Immunity is an interesting thing.
First, we (aniumals)have an immune system with a memory. THat once exposed, specialized cells go into storage to wait for the next exposure. THese cells vary in how long they are viable. So think about how we are vaccinated for tetnus. It lasts a good 10 years, then you need another one. If you get a puncture type wound with in 5 years you are good, if you get a puncture in years 5-10 you need a booster. At 10 years you need a booster no matter what. With other vaccines, the duration may be batter than this, or not. In the case of the influenza vaccines, annual injections are designed to keep up the the most likely latest strains running around, so annual vaccination is necessary. THere are some things I don't vacinate my horses for because they are not around other horses that might have the disease. So I don't vaccinate for the flu/rhino every THREE months like they do in the big barns with horses out to shows, and going here and there. So . . . . immunity in a specific bird can be aquired by exposure thru vaccination, or thru disease exposure.
For offspring, they cannot benefit from the immunity described above. What they will benefit from is when a parent has a genetic immunity. For example, when the , oh I can't remeber if it is the black plque or the bubonic plague, shoot, well my point is that during this plague a stunning number of humans became sickened and died. Like 25-40% of the human population. BUt some people did not even get sick despite exposure. ( NOt all peole that were exposed died either. ) THe people that survived had the genetics to fight the bug. ANd when these people reproduced, they passed on the ability to fight the bug. So the next generations were not suseptable to this disease. This is also applicalbe to the birds. Basically, don't keep a sick bird. Eliminate it from the breeding pool.
Hope this makes sense. ONly in mammalian animals which drink their mother's milk can specialized cells be passed to the infants/babies. ANd then the time is limited. These are BIG molecules and the intestinal lining allows these to be abosorbed only with in the first 24 hours. Colostrum is sticky as it is made up of these sticky proteins. My children do not get ear aches because they acquired passive immunity from me. My body copied all my stored immunity and made that available to my children in the first milk.
Chickens can't do this because they don't feed their babies milk. So technically they can't acquire immunity from the parents this way, only genetic immunity.
Here endith the lesson.