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=adaptive+benefits+color+accipiter&source=bl&ots=VTmKbRyo5c&sig=ME-kejl_AMCANJ9h6ywUL4uqpu8&hl=en&sa=X&ei=h4DwUJ6qHuvOmgWipYCICA&ved=0CDoQ6AEwAQ#v=onepage&q=adaptive%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.)
===============================
To unsubscribe from this mailing list,
send the message:
unsubscribe
(in the body of the message, with no Subject line)
to:
http://birding-aus.org
===============================
===============================
To unsubscribe from this mailing list,
send the message:
unsubscribe
(in the body of the message, with no Subject line)
to:
http://birding-aus.org
===============================
|
