How do you know whether a duck egg is fertile?

i have a male and female duck and she just this minute started laying eggs
If your duck or the duck has mated with another consequently it is fertile but if you don't know if it is or not than just assume it's not.
Sometimes you can candle it. Put it up against a light and you can see a small spot through the standard lamp. That is how most hatchery's do it. Read more on candling a egg on the internet, I have ducks, geese, and chickens. Oh and guineas. She the female duck will do the work for you if she building a nest. Early within the spring she will start laying but not build a nest. I pick up the eggs and either hatch them or I give them to a friend who loves to guzzle duck eggs. The female will make her nest when she is ready. Watch out for varmints ( opossums, skunks, coons , etc. ) they love duck eggs and coyotes love ducks. They are adjectives at nesting time.
candle it but you can usually only tell once its be incubataed for 3days or so before you see anything you will be looking for a little spider thing that will be its vains growing
hope this help
good luk
hold a flashlight to the other side of the egg and look through it, the difference is fairly obvious, google it and you can see different metaphors
Answers:    You can start incubating the egg in a small incubator ( google incubating duck eggs if you be aware of that this is what you want to do ) and then wait 7 days then candle the egg ( google candling duck eggs whether you want to find out how to do that ) if you see small spiderweb like veins inside the egg when you candle it, afterwards its fertile. If its clear or has blood rings or blood spots then its either infertile or dead-in-shell.

But, the likeliness of this exceptional egg being fertile is very unlikely, since most males are incabable of fertilizing this time of the year.

on the eight day after the hatching hold it surrounded by front of a flash light and if you see a black spot and some blood vessels afterwards it is firtle
If you have seen them mating after the eggs should be fertile.
They are better nutrition-wise than unfertilized eggs.
You will not see any difference when cracking them open so don't worry about that.
Treat them as you would chicken eggs, collect the eggs from where on earth your duck lays them everyday and put them in the frig.
She may decide to hatch her own eggs at some point. If this happens, she will lay eggs and not vacate the nest till they all hatch.
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