Every year around January or February, someone usually asks me about a strange currawong they saw with a striped tail. If you’re familiar with currawongs you’ll know they don’t normally have stripes or spots down the tail, in the way that cuckoos and kookaburras do. So what’s the story with the ones that appear in summer, tail all stripy like the bird in the photo below? The explanation is quite simple.
In case you’ve never met one, the Pied Currawong, Strepera graculina, is a largish bird, common in forests, gardens and picnic grounds of eastern mainland Australia. It has more black and less white in its plumage than the Australian Magpie, and the adult has piercing yellow eyes. It’s smart and adaptable, which brings it into frequent contact with humans. Normally, its tail is mostly black with a white base and neat white tip.
The barred tail you might see in late summer doesn’t signify a rare subspecies or a congenital abnormality, or even wet paint on a fence. It’s due to the feathers regrowing after moult.
Most birds moult their feathers at least once a year, making way for a new set to grow. (The developing new feather actually pushes the old one out.) This usually happens in summer when nesting has finished, the weather is warm and there’s plenty of food available. Moult (or molt to our US friends) is probably an uncomfortable time for a bird (see Why Molting Makes Our Feathered Friends Grumpy) and growing feathers require a lot of energy.
The feathers don’t all fall out at the same time but are lost in a sequence, so that new feathers are already growing as the next ones are lost. And luckily the bird is never completely bald, which would mean it couldn’t fly, couldn’t regulate its temperature and would look very ugly!
A currawong usually moults and regrows its tail feathers (retrices) symmetrically, starting with the two central ones, followed by the feathers either side of those, then the ones either side of those…with the outer feathers last. So when all the old retrices have fallen out and the new ones are still growing, there’s a period when the feathers are longest in the middle, getting progressively shorter towards the outer edge.
Now, a bird normally holds its tail folded with the central feathers at the back and the others filed in front. That is, except when it spreads its tail wide, for example in flight when turning or landing. Remember that each feather in the currawong’s tail is black with a white tip (and a white base, not necessarily apparent at this stage). The white tip emerges first in the growing feather.
So the effect of these progressively shorter feathers in front is that the tail looks striped or spotted — but only when seen from underneath. This lasts just a short time until the feathers grow to their normal length.
I’ve seen the same thing happen in the Grey Currawong although the striping effect isn’t usually as marked, with the white tips overlapping more. There’s an example in the first photo on BirdLife Australia’s Grey Currawong page. Wattlebirds, with their long, white-tipped tails can also display a nice barred effect on the underside, even to some extent outside of moult as the feathers are naturally graduated in length.
The moulting process can be responsible for a variety of unusual markings or oddly-shaped tails in many species. So next time you see a bird that doesn’t quite match the field guide, ask yourself: Could it be moulting?