Friday, 1 December 2006

In black and white

My office overlooks the car park on one side and a spread of daisies that passes for a lawn on the other. (Really, I prefer the daisies to grass—the butterflies and other insects just love 'em.) A palisade of gum trees runs along the edge of the car park. Because I'm on the top floor, one of my windows opens onto their highest branches.

Not a bad view, you know. The gums are flowering so they're usually full of white-plumed honeyeaters and red wattlebirds, with the occasional silver eye and New Holland honeyeater. For some reason the lorikeets are ignoring these trees but eastern rosellas pop in from time to time. Just the young ones, still with their green feathers. And there are always magpies.

I love magpies (Gymnorhina tibicen). When I introduced an overseas friend* to their glorious carolling he told me they sounded like R2D2. Which is true—they do. But it's a very musical version of R2D2.

Geographical variation in colour pattern has led to speculation that there are several species of magpie but all are now regarded as forms (morphs) of a single widespread species. Each morph is divided into several subspecies.

The most abundant morph is the black-backed form, which occurs over much of Australia. This includes one western and three eastern subspecies.

The white-backed form is restricted to coastal regions of SE Australia, from the eastern end of the Nullabor Plain to the far eastern Victoria – New South Wales border. And, of course, there's more than one subspecies. Apparently three of 'em are jammed into that small area.

Just before you think that you've got it all under control, there's a third form, Gymnorhina tibicen dorsalis, in SW Western Australia. In this subspecies, the males have white backs and the females black. Where the various forms meet, there is intergradation from one to the other.

So why is there such variation? Scientists have proposed two hypotheses. The pattern is either due to:
  • Past isolation resulting in differentiation of plumage local populations, followed by range expansion that has brought the populations back into contact
  • A continuous distribution (no isolation) with local conditions affecting plumage

Genetic studies on these populations shows that there's not a lot of evidence for isolation alone as the diversifying force. Colour patterns seem to be maintained by one or more selective pressures acting on different populations. Researchers in Queensland have found some indication that back colour in males is related to success in raising fledglings. If predators can't spot the nests in tree crowns, they can watch the parents returning to feed their chicks. Black-backed males may be less obvious to predators in open vegetation than are the white-backed males. If this is the case, then populations with white-backed male will be restricted to those areas where vegetation is dense—the SE and SW.

It's not known whether this is the explanation for the colour morphs but it is an explanation. And one open to further study.

I just think magpies are wonderful singers. And to hear one of these fabulous warblers doing her stuff, check out the link in the second comment below.

[Photos from top to bottom: WB juvenile; BB adult; WB adult; BB adult. White-backed birds photographed in Melbourne. Black-backed birds photographed in northern New South Wales. Many thanks to Tapperboy for taking those photos.]


References

Hughes, J.M., Lange, C.L., Mather, P.B. & Robinson, A. (2002). A comparison of fitness components among different plumage morphs of the Australian Magpie, Gymnorhina tibicen. 102: 331 – 338.

Toon, A., Hughes, J., Baker, A. & Mather, P. (2003). Discordance between morphology and genetic structure among three plumage forms of the Australian Magpie. Emu 103: 337 – 343.

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*You know who you are!

9 comments:

Duncan said...

Great post, I love Maggies too. Will never forget being rewarded with a burst of song after feeding one over at Two Peoples' Bay in WA.

Darky said...

What about the opposite hypothesis? That increased dimorphism - more white on the cocks' backs protects the southern forest dwellers.
When I look up into a forest canopy, everything is dappled bright white and shade. The Tassie race has females with the most white on their backs, while the SW race females lose almost all of their shingling with age. What point of view and which predators ? Looking down onto a canopy gets you a lot of reflected light during spring with new growth flush.
I also seem to remember Gisela Kaplan, of "Australian Magpie" authorship, made a point in an interview somewhere about moonlight also needing to be taken into consideration with camouflage.

Just saying. I have no hypothesis.

All maggies get blacker in their black feathering with age. I suppose that was corrected for in the Q article wrt breeding success.
In fact, all birds increase the density of colour with age. White is very expensive to maintain.

Would you like to use a clip I recorded of a young female practising on her own outside the study window this spring?
She was just getting the hang of making the modulation along harmonics fast enough, without losing her way, and she was still vocalising on her own - away from the others - and trying to keep it quiet. But she was nearly ready to join in with the adults and a honeyeater's territorial call provoked a nice loud agonistic bit at the end. Location is about 100km S of Perth on the Southwest Highway, in mixed forestry farming land.
I've posted it on a google page and you're welcome to edit and use it any way you like.

Snail said...

I just wrote a great big long comment and it disappeared ... I'll try it again but in shorter form.

I'm not convinced by the 'black backs do better than white backs when there's not much cover' hypothesis. The evidence is pretty weak but you can see how it would be an attractive idea. Especially if thinking about aerial predators.

I'd like to see reflectance data from plumage---over the whole range of wavelengths seen by birds not just our miserable range. It may be that the black feathers reflect the UV so they stand out like neon signs. I dunno.

Thank you very much for the audio clip. I've mentioned it up there (points to top of screen) rather than download and edit---because I've got no idea how to do that yet. I'm sure it's not all that difficult but I've only just come to terms with uploading photos and putting in links. I'll get the hang of it soon but it's one new trick at a time for me ...

Kyan gadac said...

Being a push bike rider I have a love hate relationship with those old boys in September/October when there testosterone's up - but the rest of the year they make fine singers, it's true.
The Noongar word for magpie is kulbardi or coolbaroo which is wonderfully evocative of their song and I propose that it should be forwith adopted as their proper name! The Noongar word is quite well known in Perth because it has featured in the name of a number of Noongar organisations.

Oh well - that didn't work - could somebody please tell me how to change the world... I'm sure there's a switch somewhere....

Darky said...

Kaya weijela! Great to see some trying about this bidness of nulla koorliny.
There's a better name than that other black and white one - the weijela "willie wagtail", Rhipidura leucophrys, too - - "jiddy-jiddy", unna?

Darky said...

I've mentioned it up there (points to top of screen) rather than download and edit

Whatever :-)
I don't know how long the file will stay where I've parked it, so I only mentioned it in case you don't want broken links.

Snail said...

You two are reminding me how much more there is to know. About everything. I'm almost tempted to mention known knowns and ... well, you know. I'm really enjoying this.

On the swooping—our campus magpies are relatively benign. They flap a bit but don't get too aggressive. Unfortunately, one year a pair of ravens decided to take up the cause and we had to cordon off a corner of the campus to let them go about their business while ensuring that student's brains remained inside their skulls. Nevermore.

...

I don't know how long the file will stay where I've parked it

I'd better do something quick smart about the link then. Where's that young person who knows about these things when you need him? Working at home, huh? (You picked a good day for it.)

Snail said...

Darky, I've downloaded the sound file. I'll transfer it to the blog ASAP. (Bearing in mind my lack of techno-savvy.)

Thanks once again for your generosity!

Anonymous said...

This NYT article
http://www.nytimes.com/2008/02/19/science/19camo.html
has a pointer to another way to think about camouflage - with specific reference to the black and white distribution in maggies.

The researcher has squashed ways to fool predators with patterns into 3 categories.
The one that would cover maggies I think may be the third one:
"Dr. Hanlon calls the third category disruptive patterning. A cuttlefish creates large blocks of light and dark on its skin. This camouflage disrupts the body outlines…To use disruptive patterning, cuttlefish need to make sure that their color blocks are on the same scale as the objects around them."
The author alludes to Panda's B&W patterning as an example of disruptive patterning, and that it may give camouflage equally on snow as in trees.

I like this approach, and am now thinking about including the ground on which maggies forage in thinking about the camouflage mix. Especially since I now recall that I am still very often surprised by the sudden appearance of a trio of maggies doing their ground foraging; they've been there walking slowly all the time, and it's me who's just seen them.

tcfra d**ky