On a regular basis, it's important to peek inside the nestbox, and watch the activity around the box, to see how its residents are doing. If there's a problem, you can try to solve it. Monitoring is interesting, and it also increases the likelihood of a successful clutch.
Bluebirds are tolerant of humans. Opening the box will not hurt the birds, and some brooding mothers will even remain on the nest when you look inside. Songbirds do not have a good sense of smell, so leaving your scent will not deter them.
Bluebirds lay their eggs in the morning, so the best time to do a quick check is in the afternoon. You can wait until you see the birds leave the nest, and then monitor the box. If the box opens from the front or side, you can use a small mirror to see inside the nest better. You can monitor once or twice a week. Check for insects and make sure none of the nestlings feet are caught in the nesting material which could injure their legs and prevent them from fledging. The eggs take about two weeks to hatch, and the hatchlings take about three weeks to fledge (see nesting behavior). To keep the nestlings from fledging prematurely, it may be best not to monitor after the 14th day of hatching. After the bluebird chicks have fledged and left the box, remove the old nest and clean out the box. Western Bluebirds usually have two broods (occasionally three) during each breeding season from late March to August.
It's also a good idea to take notes while you're monitoring, to keep track of when the eggs will hatch and the hatchlings will fledge. You can record your information on this data sheet as you monitor your nestbox. And you can enter your data online at the Cornell Lab of Ornithology NestWatch. It's fun and NestWatch uses the data for current bird population studies and trends. You can also enter data at the CBRP online program. Unfortunately, CBRP is unable to share data directly with NestWatch, so you might choose to enter your data at both sites.
Depending on where you live in San Diego County, California, some other native small cavity-nesting birds that you might attract to your nestbox are: Bewick's or House Wren; Oak Titmouse; Ash-throated Flycatcher; Violet-green or Tree Swallow; Mountain Chickadee; and possibly White-breasted Nuthatch. Non-native House Sparrows might also be attracted to your nestbox. House Sparrows are very aggressive birds and may destroy bluebird eggs and young in order to take over a nesting box. Because House Sparrows are not native, it is legal to control them by removing their eggs and nests.