Revista Chilena de Ornitología 26(1): 33-36 Unión de Ornitólogos de Chile 2020 COMUNICACIÓN BREVE 33

COMPOSITION AND STRUCTURE OF FLOCKS IN A TEMPERATE FOREST OF CENTRAL CHILE

Composición y estructura de bandadas en el bosque templado de Chile central

FERNANDO MEDRANO1,2, MARÍA A. VUKASOVIC3,4, ROMINA CHIAPPE3,4 & CRISTIÁN F. ESTADES3,4

1Departamento de Biología Evolutiva, Ecologia i Ciències Ambientals, Institut de Recerca de la Biodiversitat (IRBio), Facultat de Biología, Universitat de Barcelona, Barcelona 08028, España. 2Red de Observadores de Aves y Vida Silvestre de Chile. Santiago, Chile. 3Unión de Ornitólogos de Chile (AvesChile). Santiago, Chile. 4Laboratorio de Ecología de Vida Silvestre. Facultad de Ciencias Forestales y Conservación de la Naturaleza. Santa Rosa #11315. La Pintana, Santiago, Chile.

Correspondencia: F. Medrano, [email protected]

RESUMEN.- Formar bandadas es una adaptación de las aves para aumentar la eficiencia de forrajeo y disminuir la depredación. Pese a que en bosques tropicales existen numerosos estudios sobre la formación de bandadas, en bosques templados de Sudamérica esta conducta ha sido escasamente documentada. En este estudio describi- mos la composición de 77 bandadas encontradas en un paisaje forestal de la región del Maule entre 1999-2002, y analizamos si existen relaciones entre las abundancias dentro de las bandadas entre especies. Usando simu- laciones de Monte Carlo analizamos la prevalencia de diferentes especies a formar bandadas y relacionamos la abundancia de las especies entre sí dentro de las bandadas. Cada bandada tuvo en promedio 10,5 individuos y la especie más frecuente fue Aphrastura spinicauda. Pese a que se ha señalado a A. spinicauda como especie nuclear, su abundancia sólo se relacionó con la de Pygarrichas albogularis. Por otro lado, se sugiere que la pre- sencia de Anairetes parulus estimularía a otras especies a unirse a la bandada. Nuevos estudios son necesarios para conocer mejor la composición y estructura de las bandadas en otros sitios en Sudamérica.

Manuscrito recibido el 7 de diciembre de 2019, aceptado el 17 de mayo de 2020.

Bird flocks can be defined as groups of foraging to- ovenbirds, woodpeckers, tanagers, wrens, and tyrant-fly- gether within and among resource patches (Morse 1970, catchers being the most frequently studied species (e.g., Srihdar 2009, Colorado 2013). This behavior is consid- Davis 1946, Short 1961, Wiley 1971, Munn & Terborgh ered a strategy by individual birds to increase their forag- 1979, Alves & Calvacanti 1996, Maldonado-Coehlo & ing efficiency by reducing the risk of predation, despite Marini 2004). However, in the temperate forests of South the fact the individuals could compete among themselves America, there are only two of such studies, both in the (Morse 1977, Sridhar 2009, Colorado 2013). These flocks Argentinian Patagonia (Vuilleumier 1966, Ippi & Trejo can be monospecific (i.e., composed of individuals of one 2003). These flocks were composed of ovenbirds, tyrant- species) or mixed, in which case there are two or more flycatchers, woodpeckers, siskins, and tanagers - (Vuil species involved (Hino 2000, Sridhar et al. 2009). Mixed leumier 1966, Ippi & Trejo 2003). This note contributes flocks are believed to function as a mechanism to obtain to reducing this information gap by describing and ana- the benefits of participating in a bird flock while avoiding lyzing the composition and the structure of flocks found intraspecific competition for food (Morse 1977, Powell in forests in the Mediterranean region of central Chile. 1985, Sridhar et al. 2009). The study was conducted in the Maule region coastal The composition and structure of flocks in tropical re- range, Central Chile. The landscape is dominated by pine gions of South America has been widely researched with plantations (Pinus radiata), with the presence of a few 34 COMUNICACIÓN BREVE Medrano et al.

Table 1. Comparison between the observed and the expected abundances within winter flocks (N=77) in the bird communities of tem- perate forests of the Maule Region between 1999-2002. The expected abundance was modeled through a Monte Carlo analysis. In the “Relative abundance in flocks” column, numbers in bold represent different values than those expected by chance in the Monte Carlo simulation. Mean abundance of in- Expected mean abun- Common name Scientific name dividuals per flock (N) dance in flocks (5-95% CI) Chilean Flicker Colaptes pitius 0.05 0.07 (0.01-0.12)

Thorn-tailed Rayadito Aphrastura spinicauda 4.46 1.41 (1.20-1.61)

White-throated Treerunner Pygarrichas albogularis 1.28 0.24 (0.14-0.33)

Des Murs’ Wiretail Sylviothorhynchus desmursii 0.37 0.32 (0.22-0.42)

Plain-mantled -spinetail Leptasthenura aegithaloides 0.25 0.14 (0.07-0.21)

Dusky-tailed Canastero Pseudasthenes humicola 0.07 0.18 (0.09-0.26)

Tufted Tit-tyrant Anairetes parulus 3.29 4.54 (4.23-4.84)

Patagonian Tyrant Colorhamphus parvirostris 0.04 0.56 (0.43-0.69)

Dusky Tapaculo Scytalopus fuscus 0.04 0.41 (0.29-0.53)

Ochre-flanked Tapaculo Eugralla paradoxa 0.04 0.14 (0.07-0.21)

Chestnut-throated Huet-huet Pteroptochos castaneus 0.01 0.10 (0.04-0.16)

Southern House Wren Troglodytes musculus 0.25 0.92 (0.74-1.09)

Patagonian Sierra-finch Phrygilus patagonicus 0.51 0.21 (0.12-0.29) scattered deciduous Nothofagus forest patches (Donoso ized linear models assuming a negative binomial distribu- & Lara 1995, Estades & Temple 1999). While conduct- tion. The latter was due to the overdispersion (variance ing point counts during the Austral winters (May-August) >> mean) of count data for flocking birds (Zipkin at al. between 1999-2002, we recorded all bird flocks we en- 2014). countered, and characterized them by the number of in- During our study we recorded a total of 77 bird dividuals of each species. We considered individuals as flocks, with a mean size of 10.6 individuals (3-39). The participating in a flock when they were observed mov- species mean richness was 3.5 species per flock, with a ing together with a group of foraging birds (Sridhar et minimum of 1 (monospecific flocks) and a maximum of al. 2009). We usually lost sight of the group after a few 7 species. minutes (up to 10 min), which we considered to be a sign We recorded a total of 13 species in the studied of a moving flock, rather than a collection of birds behav- flocks, the most frequent being the Thorn-tailed Rayadito ing independently. Point counts had a 50 m fixed radius (Aphrastura spinicauda), present in 94% of the flocks and were conducted during 5 min. (mean = 4.5 individuals per flock). The second and third To analyzed whether the relative abundance of birds most frequent species were, respectively, Tufted Tit-ty- within flocks was related to their relative abundance in rant (Anairetes parulus), recorded in 91% of the flocks the study area. We used the average winter abundance (mean = 3.3 individuals per flock) and White-throated (ind/ha) for the mosaic of pine plantations and native for- Treerunner (Pygarrichas albogularis), present in the 56% ests for the aforementioned years. For field methods see of the flocks (mean = 1.3 individuals per flock). We only Estades & Temple (1999) and Estades (2001). We used recorded three monospecific flocks (3.9%), one with 5 Monte Carlo simulations to estimate confidence intervals Patagonian Sierra-finchesPhrygilus patagonicus and two for the expected frequency of birds within flocks (Manly with 5 and 6 Thorn-tailed Rayaditos, respectively. 1997). Our simulations were performed in Basic and, for The results of the Monte Carlo simulations show that each case, we used 10,000 iterations. the average abundance of most species within flocks dif- We further explored the tendency of the different fered from what was expected by chance (Table 1). The species to join flocks in consideration of the abundance Thorn-tailed Rayadito and White-throated Treerunner of the other species, by correlating their abundances in were significantly more abundant in flocks than what was the observed groups. With this purpose, we used general- expected by their relative abundances in the habitat. On Revista Chilena de Ornitología 26(1): 33-36 Unión de Ornitólogos de Chile 2020 COMUNICACIÓN BREVE 35

Table 2. Significance of the association between the abundance of different species within flocks. *, ** and ***: p<0.05, p<0.01 and p<0.001, respectively. All relationships are positive. Only species for which a significant relationship was found are shown.

Thorn-tailed White-throated Des Murs’ Plain-mantled Tufted Southern Common name Rayadito Treerunner Wiretail Tit-spinetail Tit-tyrant House Wren Thorn-tailed Rayadito **

White-throated Treerunner

Des Murs’ Wiretail *

Plain-mantled Tit-spinetail *

Tufted Tit-tyrant ***

Southern House Wren the other hand, even though the Tufted tit-Tyrant was the flock was only correlated to each other’s abundance, but second most abundant bird in the recorded flocks, it was did not seem to influence the abundance of the rest of the most abundant species in the habitat and, therefore, the species in a flock (Table 2). On the contrary, Tufted the Monte Carlo analysis indicated that its presence in Tit-tyrants joined flocks with a lower abundance than ex- flocks was lower than that expected by chance. pected by chance. However, when this species is abun- The abundance of some species within flocks was dant, it might be an incentive to forming flocks of birds positively correlated with the abundance of other species with similar foraging strategies (Table 2) and, therefore, (Table 2). Two groups of associations among birds were this species could be considered “nuclear” for foliage evident. First, the positive correlation between the Thorn- gleaning birds. This agrees with the results of Sridhar et tailed Rayadito and the White-throated Treerunner, two al. (2012), who, by using meta-analysis techniques found species that forage mostly on trunks and branches (al- that species within mixed flocks usually have similar for- though they present differences in the micro-habitat use, aging behavior. as shown in Thomson et al. 2014), but which could be Species from the Rhinocryptid family (Dusky Tapa- foraging on different items. The second group suggests culo Scytalopus fuscus, Ochre-flanked TapaculoEugralla the potential importance of the Tufted tit-Tyrant as an at- paradoxa, and Chestnut-throated Huet-huet Pteroptochos tractor for other foliage gleaners such as the Des Murs’ castaneus) rarely joined mixed flocks, which agrees with Wiretail (Sylviorthorhynchus desmursii), the Plain-man- the lack of records by Vuilleumier (1967) and Ippi & Tre- tled Tit-spinetail (Leptasthenura aegithaloides), and the jo (2003) of these species or their sister taxa (Magellanic Southern House Wren (Troglodytes aedon musculus). No Tapaculo Scytalopus magellanicus and Black-throated negative correlations were detected between the abun- Huet-huet Pteroptochus tarnii) in the flocks studied by dances of species. them. The fact that all these birds are. These are highly In the studies conducted in southern temperate for- territorial birds all year long. might explain their relative ests by Vuilleumier (1967) and Ippi & Trejo (2003), both reluctance to join a moving flock, instead only following Thorn-tailed Rayadito and White-throated Treerunner the other birds when they pass through their territories. were the most frequent and abundant species in winter Finally, we did not record some within the mixed-species flocks. Interestingly, the Tufted Tit-tyrant flocks, such as the Fire-eyed Diucon Xolmis( pyrope) and was not recorded in the flocks studied by these authors, Austral Thrush (Turdus falcklandii). Probably flocking and in a similar study conducted in Polylepis besseri for- by these species does not present any adaptative benefit ests in Bolivia, the species was present in only 3% of the in our study area; the Fire-eyed Diucon was already re- recorded flocks (Herzoget al. 2002). These observations, corded in mixed flocks in Bariloche (Vuilleumier 1967). plus our results, suggest that Tufted Tit-tyrants may join However, it would be necessary to gather information mixed flocks opportunistically. Thus, the presence of this in other environments to understand if the behavior of species in 90% of the flocks recorded by us is likely due avoiding flocks is consistent. to the fact that, roughly, one out of two birds in our study This note gives new insights on the forest birds that site corresponded to Tufted Tit-tyrants. join mixed flocks in Chile, providing additional informa- Although the Thorn-tailed Rayadito and the White- tion on their composition and structure. However, we still throated Treerunner fit the general definition of “nuclear” lack information for analyzing the differences between species (sensu Moynihan 1962), their abundances in a our study area and other habitats. 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