Both occur in dense forests along the east coast of Australia, but only the bassian thrush is found south of Sydney. In northern NSW, and along the QLD coast, where both species occur together the russet-tail is more of a lowland bird, while the bassian inhabits the high forests.
According to Michael Morcombe’s “Field Guide to Australian Birds” the bassian is found above 500 metres, and the russet-tail below 750 metres in altitude, and “therein lies the rub”. There is a band in the middle where a thrush seen could belong to either species.
Apparently, the best way to tell the difference is by their calls. The russet-tail has a sort of two note drawn-out mid-tone whistle which you can hear here. The bassian has a more complex three-note call which you can hear on this website. But this doesn’t help if the bird is not calling, or you are looking at photos.
While we were at Eungella, a group of bird watchers from Mackay were trying to determine if the thrushes that we were seeing around the camp and picnic spots, were Bassian (Zootheria lunulata) or Russet-tailed (Zootheria heinei).
Bird guides have not been much help. Michael Morcombe, in the guide mentioned above, has almost identical pictures for the two species. He says that the colouring of the russet-tail is slightly more rufous, especially on the rump and over the tail, and that it has a bigger white patch near the tip of the outer tail feather.
But really, unless you can directly compare the two species, you need definitive features to look for, not ones that are more-or-less of some aspect, so I have been searching for them on the internet, and a few more differences have turned up.
By comparing photos of a Bassian Thrush, taken at Washpool NP, NSW in 2014, with the photos taken of thrushes at Eungella, that I now believe were Russet-tails, I am hoping to show some of the differences here.
Firstly, I want to point out that the supposed slight colour difference, the russet-tailed thrush being a little more russet, cannot be reliably assessed, here, because the camera and white-balance differed between the photos. Also, the russet-tail looks fatter, but this is also not a reliable point of difference. The two birds in the photos above really look quite similar, so what are the distinguishing features?
The things to look for are shown in the two photos here, above and to the right.
Firstly, look at the secondary wing coverts, feature A in the photos above. In the bassian thrush the pale colouring of the feather-tips continues along the shaft of the feathers. In the russet-tail, it doesn't.
The next feature to look for, feature B above, is the dark banding on the feathers of the rumps of the birds. In the bassian, the banding is as wide and dark as in the feathers on the back. In the russet-tail, the banding is narrower and paler than on the back feathers. This may be what causes the more russet colour of the rumps of the russet-tails, hence their name.
The next distinguishing feature, C, can clearly be seen in the photos of the whole birds, above. The bassian thrush has a longer tail.
The last feature, D, relates to the size of the white spot on the edge of the outer tail feathers, which is bigger in russet-tailed thrushes. It is smaller, paler, and not easily seen in bassian thrushes.
However, in spite of these differences between the species, the picture is somewhat confused by the juvenile bird at Eungella, pictured near the top of this article. This bird, a russet-tailed thrush, had the secondary wing covert pattern (shown in close-up on the right) of a bassian thrush .
Whaaa....aaat! Confusing! I don't know what the story is.
Perhaps the immatures show intermediate characteristics, as suggested for bassian thrushes here.
I hope this comparison has been useful for some people.
I think that the bird-watchers from Mackay, mentioned above, also concluded that the thrushes at Eungella were russet-tails. But it is certainly very difficult to tell when you are looking at the birds in the wild!