Why are the C. Sparrowhawk & B. Goshawk so similar?

Subject: Why are the C. Sparrowhawk & B. Goshawk so similar?
From: David Adams <>
Date: Sat, 12 Jan 2013 08:50:56 +1100
> 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) 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):

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.)

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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