Why are some birds eggs blue? 

This time of year always makes me think of eggs and not just chocolate ones. 

Bird eggs come in all different shapes, colours and sizes. The Cetti’s warbler lays gorgeous, brick red eggs. The Golden plover lays perfect little chocolate brown and cream speckled eggs. The dunnock lays eggs the colour of a blue Smartie (other blue sweets are available). But when you think about it, blue doesn’t make a whole lot of sense. Blue isn’t an especially good colour for camouflage, very few things in nature are blue. So why blue eggs?

Questions about colourful eggs

In my quest to find out why birds lay blue eggs, I first needed to figure out why eggs are colourful in the first place. It turns out, only birds who nest in open nests have colourful eggs. Birds that lay their eggs in cavities or caves typically have white or light in colour eggs. Birds where both parents sit on the nest also have reduced colour eggs. Assumedly, with two parents sitting on them, there is less time when the eggs are in the open. So, we know that the colour has something to do with the egg being outside and visible. So why blue? Naturally, my next question was…

What colour were dinosaur eggs?

Therapod dinosaurs are the early relatives of birds. The therapods or three toed dinosaurs include t-rex, velociraptors, and oviraptors. Scientists have studied oviraptor fossils and have discovered some interesting things. The oviraptor eggs studied were blue green in colour and were laid in open nests. Not only that but dinosaurs that buried their eggs had white eggs. The blue green colour of the eggs seems to have evolved in response to this change to open nesting. 

So we know that blue eggs have existed for millions of years and have something to do with nesting outside but why? 

Nature’s sun-tan lotion – and anti-bacterial?

Blue-green eggs reflect blue green light. Reflecting blue-green light not only keeps the egg cooler but also protects it from harmful UV radiation! Neat. Blue green colour may also kill some bacteria. Scientists found that when blue chicken eggs reflect UV radiation, the reflected light also kills g bacteria on the eggshell!

Blue green colour in eggs may also have evolved to give signals to males.

Choosy Dads Prefer Blue Eggs

Birds have to expend a lot of time and energy bringing food for the newly hatched chicks. The female has invested so much energy into making the eggs that it makes sense for her to try her best to raise them into healthy chicks. However, the male doesn’t invest any energy into the eggs and so must make a choice, stay and help the female raise the chicks or ditch the eggs and try again. How do males decide which eggs have the best chance of surviving? Egg colour might be a clue. In Eastern Bluebirds, egg colour can be a reliable indicator of the health of the mother. Healthier females lay healthier, more colourful eggs. Healthier eggs have a better chance of survival. Males have a better chance of figuring out where it’s best to spend their energy by inspecting the eggs. American robin males also give more food to chicks who hatch from brightly coloured eggs! 

Choosy dads are not the only ones inspecting eggs.

A cuckoo in the nest…

Brood parasites are birds that lay their eggs in the nests of other birds. There are over 200 birds worldwide that use this sneaky egg laying strategy. Imagine spending time and energy building a nest just to raise a baby that’s not yours! Bright colours and patterns in eggs may help bird parents work out who's eggs are who's. The Tawny Flanked Prinia is an amazing example of egg colour and pattern variation. Tawny Flanked Prinia eggs have endless different combinations of patterns and colours. Prinia eggs can be red, blue, white or anything in between. The patterns of prinia eggs can be spotted, marbled or squiggly! The amazing variation in Prinia eggs may have evolved to prevent brood parasitism. If prinia parents can easily tell which eggs are theirs, they can eject the fake eggs from the nest. The parasite of the prinia is the parasitic weaver. Parasitic weavers also have some tricks up their sleeves. Over time, the eggs of the parasitic weaver have become almost as variable as the prinias! This process is a never-ending race for prinias to lay more complex eggs and the weavers to copy them. 

Honestly, why not blue?

Thank you for joining me on our most egg-cellent adventure. We’ve learned that blue eggs have been around for millions of years. Blue colour may protect eggs from sun damage, bacteria, and give males a clue about the quality of eggs. Not only that but blue eggs may make it easier to tell if an egg belongs to you or a sneaky brood-parasite. After all that, it begs the question, why aren’t all eggs blue?

Have a pressing question about nature? Email me at sarah.morgan@welshwildlife.org