Thanks, David, for your comments on my comments. Very interesting stuff.
One minor point (I am also a Northern Hemispherian): Which birds are so
difficult to identify in Europe and North America? Apart from maybe some
Empidonax in North America and maybe some Phylloscopus, Acrocephalus and
Hippolais in Eurasia/Africa I can't think of many species that are that hard to
ID there. "Hard" birds are certainly not the norm there.
Nikolas
----------------
Nikolas Haass
Sydney, NSW
________________________________
From: David Adams <>
To: COG line <>; Birding-Aus
<>
Sent: Saturday, January 12, 2013 12:56 PM
Subject: [canberrabirds] Why are the C. Sparrowhawk & B. Goshawk so similar?
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 obvious...it 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:
http://www.hdouglaspratt.com/Art_Gallery/HandbookoftheBirdsoftheWorld/Zosteropidae_Plate1.html
http://www.hdouglaspratt.com/Art_Gallery/HandbookoftheBirdsoftheWorld/Zosteropidae_Plate2.html
http://www.hdouglaspratt.com/Art_Gallery/HandbookoftheBirdsoftheWorld/Zosteropidae_Plate3.html
http://www.hdouglaspratt.com/Art_Gallery/HandbookoftheBirdsoftheWorld/Zosteropidae_Plate4.html
http://www.hdouglaspratt.com/Art_Gallery/HandbookoftheBirdsoftheWorld/Zosteropidae_Plate5.html
http://www.hdouglaspratt.com/Art_Gallery/HandbookoftheBirdsoftheWorld/Zosteropidae_Plate6.html
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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