Females are sneaky going to and from the nest. Our recommendation for incubation is systematic sweeping but be aware of far away flushes to mark the general area.
Males are generally quicker and less cautious when approaching the nest.
If the female flies far away while observing her with food, hold your ground because she is probably trying to throw you off the trail and will return. He becomes very agitated, flying near you and chipping loudly if you attempt to approach an area where there is likely to be a nest. It's better to be patient and wait for the male to leave the area, so you have a better chance at catching the female unaware. They are also quick to catch on to you during observation, and if you've gone over to the nest area a few times they may stop going to the nest completely, and become less likely to fly down close to the nest. So if you are going to approach an area, try to have a good idea of where the nest might be.