Nesting sparrows have held up the completion of a #163;32 million new school building in Nailsea, near Bristol.
Construction work on the classrooms is delayed because of the pair, which have made the site their home.
The work was meant to be finished by the end of March, but this was extended until May so a state-of-the-art synthetic sports pitch could be added.
The sparrows are nesting in the old caretaker's cottage. They were spotted while on-site safety inspectors were checking for asbestos. Plastic sheeting has been put up to stop more birds joining them.
Headteacher David New said: "Nailsea School's facilities are now undeniably the best in north Somerset with the completion of the synthetic pitch.
"It was impossible to foresee nesting birds getting into the old cottages but it is only the completion of the landscaping which will be delayed." km.