Carrier pigeons were already a vital part of military communications, so why not use them to for photo reconnasance?

Balloons and kites were easy to shoot down, but pigeons were not. Small light weight cameras were attached to the birds and a timer was set to take pictures ever 30 seconds as it flew (note the bird's wing tips on the edges of the photo in the lower right).

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The technology behind Google's great results
http://www.google.com/technology/pigeonrank.html