Chickadees are rather the Norwegian Bachelor Farmers of the bird world. They are very sociable--ya, sure, you betcha!--but they aren't comfortable getting TOO close to other chickadees. They maintain a rather large personal space, which is why they never sit side-by-side at your feeder. If one chickadee does approach another too closely, the dominant one between them will make a little gargle call which effectively shoos the other bird away like a discreet "ahem!"The trick is, at least once a year a pair of chickadees must draw very close together indeed, or we'll never have baby chickadees. So chickadees sing, which revs them up hormonally so they can overcome their inhibitions and reserve to finally do what birds and bees and educated fleas do. During the song period, female chickadees get so revved up that they become like teenaged girls at a rock concert who throw their panties on stage--if a particular male chickadee is exceptionally tuneful, many or most of the chickadee families in his neighborhood will have at least a few babies who share his genes. Fortunately, father chickadees never demand paternity tests before raising their babies--they spend weeks feeding all the babies in their brood regardless of parentage.
It takes almost 2 weeks for chickadee eggs to hatch after the brood is complete. The young remain in the nest for another two full weeks, and remain with the parents for another month or so. By the time the young are on their own, there isn't enough time remaining in summer to raise another brood. But many things kill chickadees, so to maintain their numbers, broods contain an average of 6-8 eggs, and some have as many as 13!
If you think this is extreme, the even tinier Golden-crowned Kinglet's average brood normally contains 8 or 9 eggs. In both cases, the eggs are so tiny that they can be brooded in one layer. I love how the eggs are arranged in that chickadee nest!