Hi from Perth, WA. Where in Australia are you? Chickens 1-4 look like Isa browns or Hy-line browns, which are the commercial hybrids used for egg production in Australia. Sex-linked means that you can tell what sex they are from the time of hatching. They are produced like this so the males can be euthanased when they hatch (in the commercial hatcheries). Females are brown and males are white. The last chicken pictured is quite dark, it doesn't look like a commercial egg layer, I'm not sure about that one.