Hi, welcome to BYC!
Both other posters gave great information.
I would avoid those 4 year old hens unless you are planning a retirement community. The whole point for most people to expand their flock is to continue egg production... aside from just shear joy of course. Those ladies are nearing the end of their production if they haven't already quit laying..... And possibly nearing the end of their days all together.
The stress of moving and adding them to another flock might even induce underlying conditions.
If you would really like to expand... I suggest adding a few new layers (chicks) each spring. Every bird that is over a year old will molt come fall winter, during which time they will lay ZERO eggs, usually for a couple months at least. By adding a few new layers each year, you will always have eggs.... And you will be expanding your flock!
Also, since you have hens and roosters.... could probably get your own chicks to expand your flock and keep it closed if you like. I would think in a flock your size..... losing the entire flock would be costly aside from the stress of it happening. Another coop isn't the issue,
if your original has plenty of space. Many people do integrate birds. Many people try their best at quarantine before hand. And many others just toss them in and take their chances because they either don't have the understanding or aren't too worried about a loss. And plenty of people lie about doing it! My flock is usually over 40. So a big illness could devastate the flock and hurt my wallet. Plus the time I have invested into my current flock... picking them out, raising them, bonding... All of my birds have names and are recognized as individuals. The emotional toll that would take (even though we do eat our chickens), might ruin the whole chicken keeping experience for me.
So pluses and minuses weighed.... doesn't sound like you come out very far ahead with this set of Australorp, which are supposed to be nice birds. I am getting a couple as the LFS is getting them in their first shipment as well as Welsumer.
My final suggestion would be.... when expanding your flock, pick breeds YOU are interested in. It makes for much more fun! I enjoy having a diverse flock of eye candy in my yard and being able to tell them apart. And it makes me feel like a kid again when I get to collect different colored eggs! Plus it helps me to know if an individual is having health or laying issues if I can identify who's egg I am inspecting or who didn't lay. I would like to try a couple New Hampshire, but they aren't really at most feed stores here and shipping doesn't do great to my area, aside from the usual minimum of 15-25 being too many for me right now.
I hope you enjoy BYC as much as I have. It has changed my chicken keeping life fur ever!
