When it comes to breeding, some birds, like blue tits, have just one brood in a year and others are multi-brooded such as swallows, which mostly feed on flies. Single brooded species are generally constrained by the availability of suitable food to feed their nestlings. In the case of blue tits this is green caterpillars that are most abundant in oak trees. One brood of about ten blue tit nestlings and their two parents require from 700 to 1000 caterpillars every day. These caterpillars are only available from about mid-April to about mid-June. Consequently, British blue tits cannot breed during months outside these three.
It takes about two weeks for a female blue tit to build her nest out of honeysuckle bark, moss, feathers and fur. Birds can only lay one egg per day, so when she is ready, the female will take about ten days to lay her clutch of eggs. Eggs can remain cold and dormant, but still viable, for several weeks if necessary. On average, the hen blue tit will start to incubate the eggs just before laying the last one, but it can start as early as five days before this time or even six days after she finished egg-laying. It all depends on when the local caterpillars start to appear. It then takes two weeks to incubate the eggs before they hatch, which is usually during May. The blue tit is trying to synchronise most of her brood to hatch at the best time for catching caterpillars. If she incubated some days before clutch completion, her nestlings will hatch over several days. Or if she waited until all her eggs were laid before warming them, they will all hatch on the same day. It will be a minimum of 18 days before her oldest nestlings are ready to leave the nest, and when they do, the others will usually follow on the same day, however well-developed they are.
So that is 14 days for nest-building, then a rest for about six days, 10 days for egg-laying, 14 days for incubation, then 18 days for the nestlings to grow. That is a total of 62 days from the start of nest-building to the fledglings leaving the nest. In Britain, it is impossible for blue tits to produce more than one family per year, because the caterpillar season is so short and the clutch size so large. However, some blue tits on the island of El Hierro in the Canary Islands frequently find time to raise two small broods of about five every year.