Male and Female birds

A very interesting question, Karen, and one to which I have no answer. In some ways sexual dimorphism is easier to understand. Males often need to be more colourful to attract females or defend territories. On the other hand females who are often the main carers of offspring need to be less obvious and have subdued markings to avoid predators.

Even with birds that look superficially similar like Great Tits, Great Spotted Woodpeckers and Kingfishers there are subtle differences which enable us to tell them apart but why is another question.

Good topic for discussion. Let's hope someone with more scientific knowledge than me has some ideas

I'm certainly no expert, so these are just my musings on your interesting question.
As others have said, it is perhaps easier to explain why males (in general) might be more colourful. A theory suggests it is to show they can survive the obvious hindrance of not being camouflaged and therefore have 'good' genes. However, it is not so easy to explain why some species go in for that, while others don't. Either those that do go in for strong displays have an easier life, so they aren't hindered so much and can get away with it … or, and this would be my guess, most species started out with the sexes the same. In some, the sexes begin to differ after a freak mutation and if no significant disadvantage occurred, the difference would get greater over time. Other species who don't suffer the mutation that begins to change the sexes, just carry on looking similar. I realise that just makes it random, but that's what evolution is. A possible flaw with that, is why does it happen so often in birds, but less in other animals? Is it that feathers are easier to change than skin or fur? I think it is also true that we are trying to understand after the event, so it would be difficult to actually know if any particular theory is actually the right one.

Thank you all very much for your interesting replies. The more I learn about birds, the more I want to know!