Foraging activity of the snail kite, Rostrhamus sociabilis (Aves: Accipitridae) in wetlands of southern Brazil

Abstract

The snail kite (Rostrhamus sociabilis) is widely distributed in the American continent. Its specialised diet consists mostly of the gastropod mollusk Pomacea sp and its foraging strategy probably varies depending on the season, prey availability, and climate factors, which can be reflected in its semi-nomad behaviour. This study was aimed at examining the hunting strategy of the snail kite, and its association with climate factors and habitat heterogeneity. Direct observations of birds between January 2010 and March 2011 in southernmost Brazil revealed that hunting was still the predominant foraging strategy (79% of records) to capture mollusks. Despite morphological specialisations to extract mollusks from the shells, the handling time (average = 92.4 s) was twice as much the time between prey search and capture (average = 55 s). The increase in the number of mollusks ingested apparently occurs when the resting time on perches or any other substrates near the hunting sites decreases between successive unsuccessful attempts. The correlation between the number of consumed preys and the climatic variables examined was low. Regarding habitat heterogeneity, our findings suggest that birds forage preferentially in marshes with low vegetation, which may increase the access to mollusks. The hunting efficiency of the snail kite was high (76 % successful attempts) compared to those of other birds of prey.Keywords: feeding, wetlands, predation, trophic ecology. Atividade de forrageio de gavião-caramujeiro, Rostrhamus sociabilis(Aves: Accipitridae) em áreas úmidas do sul do Brasil ResumoO gavião-caramujeiro (Rostrhamus sociabilis) possui ampla distribuição no continente americano e apresenta dieta especializada no molusco gastrópode Pomacea sp. Possivelmente, a estratégia de forrageio da espécie responde às variações sazonais na disponibilidade de presas, bem como aos fatores climáticos, o que pode refletir em seu comportamento seminômade. O objetivo desse estudo foi avaliar a estratégia de caça do gavião-caramujeiro, buscando suas associações quanto aos aspectos climáticos e à heterogeneidade do habitat. Observações diretas das aves, feitas entre janeiro de 2010 e março de 2011 no extremo sul brasileiro, revelaram que, para a captura dos moluscos, as aves usaram predominantemente a estratégia de caça do tipo still hunting (79% dos registros). Apesar de apresentar especializações morfológicas para a extração dos moluscos da concha, a avaliação do tempo empregado na manipulação (média = 92,4 s) foi quase o dobro do desprendido entre a procura e a captura (média = 55 s) das presas. Aparentemente, o aumento no número de moluscos ingeridos ocorre por meio de redução no tempo de repouso entre sucessivas investidas de captura mal sucedidas, tal como pelo uso de poleiros ou quaisquer outros substratos próximos aos locais de captura. O número de presas consumidas apresenta baixa relação com as variáveis climáticas testadas. Quanto à heterogeneidade do habitat, os dados sugerem que as aves forrageiam preferencialmente em ba...

Variable Seasonal Movement Dynamics among Individual Snail Kites (Rostrhamus sociabilis) in South America

“…data). Given that snails can be difficult to detect, excessive vegetative cover may JUNE 2021 161 SOUTH AMERICAN SNAIL KITE MOVEMENTS inhibit foraging performance (Bergmann et al 2013), which may explain the smaller numbers of tracked Snail Kites that returned to the study site in 2018. Furthermore, Snail Kites can be locally absent during prolonged droughts in northern Argentina (Contreras et al 1990, Di Giacomo andKrapovickas 2005), an observation that agrees with our finding that some individuals moved more in one year than others and that suggests a promising direction for future research into the proximate drivers of kite movements.…”

“…Snail Kites are highly social semicolonial breeders, nesting and foraging in freshwater wetlands, including shallow lake edges and freshwater marshes (Reichert et al 2020) that are usually characterized as palustrineemergent wetlands with long hydroperiods (Cowardin et al 1979). Vegetation typically is interdigitated throughout such wetlands, with low trees and shrubs often interspersed (Cowardin et al 1979, Bergmann et al 2013. Snail Kites search for prey, primarily Pomacea spp.…”

Microhabitat use by three species of egret (Pelecaniformes, Ardeidae) in southern Brazil

“…Some fishes, including a potential prey of A. alba at study site (Rhamdia quelen; Garcia et al, 2006), demonstrate lower levels of mobility during warmer days (Gomes et al, 2000). Despite being a little speculative, we believe that egrets should reduce the foraging activity during hottest days as a consequence to the effect of climatic conditions on their prey (see Stevens, 2002;Bergmann et al, 2013).…”

Variable Seasonal Movement Dynamics among Individual Snail Kites (Rostrhamus sociabilis) in South America

“…data). Given that snails can be difficult to detect, excessive vegetative cover may inhibit foraging performance (Bergmann et al 2013), which may explain the smaller numbers of tracked Snail Kites that returned to the study site in 2018. Furthermore, Snail Kites can be locally absent during prolonged droughts in northern Argentina (Contreras et al 1990, Di Giacomo andKrapovickas 2005), an observation that agrees with our finding that some individuals moved more in one year than others and that suggests a promising direction for future research into the proximate drivers of kite movements.…”