From World Bird Sanctuary

How do you tell if an egg is fertile? You look for the veins!

To see the veins, we do something called "candling." It's when you hold a bright light up to the egg in a dark room to see through the shell. You can get special egg lights, but a cell phone light usually works just fine. Veins usually start becoming visible as early as 3-5 days from laying, but the best time to check is usually closer to the 7 day mark. Fertile eggs will continue to darken as the embryo grows, and eventually will be too dark to really see anything as the embryo takes up more and more space in the shell.

You can see the difference between the infertile and fertile Great Horned Owl eggs pretty easily in these pictures.

Infertile

Fertile

Thanks. Very satisfying to see the thing I've read about a lot.

Agreed. I've read and watched egg videos, but when I found this picture it felt like something new to me.

You guys hadn't shown much interest in the egg stuff in the past, and we don't have incubators at the clinic, so I had moved on from the subject, but this has renewed my interest.

Here's a growth chart showing development inside the egg. It's a little creepy looking and I'm not sure if these are disected specimens or some type of imaging of viable embryos, so I'll spoiler tag it as a precaution.

Also somewhat interesting to note, this seemed more poultry specific, but for fertilized eggs that stop growing at some point and become non-viable, the common terminology seems to call them "quitters." Not sure how I feel about that term, but I thought it was interesting in a "how humans handle things" way. 🐣

Lol perfection!

Whoop whoop whoop whoop whoop 🤣