Every Spring, when the buds open and the young oak leaves grow, the caterpillars hatch.
They mainly feed on the oak leaves and start to get fat.
At the same time, the eggs, that the tits had laid a few weeks earlier, hatch too.
The tits can now feed the fledglings with the young caterpillars.
The caterpillars’ small size perfectly suits the dietary needs of the fledglings.
Both species begin to develop at the same rate.
On the one hand, the caterpillars eat big quantities of leaves and get fatter…
and on the other hand, the fledglings grow and their dietary needs increase.
Later, when the caterpillars turn into a chrysalis, they have grown big enough to feed on other insects and can no longer serve as food for the fledglings …
This synchronization of the biological rhythms of both species is very favorable to the reproduction of the tits.
However, today, because of global warming, buds open and caterpillars hatch almost two weeks earlier.
But the tits’ eggs still hatch at about the same time.
Thus there is a time-lag between the biological rhythm of the caterpillars and that of the tits.
When the tits’ eggs hatch, the caterpillars are already fat. They also change into a chrysalis much sooner while the fledglings are still too small to fly.
The tits cannot find enough food for their young and this results in a high mortality rate.
The decrease of the tit population is thus one of the unexpected consequences of global warming.