adaptive radiation in birds

The 2005 Schol Bio paper included the following question. The three examples shown represent just some of the diversity found in bony fish. Use the diversity of the fish and/or any other named group(s) to discuss the following statement: 'Diversity is the end product of evolution.'

How do I get from that to a blog about adaptive radiation in birds? Because a number of students chose to answer the question using birds as their example. Good? No – because they chose to focus solely on the ratites, a group which they'd have studied as an example of adaptive radiation.

Now, you may think that I'm contradicting myself here. After all, I've just said that ratites show adaptive radiation. But the examiner's original example included species of fish (from memory, seahorses, flounder, and anglerfish) that differ significantly in morphology & habitat. Their expectation was that students would recognise this and select their other examples accordingly, discussing other groups with similarly high diversity in morphology & lifestyle. But ratites are actually fairly similar in body form. The examiner was, I think, hoping that students would go for the big picture: i.e. 'birds' as a whole.

And birds – the taxonomic group, Aves – do show considerable diversity. But only in some features. The fact that birds fly places considerable constraints on size & shape, for example. The upper weight limit for flight is around 15-16kg (something that was approached by NZ's own [extinct] giant eagle). Truly big birds (the ratites, for example) are those that have lost the ability to fly. And of course penguins, superbly adapted to an aquatic lifestyle, have done the same – although if you've ever visited Kelly Tarlton's, you'll have seen how they appear to 'fly' under water.

So where's the adaptive radiation in birds? Well, aside from the obvious differences in size (from hummingbirds to albatrosses), a lot of it's in their beaks and feet. Beaks shaped by natural selection to tear flesh, sip nectar, crunch nuts, filter minute algae from soda lakes or scoop fish from the sea… Feet with talons, rough scales, very long toes, webs and lobes and flaps, on legs short or stilt-like… A focus on all that – including a discussion of how natural selection operates -would have been the basis for a most interesting (& relevant) answer.

Of course, all this is really an excuse to show a holiday photo: I snapped this shoebill at the Jurong Hill Bird Park in Singapore.


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