Re: [Birding-Aus] [canberrabirds] Why are the C. Sparrowhawk & B. Goshaw

To: COG line <>, Birding-Aus <>
Subject: Re: [Birding-Aus] [canberrabirds] Why are the C. Sparrowhawk & B. Goshawk so similar?
From: David Adams <>
Date: Sat, 12 Jan 2013 12:56:21 +1100
Thanks very much for the comments.

> 2. "Unrelated but similar looking" = Convergence (adaptation of unrelated taxa
> to similar ecological niches). E.g. raptors - falcons, raptors - owls, owls -
> frogmouths, swifts - swallows, herons - cranes, storks - cranes,
> diving-petrels - auklets, quail - button-quail...

I don't know the theoretical thinking, but convergence has always struck me as being applied way too freely. Sometimes, sure, form follows function...but that only goes so far so often. I'm not discounting convergence as a real-world factor of importance, I'm just reluctant to pick it up off the shelf quickly.

> 3. "Related but different-looking" = Divergence (adaptation of related taxa to
> different ecological niches). E.g. ibis - spoonbills,
> crakes-rails-swamphens-moorhens-coot, frogmouths - nightjars, owlet-nightjars
> - swifts - hummingbirds, grebes - flamingos...

That may also depend. Taxa can radiate in ways that make them highly distinctive visually or not. You can get as much - or far more - morphological divergence from sexual selection than an environmental pressure or radiation into available niches. You also need unoccupied niches to radiate into - easier when species hit an impoverished landscape or island than when hitting a mature (and relatively intact) continental ecosystem.

> 4. The paragraph starting from "For what it's worth, Australia has fewer" to
> "could be more "recent" self-introductions that haven't had as long to
> diverge?" Where does all that come from? Divergence is adaptation of related
> taxa to different ecological niches. I don't think that this is explained by
> your thoughts. 

Just my opinions. I grew up in the northern hemisphere where "hard" bird groups are seemingly the norm. Most of Europe and non-tropical North America were crushed under huge glaciers not so long ago wiping the ground free (apart from various known and proposed refugia). I think I grew up with about 40 species of trees. Total. Australia is striking because, with limited exceptions, if you can get a good look at a bird, you can name it to species. That's not the case in Europe, North America, the Neo-tropics, Africa or Asia, so far as I know. (I guess you can get that at very high latitudes if you leave out seabirds.) It just feels like many groups elsewhere have had so much less time to become readily distinguishable, even if they don't interbreed (or not much.) The situation in South America is completely over-the-top where, based on one theory, climactic changes have lead to species getting geographically split and later rejoined over and over. (Much as you would see in continental island archipelagos like the ones north of Australia from changes in sea level.) I guess the short way of saying it is that if the species have had less time to completely diverge, you would expect to see lots of closely-related sister species and if they've had longer to diverge then you would expect more distinctive species. Over the long term, Australia has also (I'm told) had far less climactic variability.

As to adaptive radiation or speciation based on niche-filling, that's only one of several mechanisms for speciation. Some selective events and changes in, say, food preference can lead to speciation without any change in gross anatomy. In the tropics, you can even get birds that diverge because they time their breeding different to exploit different fruit resources. To be sure, niche-filling is the mechanism that would likely lead to morphological divergence of some kind - like bill size, wing shape/length, mass, coloration, or hours of activity. Down here, the Ninox all look to be built to the same plan but on different scales, for example. Then again, some divergence might not be visually doesn't have to be.

> Why are hawks, pigeons, parrots, rails and so on excellent
> dispersers? Yes, some raptors are nomadic, some are migratory, but an awful
> lot is sedentary.

As to good dispersers, I picked up some sense of that while living in the tropical Pacific for seven years. Only good dispersers make it out to the oceanic islands - and it got me interested in islands and island biogeography generally. Why are certain families good dispersers? Probably because they're naturally migratory (or at least irruptive.) And fat. Fat is good when you're flying a long way. You're also more likely to succeed as an immigrant if you've got broad habitat tolerance and broad breeding location preferences. Depending on where a place is on the globe, self-introductions are going to depend on your founding populations....and, of course, those founding populations are in the past so only their descendants remain in the original homeland, if at all. 

> 5. In both IOC and Clements Brown Goshawk and Collared Sparrowhawk are not
> each other's closest relatives (I haven't had the chance to look up the
> original work behind that). That suggests rather convergence than lack of
> divergence. I am happy to be corrected here.

Haven't seen the lists - but that's good to know. All that really says is that you've got genetically diverse birds with similar appearances. That could mean that they diverged and reconverged (although such a process seems overly complicated). It could be that some bird self-introduced, say the ancestor of the Brown Goshawk. Time goes by. The original source population has shifted slightly through natural genetic drift and modified conditions to become a bit smaller. Then you get another self-introduction from the descendants of the original founder population but it's now different enough to the Brown Goshawk that you've got the ancestor of the Collared Sparrowhawk. I think that kind of speciation-through-reproductive isolation is pretty traditional thinking (it's in The Origin of Species) and still considered a dominant force. Then again, it could also be that there was one introduction here and the original population split through specialization.It may not take a big change in overall anatomy to have a strategic advantage for specific pretty times. It's not as though animals _have_ to change a lot physically over time (Horseshoe Crab.) It's fair to assume that morphological plasticity is *itself* a heritable and selectable trait. You've got animals (and groups of animals) that haven't changed much at all for uncountable millions of years and then you've got the Zosterops, said to be the fastest evolving bird genus on the planet. Below are links to Doug Pratt's plates on the birds - so many little green birds:

Pratt, as one of the top ornithologists for the Pacific has named a few of these, I'm pretty sure. A lot of them look very, very similar on plates and in the field. (I've tried and failed to see lots of Zosterops. Grrr.)

> 6. Some of these Accipiter pairs may be just a coincidence because there are
> many similar-looking Accipiters distributed world-wide. In other words I could
> 'put together' many further "Accipiter pairs". I guess the Cocos Keeling
> Island birders can tell you some stories...

Perhaps Accipiter's look pretty similar world-wide because their basic form is already pretty much ideal for their way of life? At that point, what would make them look different? Nothing. Unless outliers are selected for, they'll be selected against and the basic shape should remain unchanged. 

> 7. Yes, a Harpy Eagle with its short rounded wings is adapted to hunting
> in/under the canopy!

Not a bird I actually know - I thought that the plucked monkeys off the tops of trees. Something I hope to verify one day ;-)
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