Changing temperatures are initiating plant growth earlier and earlier every year. In the park, there are already primroses in flower.

While we might find the early flowering of daffodils and snowdrops encouraging, there are other species in the park for which it might be a disaster,

All kinds of creatures try to time their reproduction to coincide with maximum food availability. Winter moth eggs hatch when the oak trees’ leaf buds open; blue tits time the hatching of their eggs to the arrival of the winter moth caterpillars; a stoat raises its large brood when the blue tits’ nests are full of babies.

• 

• 

1.Winter moth caterpillar; 2. Bluetit feeding nestlings on winter moth caterpillars

But it takes time to reproduce and just waiting on the arrival of seasonal food to begin breeding is not always a good strategy. Some species need a more reliable indicator that the season has changed, that their optimal food source is on its way and that it’s time to breed. Many organisms, therefore, have evolved responses to day length in order to judge these seasonal changes.

Day length is a fixture. The Earth’s tilted axis and its orbit around the sun make the seasons predictable: we know exactly when the days are at their shortest, exactly how much more daylight each day gets as the year advances, we can time the equinoxes, when day and night are of equal length, to the second. Our unscientific concept of spring, snowdrops and the arrival of the chiffchaffs, is that it comes earlier ever year but the vernal equinox remains the same.

• 

• 

… snowdrops and the arrival of the chiffchaffs …

Unfortunately, man-made temperature change is happening so fast that there is now a serious mismatch. Species that use day length to judge their breeding season are finding it no longer coincides with the food plants, prey species or predators that begin to grow or breed when the season reaches a certain mean temperature.

The established pattern has been disrupted. New behaviours will evolve as species adjust to the new patterns but evolutionary time is very slow while the changes that planet Earth is undergoing are becoming frighteningly rapid.

---

Blue tits can adjust the hatching of their eggs in several ways: