Digestibility of prey by the White Stork (Ciconia ciconia) under experimental conditions

Abstract

Food composition of the White Stork Ciconia ciconia has been widely studied by pellet analysis throughout Europe, but the effect of digestibility on pellet contents has remained unclear. We studied the effect of digestion on the composition of pellets (N = 480) under experimental conditions in Poznañ Zoological Garden in 2004–2005. Twenty-four captive wild-born White Storks were fed mammals, birds, amphibians, fish, insects and earthworms. Only the remains of mammals, birds and insects were found in the regurgi-tated pellets. Bones were present in 13.1% of pellets. Three chosen osteological pellet components (skulls, mandibles and innominate bones) were analyzed to determine the amount of bone loss by digestion. The digestive efficiency was greater than 95%. Based on a literature review, we compared results of pellet analyses with methods not affected by digestion (direct observations and prey remains). The invertebrate : vertebrate ratio in the White Stork diet differed significantly between methods. Prey biomass rather than the number of prey items should be considered a better reflection of the diet.

Intersexual patterns of the digestive tract and body size are opposed in a large bird

“…Furthermore, during the breeding season, males spend more time outside the nest, so their poorer food digestibility can be compensated for by a longer foraging time and access to a more diverse diet. This is supported by the positive correlation between digestive efficiency and changes in body mass in males, and the opposite, negative relationship in females, which suggests a different feeding pattern between male and female White Storks 8 , 9 , 33 , 44 . Alternatively, the nutrient absorption capacity of the digestive system may be reduced in species with shorter intestines, resulting in longer absorption processes 47 .…”

“…Experimental studies on the digestive efficiency and food preferences of male and female White Storks show that digestion is significantly more efficient in females than in males. Moreover, males produce more pellets than females, even though they spend less time foraging than females 8 , 9 . This suggests that males, with their shorter intestines, transport less digestive content than females, thus regaining their optimal body mass more quickly for further foraging activities; this is an adaptation to ensure optimal body mass for hunting 1 , 21 .…”

“…Similarly, female White Storks digest food more efficiently thanks to their longer intestines. This is particularly important during the production of large eggs and the incubation period, when females, on the one hand, spend most of their time in the nest and cannot hunt, and on the other, need sufficient resources and energy to perform their parental tasks 8 , 9 , 34 , 48 . Furthermore, experimental studies have shown that female White Storks have different prey preference patterns than males, being more selective in choosing bird prey while foraging and preferring birds as food to a greater extent 9 .…”

“…Furthermore, experimental studies have shown that female White Storks have different prey preference patterns than males, being more selective in choosing bird prey while foraging and preferring birds as food to a greater extent 9 . Thus, the preference of female White Storks for bird prey may be an indication of a strategy to rapidly supplement calcium and other nutrients, which are more readily available from avian than from mammalian skeletons 8 , 49 – 51 . This could be an important strategy during spring, which requires significant amounts of micronutrients and energy for activities such as egg laying and incubation 52 – 58 .…”