How do we know, could there be another reason that birds lay brightly coloured eggs?
The short answer is that we don’t. It can be very difficult to apply rules and reason to the natural world. All we can do is look at their environment and formulate ideas, and then perhaps cross reference this with the animals observed behaviour and compare with what we have seen in other species.
A good example in this case is by looking at the difference between birds’ eggs and reptile eggs.
Most reptiles lay eggs that have a soft shell, in comparison to a bird’s hard-shelled egg. This is because of the way they are incubated. Reptiles will often bury their eggs underground or hide them away in crevices to stay warm so the eggs do not have to withstand the weight of an adult incubating them with body heat as a bird egg does. This links into camouflage, because they are hidden away they are usually white in colour, as they have little need for camouflage. Whereas a bird will lay its eggs in a nest in plain sight, almost advertising their position, therefore it’s likely their eggs need to blend into the surroundings a bit better.
Photographs: Nottsexminer, Jacqueline Lege
Image from www.flickr.com/photos/stuutje