As bevedfelker has stated: When the system is working properly, the BIRD relay will connect the house batteries to the chassis battery when either battery is seen to have a voltage greater than 13.1V (approx). The 13.1V is key because the either house or chassis battery output rest voltage under no charging conditions is below this voltage. The BIRD system senses when either battery is being charged and connects the battery circuits together. Once hooked together, the BIRD does not know where the charging is coming from but will remain connected until the voltage sensed between the two battery circuits drops below the 13.1V.
It is VERY important to know what items use each of the two battery systems so that both battery systems discharge below acceptable levels. You can identify these by turning your ignition key "on" with the engine off and with use/store in the store position. The working systems are running off of the chassis battery (dash radio, heater fan, chassis lighting systems etc. The other system work off of the chassis battery.