Great tit nestlings won’t thrive in nests of the pied flycatcher, even though the opposite has been proven successful. In order to explore some of the reasons why, we performed cross fostering experiments where great tit nestlings were temporarily placed in nests of the pied flycatcher. The experiments were conducted in two sessions, at three and ten days of age. To be able to compare nestlings of the two species directly, we created mixed groups consisting of nestlings of both species, in addition to groups consisting of great tit nestlings only. This ensured grounds for comparison with regards to prey types and feeding rate featured when parents encounters great tit nestlings only.Filming inside the nest boxes enabled us to assess properties of food loads brought to the nest by the parents, such as type and size of prey, and comparing the nestlings of the two species with regards to swallowing time.Due to the fact that great tits are considered to be caterpillar specialists, we expected great tit nestlings to measure up to pied flycatcher nestlings in their handling time of caterpillars. We also expected the great tit nestlings to experience difficulties when handling other prey items such as flies and wasps.This held true, the species differed markedly in their handling time of prey; both when considering all prey types as a whole and when segregated into groups. Great tit nestlings were slower than their nest mates in both age groups, and this difference was correlated to prey type. While the pied flycatcher nestlings improved their ability to swallow flies with age, this did not apply to great tit nestlings. Although the species differed in their swallowing time of larvae at three days of age, this difference was non-existant at ten days of age, which was as predicted from the assumption that great tits are adapted to swallowing larvae. In addition, testing, cases where the parent lowered its bill into that of the nestling and then raised it again without delivering the food, was by far more extensive during feeding attempts to great tit nestlings than to pied flycatcher nestlings. This applied to both age groups, indicating that the parents assessed the swallowing ability of the former nestlings to be poorer.Problems related to prey type and swallowing constraints may help to explain why great tits do not parasitize pied flycatcher broods.