The influence of energy, nutritional value and noxiousness of prey in sex- and size-biased predation by Snail Kites in southern South America

Snail Kites (Rostrhamus sociabilis) feed almost exclusively on Apple Snails (Pomacea spp.). While field observations indicate they discard the noxious albumen gland (AG) when feeding on female snails, there is no information on the energy lost by this behaviour, the gland composition, or whether there are associated snail sex preferences. We addressed, for the first time, these aspects for Snail Kites foraging on Pomacea canaliculata in southern South America. The whole snail’s biochemical composition exhibited significant differences between sexes. Proteins and carbohydrates were the major energy-providing components. Soft parts provide ~2.91 ± 0.16 and 2.50 ± 0.14 Kcal g⁻¹ dry weight (dw) for male and female without the AG, respectively, while the AG accounts for 15% of available energy. The size and sex of prey consumed, determined at foraging perches sampled monthly, showed that Snail Kites preferentially fed on the largest snails available throughout the sampling period. Even though the remains of the female body without the AG have less energy than males, sex preferential predation towards females was observed. Our study demonstrates, for the first time, the optimal foraging trade-off between prey size/sex and nutrition/energy in Snail Kites. After discarding the AG, male and female snails of equivalent size provide different nutrients and energy, though Snail Kite foraging is generally biased towards females mostly as a result of their larger size, suggesting that Snail Kites are unable to distinguish between the sexes.