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

["Philip Veerman" <>]

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. The first bird book I ever bought was Condon's 1966 Field guide to the hawks of Australia when I could afford the 50 cents RRP. It said (snippets) that they have "the same pattern, that the Collared Sparrowhawk is a small replica of the Brown Goshawk and the female cannot be distinguished from the male Brown Goshawk in the field and they can only be picked in museum skins". Well now we know better but the point remains that their colour patterns are amazingly similar. 

 

As hawk species go, both their colour patterns in juvenile & adult plumages are an unremarkable sort of typical for the genus as a whole. But why are these two so much more the same than any others. Is it common ancestry that these two species are a more recent divergence from each other than to any other members of the genus and have changed in size and structure but not in colour. It appears to me that in structure their form is divergent along the lines of other species of Sparrowhawks and Goshawks. Why should our Brown Goshawk look so much more like our Collared Sparrowhawk than say the widespread European Northern Goshawk? Could it be colour mimicry of one species of another, in which case why? Is it coincidence, and especially that they inhabit the same geographic range. Nikolas Haass has mentioned that the Cooper's/Sharp-shinned Hawk pair causes in N America very similar confusion issues as the Brown G/Collared S pair in Australia. Does the same question apply?

 

Philip