well, they come from inland australia, and are found in low numbers (hence the Save the Gouldian Fund). In the wild they are found in 3 subspecies, all the same apart from the colour of the head. I'm not exactly sure where the lines (if any) are separating the Subspecies, but the head colours are Red, Black and Yellow (the latter being uncommon).
In captivity they are easy to house, being suited to both indoor cages or outdoor aviaries. Outdoor aviaries do tend to show them off best, especially when planted with native shrubs & grasses.
In captivity they will feed on a standard finch mix, with greens (spinach, rocket, lettuce, cucumber, etc.), soft foods (such as Wombarroo/Passwells complete soft food, egg & biscuit mix, etc), green seeding grasses and occasionally live food such as mealworms.
They can be kept as a single pair or as a flock, but I think it is important to keep the head colours separate for purity.
It has recently been found that Gouldian Finch hens have the ability to control the gender of the offspring inside the eggs. I'm not sure how exactly this works, but if you mate a any combination of head colours in a pair (apart from a pair of the same colour) you will end up with predominantly (up to 80%) male chicks.
There are over 80 mutations of the Gouldian worldwide, with new ones being developed constantly (there is a new one being bred in the US, the Blue Headed. see pics in the mutations section)
It is very hard to find pure strains of gouldians that are not split to either a different head colour or a mutation.
Breeding is fairly simple: they will usually nest in a box or wicker basket, only rarely constructing one themselves in brush. They are know as one of the finch worlds poorest nest builders! not really surprising after over 100 years of captive cultivation.
Provide soft and coarse dry grasses of varying lengths for lining, and they will sometimes use white chicken or duck feathers (available at some pet shops) to line the interior.
The chicks hatch in about 14 days, are naked and tiny, with sealed eyes. They have a rim of small glowing globules around the mouth. These are used like aeroplane landing lights, so the parent can see where to poke the food next.
When breeding extra amounts of soft food and greens should be provided.
Chicks fledge in about 3 weeks after hatching. They should not be separated from their parents until at least 7 weeks old.
Nests should be cleaned out after every clutch has become independent.
Sexing gouldians is simple: in normals, the cock will be very brightly coloured, with vibrant purple and yellow, and a very deeply coloured head. the hen will have a paler head, lighter yellow and pale lilac breast feathers. As a general rule, in most mutations also the method applies.
It can be predicted when Gouldians are ready to breed because the tip of the female's beak will turn red. This may also happen in the male.
When buying, you should look out for birds with soiled vents, fluffed feathers and dirty beaks/legs. Avoid these as much as possible.
Ask if you can hold the bird. If the breast bone is prominent, almost sharp, do not buy the bird.
Check the breeder's aviaries for signs of mice, excessive pile - up of poop, other sick birds.
I could go on, but that's a basic idea.
hope it helps
