Here is the text from the postal regs:
9.3.5 Adult Chickens
Adult chickens must be sent by Express Mail. The Express Mail containers used must pass the standards in
International Safe Transit Association (ISTA) Test Procedure 1A (detailed in Publication 2, Packaging for Mailing);
be designed to remain intact during normal handling; be constructed to totally confine the chickens; contain shavings
or other material to prevent damage to the bottom of the container; and be ventilated properly to ensure humane
treatment in transit. The number of birds in each parcel must not exceed the container manufacturer's limit. Under
the applicable standards, indemnity may be paid only for loss, damage, or rifling, and not for death of the chickens
in transit if there is no visible damage to the container. The mailing of chickens for animal fighting purposes is
prohibited under 9.3.1.
While I agree that priority usually is delivered within the same time frame as express, there is no guarantee & can take as long as 3-4 days. Postal employees should not be accepting adult birds as priority shipments, but they are seldom on the same page. I'd worry more about the handling from fed-ex at the airports for priority shipments. There's no way I'd take the risk with my birds.