Parakeet is laying eggs help?

I have 2 male and 2 female parakeets. The female that is laying the eggs really likes the yellow male parakeet. All they do is feed eachother and preen and 3 days ago I found a egg in her nest box then 2 days later she had another. I never seen them mate but like I said they are always with eachother having all the signs of a budgie that wanted to mate. What would be the chances of the eggs being fertile even though I never saw them mate? I also heard since she has 2 eggs now and is probably going to have a clutch that it means its more then likely going to be fertile eggs?