Birds are close to egg laying
Last year the birds laid their first egg on March 6th, so keep a close eye on their behavior over the nest couple of days. Since we cannot see into the nest, the best way for us to know for certain that an egg has been laid is that the birds stay on the nest for the entire night. Once the eggs are laid, there will almost always be a bird incubating.
The birds have been observed copulating a number of times on the nest in the last couple of weeks. A rule of thumb is that eggs come a couple of days later. It is also likely that the birds have been mating off camera as well, so I would expect the birds to lay eggs within the next couple of days.