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It seems like birds would want to hide their eggs, but there are hundreds of species that lay blue to blue-green eggs. Thanks to some clever experiments, we’re finally starting to understand why birds might benefit from conspicuously colored eggs.

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Sources:
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https://doi.org/10.1093/ps/85.3.546
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https://doi.org/10.1525/auk.2009.09090
https://doi.org/10.1525/cond.2008.8634
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http://en.cnki.com.cn/Article_en/CJFDTotal-XNND199404014.htm
https://www.allaboutbirds.org/news/the-beauty-and-biology-of-egg-color/
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Images:
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https://www.storyblocks.com/video
https://youtu.be/p7r_dQuQMgA
https://commons.wikimedia.org/wiki/File:Ficedula_hypoleuca_egg.JPG
https://youtu.be/4NZBreUdiOQ
https://commons.wikimedia.org/wiki/File:Black-headed_weaver_(Ploceus_cucullatus_bohndorffi)_male_nest_building.jpg
https://commons.wikimedia.org/wiki/File:Ploceus_cucullatus_MHNT_228_St_Gilles_La_R%C3%A9union.jpg
[♪ INTRO].

Much like birds themselves, bird eggs come in a wide range of colors and patterns. And some of those make sense.

Like brown or mottled eggs, because, you know, camouflage. But some birds lay bright blue eggs, which just seems weird. I mean, they aren’t being laid in blue nests or in a bed of blue flowers or anything.

There simply isn’t an obvious explanation for that color. But biologists have given this a lot of thought over the years, and thanks to some clever experiments, we’re finally starting to understand why birds might benefit from conspicuously colored eggs. There are hundreds of bird species that lay blue to blue-green eggs.

They get those hues thanks to a pigment called biliverdin that’s deposited into the eggshell while the egg is developing. But while we understand how they become blue, exactly why has proven a bit tougher to pin down. Ornithologists are still debating a few main hypotheses.

Some think the eggs are a signal of mate quality, and specifically, the quality of the female who laid them. That tracks logically, since studies have found that females who lay biliverdin-rich eggshells are in better shape. So bluer eggs may indicate a higher quality partner, one who’s worth investing in.

And field experiments support this. For instance, when researchers swapped around pied flycatcher eggs, they found that males delivered more food if their mates were incubating bluer eggs. And studies on gray catbirds and American robins have found that males provide more care to chicks from colorful clutches, all of which seems to support the idea that egg color may signal a worthy investment.

But there is an alternative explanation. Some evidence suggests that the extra effort is actually an attempt to protect the eggs, an idea cheekily described as the blackmail hypothesis. Essentially: it’s easier to see brighter eggs, so nests that contain them are at a higher risk of being targeted by predators and parasites whenever the female has to leave, like, to go catch food.

And that might put pressure on males to pull a little more weight around the nest, thereby reducing the overall time the eggs are left alone. The thing is, not all studies agree that dad birds put more effort in when eggs are bluer. So some have proposed that blueness has little to nothing to do with males, and everything to do with the sun.

This hypothesis posits that the color provides protection from damaging UV radiation that can penetrate through the eggshell. While some birds lay eggs in cavities like in old dead trees, many nests are in more open areas. And the eggs in such nests may be exposed to intense sun whenever a parent isn’t sitting on them.

So, in those cases, extra blue pigment in the shell could help filter out some of the high-energy light, kind of like sunscreen. And that seemed to be the case in one study on village weaverbirds, but there was a catch. More intense blueness also increased the infrared radiation the eggs absorbed.

So, the eggs got warmer. That would suggest the need to strike a balance, not too much blue, but not too little, depending on the nest’s location and lighting. And we’re still trying to determine all the variables at play when it comes to blue eggs.

Studies have suggested other potential benefits, like, that biliverdin itself may actually strengthen shells. So while signaling female quality, blackmailing males, and sunscreen are the big ideas floating around, there seems to be a number of surprising upsides to blue eggs, which ultimately helps explain why so many species have evolved cerulean shells. Thanks for asking!

And thanks for watching SciShow. If you enjoyed learning about the evolution of blue eggs,. I bet you’ll enjoy our episode on mind-blowing birds nests.

And you also might consider clicking that subscribe button, if you haven’t already, we put out new episodes covering awesome science like this every day! And if you subscribe, you get all of them delivered straight to your YouTube feed. [♪ OUTRO].