Thanks for the input, everyone. It seems that spikes do work.

Wysiwyg, I looked at the site for Fiske and Sons, but they are hundreds of miles from here, and I doubt if they would be interested in this small job.

Here's my long term plan:

1. Install spikes above south door
2. Renovate building.
3. When peace returns, install perch for red-tailed hawk, as    advocated by the Department of Conservation.

Yesterday I was waiting for the bus and saw a crow harrying a huge raptor with pale feathers under its wings - perhaps a red-tailed hawk, perhaps a harrier. It was a remarkable sight.