I believe that this is a 1st Summer Great Spotted Cuckoo. A juvenile would have an all black head and rufous primaries while an adult would have an all grey head with much greyer upperparts.

They eat mainly insects, especially large hairy caterpillars and tend to parasitise the nests of magpies and other crows. Unlike other cuckoos it tends to tolerate the offspring of its host rather than chucking them out of the nest.

There are illustrations in HBW, Volume 4, Pages 534, 539 and 546.

Additional information is available at the University of Michigan site.