Water depth selection during foraging and efficiency in prey capture by the egrets Casmerodius albus and Egretta thula (Aves, Ardeidae) in an urban lagoon in Rio de Janeiro State, Brazil

Abstract

ABSTRACT. This study aimed to evaluate the water depth selection during foraging, the efficiency in prey capture, and the food items captured by Casmerodius albus (Linnaeus, 1758) and Egretta thula (Molina, 1782). The work was conducted at an urban lagoon, Lagoa Rodrigo de Freitas, Rio de Janeiro. Four transects were made each month (two in the morning and two in the afternoon) for six months. When the birds were detected foraging, the water depth and the types of prey captured were recorded. There was no significant relationship between the foraging efficiencies of the two species. However, they differed in relation to the water depth when foraging, and also in the food items captured. Casmerodius albus captured mainly fishes while Egretta thula captured mainly invertebrates. The results suggest that the differences in water depth when foraging and the food items captured allow a differential use of the food resources available by C. albus and E. thula at Lagoa Rodrigo de Freitas. KEYWORDS. Foraging depth, Casmerodius albus, Egretta thula, Lagoa Rodrigo de Freitas.RESUMO. Seleção de profundidade da água durante o forrageamento e eficiência na captura de presas por Casmerodius albus e Egretta thula (Aves, Ardeidae) em uma lagoa urbana no Estado do Rio de Janeiro, Brasil. Este estudo teve o objetivo de avaliar a seleção de profundidade durante o forrageamento, a eficiência na captura de presas e os itens alimentares capturados por Casmerodius albus (Linnaeus, 1758) e Egretta thula (Molina, 1782). O trabalho foi realizado em uma lagoa urbana, Lagoa Rodrigo de Freitas. Durante seis meses foram realizadas quatro transecções (duas de manhã e duas à tarde). Quando as aves foram avistadas forrageando, foram registradas a profundidade da água e o tipo de presa capturada. Não foi encontrada relação estatisticamente significativa entre a eficiência de forrageamento para as duas espécies de aves. Entretanto, as espécies diferiram significativamente em relação à profundidade da água durante o forrageamento e também em relação aos itens alimentares capturados. Casmerodius albus capturou principalmente peixes, enquanto E. thula capturou principalmente invertebrados. Os resultados sugerem que diferenças na profundidade da água durante o forrageamento e nos ítens alimentares capturados permitem um uso diferencial dos recursos alimentares disponíveis para C. albus e E. thula na Lagoa Rodrigo de Freitas. PALAVRAS-CHAVE.Profundidade de forrageamento, Casmerodius albus, Egretta thula, Lagoa Rodrigo de Freitas.

Ecología trófica de la Garza blanca Ardea alba (Pelecaniformes: Ardeidae) en un humedal del río Paraná, Argentina

“…La Garza Blanca (A. alba) presentó una dieta predominantemente piscívora, en acuerdo con lo informado para la especie en otras áreas (Ramo & Busto, 1993;Smith, 1997;Ntiamoa -Baidu et al, 1998;Yanosky et al, 2000;Marín et al, 2003;Moreno, Lagos & Alves, 2004). Del mismo modo, el rango de tamaños de presa consumidas fue muy similar al reportado por Ramo y Busto (1993) para la especie en México.…”

Prey-capture efficiency between juveniles and adults, feeding habitat and abundance of Wattled Jacana foragers in northern Pantanal, Mato Grosso state, Brazil

“…That said, there may be variations in the spatial distribution of the macrophyte bank, since the plants may become more agglomerated in the regions closer to the margin. Thus, it seems that what characterizes the distribution of foragers on feeding habitats is not the distance from the edge and the depth, but possibly the presence of aquatic plants clustered along the banks of the swamps (Moreno et al, 2005). In any case, this idea has yet to be tested following a specific protocol and considering the factors raised above, to assess habitat selection by foraging.…”

“…Moreover, Robinson and Holmes (1982) while studying birds of forest environments, found that structural characteristics of the foraging environment, as well as the capture tactics, can also determine which and how many prey are captured. The structure of the foraging environment also determines the choice of where (spatial distribution) and when (temporal distribution) will forage (Moreno et al, 2005).…”

Temporal influence on foraging strategies, territoriality and nomadic tendencies of Snail Kite, Rosthramus sociabilis (Viellot, 1817) in an urban Neotropical wetland

“…Although they are one of the richest habitats for birds, the Neotropical wetlands have been poorly studied, mainly in areas under human influence (Alves and Pereira 1998;Moreno et al, 2004;Pimenta et al, 2007;Silva and Blamires, 2007). It is also remarkable in studies focused on freshwater bird raptors and the variation in predator densities, together with the distribution of prey availability, and their influence on territoriality and stay/leave decisions.…”