I work at a pet store that has a no breeding policy. I began to take out the finch eggs and put them in my finch's nest. I have two females who are sitting on the eggs constantly. I have added eggs every time I see them at work, which I now realize may be problematic.
Do you have a wildlife or bird rehab in your area? They'd definitely be able to advise you wrt this.
Look in a bird book about hatching eggs.then write the date you put the egg in the nest and go from there. I use to hatch duck and chicks, with them the egg is heavy if it is near hatching, if it isn't any good it will be light and will have fluid in it which you can hear. I don't know if finch eggs are big enough for such a test.
I bred all types of finches for many yrs. I had society finches that would sit on any other finches eggs and are great babysitters. I also had zebra finches and all kinds ...Lol When I would check the eggs I simply held them over a flashlight. It's pretty ez to tell. But I found this info on a site and maybe it will help. You don't want to put too many eggs under your "girls", especially if you're putting them in at different times. One a day until 5 or 6 are there is the most I would add. Maybe this will help.
Infertile eggs: these are easy to detect, as the egg is clear. Discard
early deaths: the embryo has developed for several days and then died. Candling will reveal a small dark area and disrupted blood vessels. Often deteriorating blood vessels will appear as a dark ring around the egg. Discard.
Late deaths: these are often difficult to tell apart from a viable embryo at the same stage of development. Look for the absence of movement and the breakdown of the blood vessels. Discard
viable embryos: these move in response to the light and have well defined blood vessels. Mark the air sac and the inoculation site and then return the eggs to the incubator ready for inoculation.
Just use a flash light or a candle. Hold the egg in front of the light. You should be able to see if there is anything in it or if it is just liquid. If it is liquid it is no good. It take about 28 days to hatch out.
Hope this helps you out.
Good Luck Joyce
My egg is a sparrow egg and im not sure if its alive but it is a little darker around the bottom and its fairly heavy.