserinos neotropicales. Los bordes de estos territorios tienden a ser estables, tal vez como un re¯ejo de la necesidad de defender un aÂrea que no empece a su condicioÂn, provee de su®cientes recursos alimentarios. Sin embargo, si estos territorios estables (auÂn de forma temporal) no fueran lo su®cientemente grandes, entonces pudiera haber como resultado competencia entre parejas por los recursos alimentarios, particularmente en especies en donde no hay dimor®smo sexual en el tamanÄo. Con esta competencia, pudieran surgir diferencias especi®cas en la conducta de forrajeo por parte de ambos sexos. El macho y la hembra de Cercomacra tyrannina, no son dimoÂr®cos y ambos de®enden el territoria a traveÂs del anÄo. Nuestro objetivo fue determinar si los miembros de esta especie, mostraban diferencias sexuales en su conducta de forrajeo. La conducta alimenticia de estas aves fue estudiada en la parte central de PanamaÂ, de febrero a julio de 2002, para determinar si habõÂa reparticioÂn de recursos entre la pareja. No se encontro diferencias en los patrones de forrajeo entre los sexos, con ambos individuos forrajeando a alturas similares y utilizando la misma conducta y sustrato para conseguir alimento. Estos resultados sugieren que en la especie no hay reparticioÂn de recursos, y que el cambio a otros territorios, al menos en este caso, pudiera ser la forma en donde las aves ganan acceso a otros recursos alimenticios. Key words: Cercomacra tyrannina, Dusky Antbird, foraging, sex-speci®c, territorial Long-term associations of males and females, boundaries is that territory size is a response to often on year-long territories, are prevalent potential ¯uctuations in foraging conditions among passerines in tropical habitats (Willis (Holmes 1966), and territories include the and Oniki 1972; Buskirk 1976; Freed 1987). minimum area necessary to provide suf®cient Among insectivorous passerine species, 65% in foraging habitat (Greenberg and Gradwohl Panama (Morton 1980) and 40% in tropical 1986, 1997). Energetics (i.e., foraging habitat South Africa (Rowan 1966) have long-term required to meet metabolic requirements) may pair bonds and stable territory boundaries. One prevent territories from becoming smaller, possible explanation for stability of territory while competition with neighboring conspecif- ics plus the increased cost of defending larger territories may prevent them from becoming 3 Corresponding author. Email: gary.ritchison@ larger (Hixon 1980). While this interaction be- eku.edu tween energetics and competition may usually 395 J. Field Ornithol. 396 J. V. Gorrell et al. Autumn 2005 generate territories with suf®cient resources to (82%), suggesting that Dusky Antbirds occupy support a resident pair, the stability of territory territories that enhance adult survival and long- boundaries means that, if resource levels de- term reproductive success rather than short- cline, pairs cannot gain access to additional re- term reproductive success (Morton and Stutch- sources by expanding their territories. However, bury 2000). In support of this hypothesis, under such conditions, pairs may reduce com- following experimental removals and natural petition for limited resources by intersexual par- disappearances, most antbirds abandoned ter- titioning of foraging habitat. ritories and mates and paired with other indi- In temperate areas, intersexual partitioning viduals in territories with more foraging sub- of foraging habitat through variation in forag- strate (Morton et al. 2000). Such behavior sug- ing behavior has been documented in a number gests that availability of foraging substrate may of woodpeckers (Selander 1966; Ligon 1968; be a limiting factor in some territories. If so, Pasinelli 2000) and songbirds (Williamson pairs of Dusky Antbirds may minimize within- 1971; McEllin 1979; Holmes 1986; Sodhi and pair competition for food resources by means Paszkowski 1995; Kelly and Wood 1996). of sex-speci®c differences in foraging behavior. Paired males and females partition foraging The objective of our study was to determine if habitat either by vertical segregation or by dif- pairs of Dusky Antbirds partition food resourc- ferential use of available substrates or foraging es within their territories and, if so, how. techniques (Noske 1986; Kelly and Wood 1996). METHODS Intersexual habitat partitioning has been documented in tropical species, but reported This study was conducted from February± sexual differences in foraging behavior and diet July 2002 in central Panama, 30 km north of have largely been attributed to differences in Panama City, in the Soberania National Park either size or morphology. For example, male near Gamboa (9879N, 798409W) and in forest- Montezuma Oropendolas (Psarocolius montezu- ed areas in Gamboa. Both study sites were sec- ma) are much larger than females, and inter- ond growth (70±100 yr) and consisted of mesic sexual differences in foraging behavior are clear- forest with abundant edge. The area receives an ly related to differences in size rather than com- average of 2.6 m of rain per yr and the climate petition for resources (Webster 1997). Similar- is strongly seasonal (Levings and Windsor ly, female Frill-necked Flycatchers (Arses 1985). In 2002, the dry season in Panama be- telescopthalmus) sally from leafy substrates in the gan on 18 January and ended on 21 May upper canopy, while males forage on tree trunks (Smithsonian Tropical Research Institute's Ter- and vine surfaces in the understory (Bell 1982). restrial Environmental Sciences Program). But this difference is apparently due to mor- Between 3 February and 11 March 2002, phological differences (Bell 1982). Little is Dusky Antbirds (N 5 34; 17 males, 17 fe- known about the foraging behavior of males males) were captured in mist nets using play- and females in species with no size dimorphism back of conspeci®c songs, and individuals were or other morphological differences. In one of banded with unique combinations of colored the few studies of such species, Gradwohl and plastic bands to permit individual identi®ca- Greenberg (1984) found no differences be- tion. Three females and one male had been col- tween male and female Checker-throated or-banded previously (Morton and Stutchbury Antwrens (Myrmotherula fulviventris) in the 2000). Four additional birds (three males and types of leaves searched. one female) were captured and banded on 6 Dusky Antbirds (Cercomacra tyrannina)are and 7 June 2002 as replacements for birds that suboscine passerines (family Thamnophilidae) had disappeared. For each captured bird, we that occupy forest edge and forage for arthro- measured bill length (from the tip of the upper pods on vines and brush (Morton and Derrick- mandible to the gape), bill depth and width son 1996). Males and females are similar in size (measured at the anterior of the nares), and cul- (Ridgely and Gwynne 1989), and pairs jointly men length (the distance from the front edge defend territories throughout the year (Morton of the nares to the tip of the upper mandible). and Stutchbury 2000). Annual reproductive In addition, to increase our sample size, we success is low (8%), and adult survival is high measured the bills of 21 male and 16 female Vol. 76, No. 4 Resource Partitioning in Antbirds 397 Dusky Antbirds in the ornithology collection at ing some foraging attempts, multiple maneu- the Museum of Natural Science at Louisiana vers were used. State University in June 2003.
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