An ectoparasite of house-martins, sand-martins and swallows, often present in nests parasitising juveniles.  The body is flattened and the wings are reduced and about 6x as long as wide.  It has strong claws to hook into the host bird. 

Photographs of a specimen.  In the comments, state species of bird or nest in which it was found

Martins and swallows, often in nests