Although having two clutches is common behavior for black-capped chickadees in our yard (they have two clutches almost every year), in general, it is not common for black-capped chickadees. This behavior has been reported for related parids, including Mexican chickadees, and tufted titmice.
Usually, the second clutch is smaller, and fledges sooner, than the first clutch, usually closer to 14-16 days, rather than the 20- 23 days it typically takes the first clutch to fledge. Interestingly, this year, the adults re-excavated the box and removed the nesting material from the first nest- they usually just build right on top of it.
Having a second clutch with fewer chickadees makes sense, since fecundity or clutch size
is related to fitness. After the adults raise their first clutch, they are pretty worn out. That is why it is a bit unusual for black-capped chickadees to have another clutch. The reason for the second clutch might speak to the relatively easy living conditions the chickadees find in town or the result of a behavior that is learned.
Often adults will have a second clutch if there are few fledglings from the first clutch or catastrophe of some sort happens, but this is not he cases with these- there are six fledgelings from the first clutch (this is a lot- normally there are 4-5).
Soon the young from the first clutch will help collect food for the nestlings from the second clutch. This behavior may be reason the adults can have two clutches- that they are getting assistance in raising the second clutch. Though right now the adults are doing all the food gathering, and the young are following them around, picking and pecking at random things mimicking the activities of the adults.
Though a little more esoteric, the large clutch size from the first clutch, which is typically in our yard, is the result of chickadees that excavated a nest, rather than reused a cavity or nest in an already existent cavity (as a secondary cavity nester would). Unlike other parids (including the mountain chickadee that also occurs here), black-capped chickadees, will, and in many cases, prefer to excavate their own cavities for nesting.
There has been a hypothesis that clutch size was related to whether a bird excavates their own cavity and black-capped chickadees are the perfect subject for study since they may do either. The thought is that there is a caloric expense to excavation, therefore, the clutch size would be smaller for a primary excavator or a pair that excavates their own cavity, and larger for a primary excavator that uses an existing cavity. Obviously, you can't infer much from a sample size of one (the pair annually nesting in our yard), but it is pretty interesting, nevertheless.