Intestinal digestibility of great spotted cuckoo nestlings is less efficient than that of magpie host nestlings

Digestive efficiency is one of the main factors affecting nestling growth and survival. Bird species vary in their ability to assimilate different nutrients, and current theory predicts that nestlings should be able to adjust their nutritional physiology to feeding frequency, thus increasing digestive efficiency in periods of lower food availability. Here, by using a brood parasite–host study system [great spotted cuckoo (Clamator glandarius)–magpie (Pica pica)], we tested several hypotheses and predictions of the adaptive digestive efficiency paradigm. A hand-feeding experiment was carried out in which we fed both great spotted cuckoo and magpie nestlings with exactly the same diet simulating food abundance vs. food deprivation periods, and biochemical analyses including the determination of faecal endogenous excretion of N and energy were conducted. The results showed that magpie nestlings had higher apparent and true N and energy digestibility values than cuckoo nestlings during both deprivation and abundance periods, and that apparent and true N and energy digestibility values of both species increased during the period of food deprivation. These results support the current theory but contradict some previously published studies of nestling passerines based on calculations of food intake and faecal weights. Thus, the study also supports the use of biochemical analyses as a more reliable methodology for the study of digestive efficiency of nestling passerines.