Carrier pigeons, aka homing pigeons, wouldn't work too well. They have to be imprinted on what is "home". Thus they could be used to communicate back to base, but not to field agents who have to move fairly frequenty. Range can be hundreds of miles. And of course you have to transport the physcial birds from their home to the field. Still the method was used as late as WW-I by militaries, and WW-II by "embedded" reporters to file their stories when they couldn't use military radio or wire facilities.
Thank you kind kitty cat. I appreciate the info. IIRC, I remember some recent events where all the pigeons in various races disappeared. It was thought to be some magnetic field issue or other. With that in mind they wouldn't be very useful around areas where there was a deliberate disruption.