Research
INVESTIGACIÓN
Mixed-species flocks of birds
Bandadas mixtas de aves
Mangini et al 2023 MSF Types.pdfSupp. Mat.
Mangini et al 2023 MSF Types Supp Mat.pdf
A classification scheme for mixed-species bird flocks
A classification scheme for mixed-species bird flocks
The literature on mixed-species flocks references a wide variety of bird associations. These studies, however, have used an array of unstructured characteristics to describe flocks, ranging from the temporal occurrence of flocking to the identity and behavioural features of constituent members, with little consensus on which key traits define and characterize a mixed-species flock. Moreover, although most studies report species-specific roles, there is no clear consensus about what these roles signify nor how to define them. This lack of consistency limits our ability to compare flocks from different habitats, regions and species pools. To unify this sizable body of literature, we reviewed and synthesized 538 studies on mixed-species flocks.We propose 13 categories to classify mixed-species flocks using behavioural and physical traits at the flock and participant level, as well as the habitat where the flock occurs. Lastly, we discuss the historical terminology for different species roles and propose definitions to clarify and distinguish among nuclear, leader, sentinel, and flock-following species. We envision that these guidelines will provide a universal language for mixed-species flock research, paving the way for future comparisons and new insight between different regions and systems.
Bird mixed-species flock formation is driven by low temperatures between and within seasons in a Subtropical Andean-foothill forest
According to both the predation avoidance and foraging efficiency hypotheses, birds within mixed flocks increase their foraging efficiency and/or can spend more time feeding and less time looking out for predators. These hypotheses predict that birds in mixed flocks obtain benefits. Thus, mixed flock formation could serve as a strategy to cope with difficult conditions imposed on birds such as climatic conditions that ultimately result in a change in predation pressure or food resources. We evaluate the hypotheses that forming part of a flock confers benefits to its members and the associated prediction that birds will take advantage of these benefits and flock more often under cold and dry weather conditions between and within seasons to cope with such conditions. We surveyed the presence of mixed flocks, flocking propensity, number of species and individuals in mixed flocks in the Subtropical Yungas foothill of Argentina, to examine seasonality, flocking behavior of birds and their responses to two climatic variables: temperature and humidity. Bird species presented a higher flocking propensity and mixed flocks occurred more frequently during the dry and cold seasons than during the more benign seasons, and lower values of temperature within seasons triggered the flocking behavior. Although effects between seasons were expected, birds also showed a short-term response to small changes in temperature within seasons. These results strengthen the ideas proposed by the foraging hypothesis. Although benefits derived from flocking have yet to be determined, whatever they are should be understood in the context of seasonal variation in life-history traits.
Mangini and Areta 2018 MSF Temperature
Mixed-species flocking is associated with low arthropod detectability and increased foraging efficiency by Yungas forest birds in Argentina
Mixed-species flocks presumably provide birds with antipredator and foraging benefits. The foraging benefits hypothesis predicts that a reduction in arthropod abundance will trigger flocking activity; however, flocking activity may also be influenced by the difficulty of detecting arthropods, a seldom explored possibility. We found that environmental traits (temperature and foliage density) combined with arthropod abundance explained arthropod detection by birds in the Yungas foothill forest of NW Argentina. Prey detection was inversely related to ambient temperature and foliage density while positively associated with arthropod abundance. Based on this result, we built a structural equation model using a latent proxy variable for arthropod detectability, arthropod crypsis, integrating ambient temperature, foliage density, and proportion of immature arthropods. This model allowed us to compare the relative importance of arthropod abundance and the difficulty in detecting prey items as predictors of flocking propensity. After 2 yr of studying 129 mixed-species flocks, 1,351 bird foraging sequences, and 25,591 arthropod captures, we found that the flocking propensity of birds was only significantly correlated with arthropod detectability and not with arthropod abundance. Flocking propensity peaked when the arthropod community was comprised of proportionately more immature and non-flying arthropods, the temperature was low, and the foliage cover was denser; all factors are contributing to a low arthropod detectability. Finally, we evaluated whether joining mixed-species flocks provided foraging benefits such as increased foraging efficiency. Individuals benefited from joining flocks by an average increase of their prey-capture attempt rate of 40%, while the search rate increased by 16%. Our results add a new perspective on the drivers of mixed-species flocking by showing that the capacity to find prey items may have a more significant effect than prey abundance per se.
Mangini et al 2022 MSF Arthropod crypsis.pdf
Benefits of foraging in mixed-species flocks depend on species role and foraging strategy
Mixed-species flocks of birds involve species with different roles and foraging strategies and are ubiquitous in forest environments. Species roles can broadly be categorized into leaders that attract others to form the flock, as well as lead the flock’s movement; and follower species. The diversity of foraging strategies occurring in a mixed flock reflects the diversity of participant species. One of the main benefits of participating in mixed-species flocks is increased foraging efficiency. However, feeding benefits remain under debate, probably because not all participants receive this benefit in the same way. For instance, leaders and followers may benefit differentially, and particular foraging strategies might additionally influence feeding benefits. To identify leader species and evaluate foraging benefits in relation to species roles and foraging strategies, we conducted 375 h of surveys in subtropical Andean-foothill forests across 3 years. We recorded species leading the movement of the flocks and the foraging efficiency of the constituent birds. Because leader behaviour is related to the ability to attract other participants, we developed a standardized network analysis that helped us to identify them. We tested and compared whether behavioural leaders presented a higher topological position of dominance within the network. We found 35 species behaving like leaders; most could change from leaders to followers depending on the flock composition. The most frequent leader species had a higher topological dominance in the network. The foraging efficiency of leader and follower species inside mixed flocks increased compared with birds outside them. To some extent, the foraging efficiency of birds inside mixed flocks depended on their primary foraging strategy. Foliage Invertivore and Frugivore–Invertivore species tended to increase their foraging efficiency while Bark Invertivore, Aerial Invertivore and Granivore–Invertivore species did not show changes in their foraging efficiency in flocks. Although the general trend among mixed flock participants was to increase their foraging efficiency, exceptions suggest that other benefits come into play, such as predator avoidance or feeding on more profitable prey items. Our study indicates that benefits gained from foraging in mixed flocks depend on a complex combination of species roles, species-specific traits, and facilitation mechanisms linked to the primary foraging strategy of bird species.
Mangini et al 2022 Benefits and roles Ibis.pdf
Ecology of mixed-species flocks of birds across gradients in the Neotropics
Mixed-species flocks of birds have been studied for more than a century, but investigation efforts are historically unbalanced towards certain types of habitats, such as woodlands and lowland forests. Here we provide a first glance of bird flocks patterns across different gradients in recent studies conducted within the Neotropics. We summarize a symposium where a series of independent studies that approached the topic, some of them making use of techniques that were seldom applied in previous decades in Neotropical systems. We discuss bird flocks patterns across a latitudinal gradient, social network patterns in bird flocks across elevational gradients in local and regional scale, and finally, patterns of flocking response to different levels of human disturbance. Altogether, these studies offer a larger and diverse panorama of possible patterns of response and diversity of mixed-species flocks of birds in the Neotropical region, and provide a rich ground where future studies with bird flocks in the Neotropics may rely on.
Kajiki et al 2018 MSF Neotropics.pdf
Network structure of avian mixed-species flocks decays with elevation and latitude across the Andes
Birds in mixed-species flocks benefit from greater foraging efficiency and reduced predation, but also face costs related to competition and activity matching. Because this cost–benefit trade-off is context-dependent (e.g. abiotic conditions and habitat quality), the structure of flocks is expected to vary along elevational, latitudinal and disturbance gradients. Specifically, we predicted that the connectivity and cohesion of flocking networks would (i) decline towards tropical latitudes and lower elevations, where competition and activity matching costs are higher, and (ii) increase with lower forest cover and greater human disturbance. We analysed the structure of 84 flock networks across the Andes and assessed the effect of elevation, latitude, forest cover and human disturbance on network characteristics. We found that Andean flocks are overall open-membership systems (unstructured), though the extent of network structure varied across gradients. Elevation was the main predictor of structure, with more connected and less modular flocks upslope. As expected, flocks in areas with higher forest cover were less cohesive, with better defined flock subtypes. Flocks also varied across latitude and disturbance gradients as predicted, but effect sizes were small. Our findings indicate that the unstructured nature of Andean flocks might arise as a strategy to cope with harsh environmental conditions
Montaño et al 2023 Network-structure-of-avian-mixed-species-flocks-decays-with-elevation-and-latitude-across.pdfA little bit about birds and their behavior
Un poco de aves y su comportamiento
Nesting record of White-throated Quail-Dove Zentrygon frenata in Argentina
La Paloma Montera Grande Zentrygon frenata habita en la selva de montaña de los Andes desde Colombia hasta el noroeste de Argentina. Reportamos el segundo nido descrito de la especie y el primero documentado. Hallamos un nido de la subespecie Z. f. margaritae en el extremo sur de su distribución. La primera observación fue el 14 de noviembre de 2019 en el Parque Nacional Calilegua, Jujuy, Argentina, a 960 m de altitud. El nido tenía forma de plataforma densa, compuesto de pequeñas ramas secas, ubicado a c.3,5 m de altura y protegido por vegetación enmarañada. Pudimos observar el nido y el comportamiento de los adultos en cuatro oportunidades diferentes hasta 15 días después de haberlo encontrado. Los dos individuos de la pareja se turnaron para atender el nido y, aunque todo parecería indicar que contenía pichones, no pudimos confirmar el éxito del mismo.
Mangini et al_Cotinga 44 DIGITAL-mr-75-77.pdf
Notes on the nesting of the Rough-legged Tyrannulet (Phyllomyias burmeisteri): Phylogenetic comments and taxonomy tracking of natural history data
The Rough-legged Tyrannulet (Phyllomyias burmeisteri) is considered monotypic, yet two populations can be distinguished: the allopatric, apparently larger and longer-tailed Yungas population can be referred to as P. burmeisteri salvadorii, and the Atlantic Forest population as P. b. burmeisteri. Although a nest and egg from the Brazilian Atlantic Forest were assigned to this species, subsequent taxonomic use suggests they may belong to the Greenish Tyrannulet (Phyllomyias virescens). A nest of P. burmeisteri found on 8 September 1995 in the Atlantic Forest of Paraguay was a cup made of twigs, roots and covered in lichen, placed c. 20 m up on a horizontal bough. Another one found on 31 December 2013 in the Yungas of Argentina was a relatively large cup lined and covered with green "old-man´s beard" lichens in the upper outer rim, with abundant and relatively large flakes of glaucous foliar lichens on the sides, and placed 7.6 m above the ground on a mossy and licheny horizontal fork in a secondary horizontal branch of Myroxylon peruiferum. Both sexes contributed to nest construction and fed the nestlings, which were partially covered with grayish-white down and had black skin. We recorded an assortment of vocalizationsduring nesting duties. Our nests resemble that reported by von H. Ihering in 1900, but we cannot rule out whether his description pertains to P. virescens. The use of cup-shaped nests to establish phylogenetic relationships is at present limited, mostly by their simplicity and the lack of detailed descriptions.
Areta et al 2021 Phyllomyias burmeisteri.pdf
Observations on foraging behavior of Rufous-headed woodpecker Celeus spectabilis in the Ecuadorian Amazon
En este trabajo reportamos observaciones del comportamiento de forrajeo del Carpintero Cabecirrufo Celeus spectabilis en la Amazonía ecuatoriana. Esta especie es considerada especialista de bambú, pero nuestras observaciones aportan evidencia de que también utiliza áreas con Cecropia y sin bambú para alimentarse. Observamos a C. spectabilis alimentándose de la hormiga Camponotus balzani que habita dentro de los árboles de Cecropia. Estos árboles se dividen en múltiples secciones, y C. spectabilis se alimentó de numerosas hormigas en cada sección, antes de desplazarse a la siguiente. Presentamos nueva información sobre el comportamiento de forrajeo de C. spectabilis, especie poco conocida, y comparamos nuestras observaciones con otras descritas previamente en la literatura sobre esta especie y su especie hermana el Carpintero de Kaempfer C. obrieni.
Mangini and Thomas 2020 Celeus spectabilis.pdf
Lower breeding success in a new range: No evidence for the enemy release hypothesis in South American Barn Swallows
It is widely assumed that colonizing species thrive because they lack natural enemies in their new range, increasing their survival and reproductive success. Barn Swallows (Hirundo rustica) started to breed in South America around 1980 and since then have dramatically increased their population size and geographic range, in stark contrast to the decline of the source population in North America. The reasons behind the growth of the South American population are unknown. However, because this species had never bred in this area in recorded times, the lack of natural predators, parasites, or pathogens could lead to higher breeding success, as predicted by the enemy release hypothesis. Here, we test whether breeding success is higher in the newly colonized range than in the native range to test the enemy release hypothesis. We studied the breeding biology of South American Barn Swallows quantifying 7 breeding parameters: clutch size, overall breeding success, offspring mortality by predation and ectoparasites, number of fledglings per breeding attempt, number of breeding attempts per breeding season, and total number of fledglings produced per pairper breeding season. Additionally, we compared these parameters with published information from North American populations using meta-analyses. We found that, while clutch size did not differ between North and South American populations, the southern population overall had lower breeding success, with higher mortality from predation and ectoparasites. An egg laid in South America was 2.6 times more likely to fail than one laid in North America, which resulted, on average, in 1.1 fewer fledglings per pair per breeding season for the South American breeding population. These results, demonstrating lower breeding success in a newly colonized range, do not support the enemy release hypothesis and indicate that the growth of the South American Barn Swallow population is most likely caused by otherdemographic factors.
Gandoy et al 2019 Hirundo Lower breeding success.pdf
First nesting records of Lyre-tailed Nightjar Uropsalis lyra in Argentina with notes on its breeding biology
El Atajacaminos Lira Uropsalis lyra es un caprimúlgido andino poco conocido. En Argentina se desconocen aspectos básicos de su biología reproductiva. Presentamos notas sobre su reproducción en base a cuatro nidos ubicados en abruptos barrancos en la selva de yungas de la provincia de Jujuy. Solo la hembra cuidó y alimentó a un pichón durante todo su desarrollo. La hembra alimentó al pichón durante la noche y durante el día permanecieron juntos en reposo.
Mangini et al Uropsalis.pdf
Ecology and Behavior of Alder Flycatchers (Empidonax alnorum) On Their Wintering Grounds In Argentina
The Alder (Empidonax alnorum) and Willow (E. trailli) flycatchers are cryptic species, and their distribution outside the breeding season is poorly known, owing mostly to identification difficulties. Our new records suggest that large numbers of Alder Flycatchers overwinter in rivers crossing the Chaco region and in the foothill forests of the Yungas of Argentina, significantly increasing their southern wintering range. Records in northern Argentina span 3 November to 23 March. Key habitat for overwintering Alder Flycatchers in Argentina included stands of palo bobo (Tessaria integrifolia) along the R´ıo Bermejo and tributaries, either in association with sparse shrubs of chilca (Baccharis salicifolia) or more rarely with canebrake (Gynerium sagittatum). Other habitats used were old shrubby madrejones with Tessaria scrub and sacha cafe (Sesbania virgata), very dense Baccharis salicifolia scrub next to flowing creeks in foothill Yungas and dry Chaco, and riparian forests dominated by palo flojo (Albizia inundata) and timbo (Enterolobium contortisiliquum). Seasonal flooding of these habitats did not affect the presence of Alder Flycatchers. Small territories of c. 20320 or 25325 m were defended in Tessaria stands. Alder Flycatchers fed mostly on insects in flight (aerial hawking), but also on green Lepidoptera larvae (upward sally-strikes), and on insects on leaves, and ripe fruits of tala (Celtis cf. ehrenbergiana) in forest understory(clinging). Vocalizations given by overwintering birds (fee-bee-oo, zwee-oo, wee-oo, churr, pit, double-peak, and kitter) were similar to those used while breeding. Alder Flycatchers collected in Argentina had significantly longer wings and wider bills than specimens from the sympatric Euler's Flycatchers (Lathrotriccus euleri) from Argentina with which it has been confused in museum specimens.
Areta et al 2021 Empidonax.pdf
NUEVO REGISTRO DE LA BECASA COMUN (Limnodromus griseus) PARA ARGENTINA: IDENTIFICACIÓN E HISTORIA DE UN GÉNERO CONFLICTIVO
Para Argentina, existen citas tanto de la Becasa Gris (Limnodromus griseus), como de la Becasa Escolopácea (Limnodromus scolopaceus). Históricamente se ha citado a L. scolopaceus como representante del género en Argentina. Sin embargo en los últimos años algunos autores se han retractado en trabajos posteriores citando a L. griseus en lugar de L. scolopaceus, basándose seguramente en la Lista Comentada de las Aves Argentinas de Mazar Barnett y Pearman 2001. Trabajo que sugiere que los registros de L. scolopaceus no poseen evidencias sólidas y que probablemente se deban a que siguen la primer cita del genero para la Argentina de Zotta 1942, cuya piel depositada en el MACN como L. scolopaceus sería L. griseus. El plumaje invernal de ambas especies es casi indistinguible, según varios autores el carácter más eficiente para diferenciarlas es el canto realizado en vuelo. Luego de una búsqueda bibliográfica presentamos una serie de caracteres morfológicos con el fin esclarecer mediante futuros registros bien detallados, el status del género en Argentina. L. griseus a diferencia de L.scolopaceus posee el pico de una longitud menor a una vez y media la longitud de la cabeza, garganta blanca poco manchada, parte superior del pecho gris claro, seguido de la presencia de maculas para terminar en un abdomen blanco, base del pico relativamente ancha afinándose abruptamente y bandas blancas en las timoneras iguales o más anchas que las negras dando la apariencia de una cola más clara.
Gandoy et al 2013 Limnodromus.pdf
El aguilucho jote (Buteo albonotatus) en Argentina
El Aguilucho Jote (Buteo albonotatus) poseería dos poblaciones de cría disyuntas. Una en el sur de Estados Unidos y norte de México que migraría hacia el sur fuera de la época reproductiva, y otra distribuida en parches en el este de Panamá y América del Sur. Obtuvimos seis registros documentados mediante fotografías, cada uno correspondiente a un individuo adulto, en una localidad en la provincia de Chaco, dos en la provincia de Salta y tres en la provincia de Formosa. Los reportes no documentados pero con una descripción adecuada (e.g., Fraga & Clark 1999) y aquellos con descripciones insuficientes para identificar la especie (e.g., Borbolla & Marano 2005) cuya descripción no permite distinguir al ave en cuestión de un morfo oscuro de Aguilucho Cola Corta [Buteo brachyurus]) sirven para orientar futuras prospecciones en localidades donde la presencia del Aguilucho Jote debe ser corroborada. Las poblaciones de Aguilucho Jote del este de Panamá y todo Sudamérica han sido referidas a la subespecie abbreviatus El estatus migratorio de las poblaciones australes requiere ser aclarado. En Bolivia fue considerado con dudas como un posible migrante boreal (Hennessey et al. 2003) y existen registros recientes durante el invierno en la región chaqueña boliviana. Los registros documentados del Aguilucho Jote en Argentina abarcan los meses de febrero, julio, agosto, septiembre y octubre, indicando que la especie es residente en el país; y las observaciones sugieren lo mismo. Sin embargo, Hellmayr & Conover (1949) sospecharon que podría existir superposición de migrantes boreales con poblaciones reproductivas en Sudamérica. Con el material y observaciones disponibles no podemos descartar que exista alguna superposición de migrantes boreales y de residentes en el sur de la distribución de la especie.
Areta et al 2014 Buteo albonotatus.pdfAmphibians and Reptiles
Anfibios y Reptiles
Sleeping site fidelity in three neotropical species of herpetofauna
All animal species studied to date have shown sleep or sleep-like behaviour. Sleep aids in a variety of processes including recovery of cellular and endocrine systems through to learning, memory and energy conservation. Sleeping site selection must play an important role in obtaining the benefits of sleep while avoiding predation. It is likely influenced by microhabitat preferences, the need for protection from predators or exposure, and remaining within territories to avoid competition. Thus, once selected, a sleeping site may be maintained through days, weeks, or longer, to retain these benefits over time. However, to date studies of sleeping site preferences or site fidelity have focused mainly on endotherms (mammals and birds) rather than ectotherms such as reptiles, amphibians, fishes or invertebrates. To gather data, we undertook day and night-time visual encounter surveys between June and August 2019 in the the Sani Reserve, Sucumbíos, Ecuador. We report and discuss the sleeping behavior, sleeping site features and sleeping site fidelity for two reptiles and one amphibian. We recorded 2 sleeping sites for Enyalioides laticeps with the individual returning to the same site for 7 days; 1 sleeping site for Anolis fuscoauratus with the individual returning to the same site each night for 13 days, and 1 sleeping site for Rhinella margaritifera where we observed the same individual returning to the same site for 5 days. The information gathered here for these three species is a first step in understanding what sleeping sites they select and the degree of site fidelity they show. Further detailed work is needed to understand how these species select sleeping sites and the characteristics of sites to which they would show fidelity
Thomas et al 2021 Sleeping papers.pdf
Ranas y sapos del fondo de tu casa
Las ranas y los sapos se ven perjudicados por los cambios que se producen en los ambientes que habitan debido al creciente avance de los agroecosistemas, siendo común que las charcas donde se alimentan y donde se reproducen desaparezcan, así como también los sitios que utilizan comúnmente como refugio. Este libro acerca al lector al conocimiento básico de los anfibios en general, a las posibles causas de su declinio y a cuales son aquellas especies de anuros que puede encontrar en los agroecositemas de los alrededores de la ciudad de La plata, con el fin de que este conocimiento favorezca su conservación.
Ranas y sapos.pdfNEOTROPICAL PERSPECTIVES
Roteiro Integrado para o Turismo de Observação de Aves na Rota Bioceânica: Brasil, Paraguai, Argentina e Chile
Este trabalho tem como objetivo avaliar como a geografia territorial da Rota de Integração Latino-Americana contribui para a construção e a formatação de roteiro para o turismo de observação de aves, com enaltecimento de paisagens, de práticas culturais, de territorialidades e da sociobiodiversidade como um todo representadas na Rota. A área de estudo envolve municípios brasileiros, paraguaios, argentinos e chilenos atingidos diretamente por esse Corredor Rodoviário. O território abarcado pela Rota de Integração Latino-Americana é contemplado por vários ambientes propícios para a prática do turismo de observação de aves, em especial aqueles contemplados por Áreas Protegidas e arredores. O roteiro proposto percorre diferentes topografias, ecorregiões e ecossistemas singulares da América do Sul, tais como: Cerrado, Pantanal, Mata Atlântica, Chaco (seco e úmido), Yungas, Deserto de Montanhas, Campos de Altitude, Puna, Lagoas Altoandinas, Deserto e Vales desérticos, Costa do Pacífico e Tamarugales. Em cada localidade a biodiversidade compõe o espaço junto das práticas culturais e de territorialidades singulares. Tamanha riqueza e diversidade projeta esse percurso como um excelente roteiro para o turismo de observação de aves, pronto a ser percorrido, contemplado e apreciado em suas múltiplas possibilidades.
Mamades et al 2022 Ruta Bi-oceanica.pdf
Neotropical ornithology: Reckoning with historical assumptions, removing systemic barriers, and reimagining the future
A major barrier to advancing ornithology is the systemic exclusion of professionals from the Global South. A recent special feature, Advancesin Neotropical Ornithology, and a shortfalls analysis therein, unintentionally followed a long-standing pattern of highlighting individuals, knowledge, and views from the Global North, while largely omitting the perspectives of people based within the Neotropics. Here, we review current strengths and opportunities in the practice of Neotropical ornithology. Further, we discuss problems with assessing the state of Neotropical ornithology through a northern lens, including discovery narratives, incomplete (and biased) understanding of history and advances, and the promotion of agendas that, while currently popular in the north, may not fit the needs and realities of Neotropical research. We argue that future advances in Neotropical ornithology will critically depend on identifying and addressing the systemic barriers that hold back ornithologists who live and work in the Neotropics: unreliable and limited funding, exclusion from international research leadership, restricted dissemination of knowledge (e.g., through language hegemony and citation bias), and logistical barriers. Moving forward, we must examine and acknowledge the colonial roots of our discipline, and explicitly promote anti-colonial agendas for research, training, and conservation. We invite our colleagues within and beyond the Neotropics to join us in creating new models of governance that establish research priorities with vigorous participation of ornithologists and communities within the Neotropical region. To include a diversity of perspectives, we must systemically address discrimination and bias rooted in the socioeconomic class system, anti-Blackness, anti-Brownness, anti-Indigeneity, misogyny, homophobia, tokenism, and ableism. Instead of seeking individual excellence and rewarding top-down leadership, institutions in the North and South can promote collective leadership. In adopting these approaches, we, ornithologists, will join a community of researchers across academia building new paradigms that can reconcile our relationships and transform science. Spanish and Portuguese translations are available in the Supplementary Material.
Soares et al 2023.pdf