how do you know if a duck egg is rotten or fertile

[1234pekin]

i have pekin ducks and i want to hatch eggs it is the 27th day how can you tell if they are rotten or not

 

[SA Farm]

Do you mean they've been in the incubator for 27 days now? They should be in lockdown preparing to hatch on day 28. Normally you would candle before lockdown on day 25 to see how the embryo is doing and to remove any 'duds'.