During the four and a half months Siege of Paris in the Franco-Prussian War normal channels of communication were interrupted and the only way for the provincial government in Tours to communicate with Paris was by pigeon post.
French photographer and inventor René Dagron "proposed to the authorities to use his microfilming process to carry the messages by carrier pigeons. Rampont, the man in charge of the carrier pigeon program, agreed and a contract was signed on 11 November. According to the contract Dagron was to be paid 15 francs per 1000 characters photographed. A clause in the contract, signed by an official named Picard, gave Dagron the title of "chief of the photomicroscopic correspondence postal service" mentioning in French: 'M. Dagron a le titre de chef de service des correspondences postales photomicroscopiques. Il relève directement du Directeur Général des Postes,' which translates as 'Mr. Dagron has the title of the chief of the photomicroscopic correspondence postal service. He reports directly to the Director General of the Post Office.'
"After a period of difficulties and through hardships brought on by the war and the lack of equipment, Dagron finally achieved a photographic reduction of more than 40 diameters. The microfilms so produced weighed approximately 0.05 grams each and a pigeon was able to carry up to 20 at a time. Up to that point a page of a message could be copied in a microfilm approximately measuring 37 mm by 23 mm but Dagron was able to reduce this to a size of approximately 11 mm by 6 mm which was a significant reduction in the area of the microphotograph.
"Dagron photographed pages of newspapers in their entirety which he then converted into miniature photographs. He subsequently removed the collodion film from the glass base and rolled it tightly into a cylindrical shape which he then inserted into miniature tubes that were transported fastened on the wings of pigeons. Upon receipt the microphotograph was reattached to a glass frame and was then projected by magic lantern on the wall. The message contained in the microfilm could then be transcribed or copied. By 28 January 1871, when Paris and the Government of National Defense surrendered, Dagron had delivered 115,000 messages to Paris by carrier pigeon" (Wikipedia article on René Dagron, accessed 04-26-2009).
J. D. Hayhurst, The Pigeon Post into Paris 1870-1871 (1970) provides a comprehensive account, and reproduces a number of original documents including photomicrographs.