I have about 8 Cayuga eggs being incubated right now -- all eggs were shades of gray, so I know they came from my Cayuga ducks...however the drakes that fertilized them could be runners, Cayuga, or KC's. Do you think the offspring of the mixes have the potential to lay black eggs? I would think maybe if they get the black plumage...maybe.
will Cayuga mixes lay black eggs?
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This is an interesting question. I'm not sure if the black bloom that is on Cayuga eggs is linked to any plumage color or any other readily observable trait.
You may just need to wait until they're laying and very carefully track which ducks is laying which eggs. And then see if there are any consistent traits you can "link" to the bloom color. The eggs are darkest a the start of the laying season and if anything interrupts laying for more than 24 hours the next egg that is laid after the interruption may have slightly darker bloom.
My guess would be that by the end of your experiment the darker Cayuga plumage may help you figure out which crosses will have the darker bloom.
It would be very cool to get some blue or green egg shells with the darker Cayuga bloom :).