Following that: Taking the information of "In both IOC and Clements Brown
Goshawk and Collared Sparrowhawk are not each other's closest relatives" If
that is actually correct (I don't mean whether these have been quoted
correctly but whether the idea is correct). That still raises that these two
species have separately evolved into both juvenile and adult plumages of
exactly the same pattern, from something presumably different. It would make
sense that they evolve into something like the typical patterns for the
genus (because presumably that colour pattern matches the woodland hawk
lifestyle) but why go so much further than this to become almost identical?
That it has happened on the same continent would suggest there has been some
benefit in this convergence. Maybe it is mutual mimicry. Is there maybe some
territorial benefit to both species or reducing disputes between them that
the male Brown Goshawk is about the same size and appearance as the female
Collared Sparrowhawk? I have no idea how that would work.
If they are similar in appearance because they have both evolved from an
earlier double invasion of one ancestor species, that would require that
they are each other closest relatives (assuming that the ancestor species no
longer exists in that form). If so the changes that have occurred are in
size and anatomy.
About "But before considering that, the question might well be why *should*
these animals look obviously different?" Well if they are not each other's
closest relatives then their basic patterns would I assume show more
connection to their separate origins, unless there is something driving the
similarity.
As for the white morph of the Grey Goshawk that question is unrelated. Yes
the aspect of mimicry of cockatoos is a plausible advantage. Or there might
something else selected with the white gene. Or some advantage in investing
energy by not producing pigment.
As for : Does anyone know if these two hawks have sufficiently distinct
diets (say prey items of substantially different sizes) to make them pretty
much non-competitive in the same landscape? I would have thought that is
pretty obviously established.
Philip
-----Original Message-----
From:
On Behalf Of Nikolas Haass
Sent: Saturday, 12 January 2013 11:57 AM
To: David Adams
Cc: COG line; Birding-Aus
Subject: [canberrabirds] Why are the C. Sparrowhawk &
B.Goshawk so similar?
Hi David,
There are some points in your comment that need some further thoughts:
1. It is likely that prey takes the time (i.e. a tiny fraction of a second)
to identify a raptor and they are mostly much better than we are in doing
so. E.g. there are species that certainly would laugh at a male Collared
Sparrowhawk but should be quite concerned about a female Brown Goshawk.
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...
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...
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. 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.
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.
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...
7. Yes, a Harpy Eagle with its short rounded wings is adapted to hunting
in/under the canopy!
Cheers,
Nikolas
----------------
Nikolas Haass
Sydney, NSW
________________________________
From: David Adams <>
To:
Cc: COG line <>; Birding-Aus
<>
Sent: Saturday, January 12, 2013 8:50 AM
Subject: [canberrabirds] Why are the C. Sparrowhawk & B.
Goshawk so similar?
> After contributions from several people to solve the recently
> Birding-Aus
posted
> "Timeless question" of a request to identify some photos as either a
Collared Sparrowhawk
> or a Brown Goshawk, it got me wondering has anyone investigated or got
any ideas why
> these two species are so similar in the colour patterns and remarkably
> so
in both juvenile & adult plumages.
There are many other people on the list that have doubtlessly forgotten more
about the mechanics of speciation than I'll ever hope to know...but I'll
toss in a few thoughts from my layperson's perspective.
While incomplete, DNA comparisons could tell or suggest a lot about how
similar or different these species are genetically and about how long ago
they diverged from a common ancestor. [I did find a phylogenic tree that
makes this pair of birds look like they're closely/recently related.]
But before considering that, the question might well be why *should* these
animals look obviously different? We've probably had nothing to do with
their evolution. What they look like to their prey is relevant - and I doubt
their prey takes the time to distinguish between Collared and Brown
;-) There are genera and families with very similar _looking_ creatures that
are quite far apart genetically. (I'm thinking of fish here but I've heard
of similar looking birds that are very distinct genetically.) Unless there's
some functional or accidental reason for animals to change their obvious
form, they may well not - even if diverging substantially at a genetic
level. Inconvenient for us birders, granted ;-)
For what it's worth, Australia has fewer
similar-looking-and-hard-to-distinguish species than anywhere else in the
world that I can think of. I've always assumed this is because Australia
been isolated for so long that the plants and animals have had ages to
diverge and specialize. (?) Hawks are excellent dispersers (as are
Pigeons/Doves, Parrots, Rails/Coots/etc, Ducks and Geese)....so the hawks
could be more "recent" self-introductions that haven't had as long to
diverge?
Why do there seem to be similar-looking Accipiter pairs and triplets in a
lot of places in the world? No idea. But given their lifestyle/habitat,
there are probably some constraints on ideal wing shape and efficient size
ranges, I'd guess. For example, you couldn't have something the size of a
Harpy Eagle hunting under the canopy like a typical Accipiter - and you
wouldn't want the wing shape and flight style of a Harrier or a open-country
Kite if your survival depends on chasing birds through a forest (and not
flying into a tree in the process.)
I think the biggest Australian Accipiter-related evolutionary question is
how to explain the white morph of the Grey Goshawk. An all white raptor in
the forest? They shine like a beacon and other birds obviously notice them.
I've heard birds in Australia raise the alarm for various Owls, Frogmouths
and Hawks but they really seem to go nuts over the White Goshawk. It's
wildly maladaptive on the face of it. Why aren't they all grey? As far as I
can remember, I've never heard of any other perfectly white bird of prey.
(Leaving out Arctic owls in their all-white plumage, which is easy to
understand when they're in a virtually all-white habitat.) I had a squiz at
"Bird Coloration" up on Google Books but honestly didn't understand the
answer (around page 476):
http://books.google.com.au/books?id=WnJ8cSoXRMIC&pg=PA476&lpg=PA476&dq=adapt
ive+benefits+color+accipiter&source=bl&ots=VTmKbRyo5c&sig=ME-kejl_AMCANJ9h6y
wUL4uqpu8&hl=en&sa=X&ei=h4DwUJ6qHuvOmgWipYCICA&ved=0CDoQ6AEwAQ#v=onepage&q=a
daptive%20benefits%20color%20accipiter&f=false
If I followed it rightly, the dominant theory ("disruptive selection") is
that the different morphs exist because the bird lives in different habitats
that select for the different morphs. Do grey and white form birds live in
different habitats? I know that the white morphs are more common in the
south, but is their habitat, diet, or activity schedule substantially
different? There's a study cited from 2004 that compares Accipiter and Buteo
hawks with this question in mind. (Many Buteos have highly variable plumage
and wide size ranges - more than any Australian Bird of prey that I can
think of.)
http://abcdef.uni-hd.de/institute/fak14/ipmb/phazb/pubwink/2004/19.2004.pdf
I can see from a quick look that they've included four Accipiters in their
phylogenic tree, including the two small Australian Accipiter that this
thread is about. They've also included a pair of North American hawks,
Northern Goshawk and Cooper's Hawk. Cooper's and Sharp-shin are often
impossible to distinguish in the field. I spoke with a Canadian birder and
biologist yesterday that said she and a friend once tried, and often failed,
to identify these birds to species with skins in the hand. If I understand
the 2004 paper's conclusions, they seem to have observed the polymorphic
species eat more mammals. I guess that Grey Goshawk eats more mammals than
the other Accipiters here, for what it's worth. (Comments from someone with
a grasp would be much appreciated here.)
As to why the two hawks here split, there are a ton of plausible mechanisms.
As someone suggested, successive self-introductions from an original
homeland could explain it. (Because the populations have split and been
apart, the descendants of each part of the original population have
experienced different selective pressures and natural genetic 'drift'.)
That's kind of the classic answer, I suspect. Increasingly, I notice in
passing examples of species that have diverged without any kind of
geographic separation because they've specializing sufficiently in food
sources. Does anyone know if these two hawks have sufficiently distinct
diets (say prey items of substantially different sizes) to make them pretty
much non-competitive in the same landscape? (In some of the New World river
Kingfishers, you can get multiple species side-by-side because they don't
hunt the same size fish.) ===============================
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