Developing eggshells requires a key ingredient — calcium — in larger quantities than the female typically has in her bloodstream. Just how different bird species supply or store calcium for egg-laying isn’t fully known. While some species seek out extra calcium from their environment, many species simply "borrow" calcium directly from their leg bones! The eggs seen here are those of an Anna's Hummingbird.

More info and transcript at BirdNote.org. 

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