The Function of Food Stores in Bird Nests: Observations and Experiments in the Barn Owl Tyto Alba

Several hypotheses have been proposed to explain why food is found uneaten in or near nests of raptors, owls and shrikes. The ‘insurance’ hypothesis states that parents store food to buffer offspring against temporary food shortages generally due to adverse weather. Under the ‘large prey’ hypothesis, the offspring’s inaptitude in dismembering large items might explain the presence of uneaten items. In the present paper, I review these hypotheses and propose a novel hypothesis called ‘feeding time’. It postulates that parents accumulate food items in their nest to allow offspring to eat at any time. Using Barn Owl Tyto alba nests, I examined predictions of these mutually non-exclusive hypotheses. Parents did not change prey delivery rate when prey remains accumulated in the nest, only 1.5 prey remains were wasted per nest during the entire rearing period, and fledging success was not greater in years when parents stored more prey items in their nest. Without being consistent with the insurance hypothesis, this observation did, however, not refute it either. In accord with the large prey hypothesis, nestlings ate small prey items before large ones. A test of the feeding time hypothesis showed that nestlings provided with food ad libitum at night, continued to spread their meals over 24 h. Hence, the large prey and feeding time hypotheses can both explain the presence of uneaten prey remains in Barn Owl nests.