Living with Birds Caterpillar Countdown Tweetapedia

Spring in the UK is a fast-moving sequence of events where timing is everything. Nature relies on a pattern of biological events to ensure that when the hungry mouths arrive, the food is ready.

 The first larvae   

April is the peak of the greening. Oak and Beech are typically the last of the major trees to leaf out, often waiting until late April to avoid late frosts. A single mature oak tree can support over 200 species of caterpillar. This makes native deciduous trees far more valuable to breeding birds than non-native evergreens or garden shrubs.

As Oak leaves begin to unfurl, the tiny larvae of the Winter Moth and Green Oak Tortrix hatch. Caterpillars are the superfood of the bird world, and the primary food source for almost all garden songbirds, providing a high-protein diet essential for rapid growth. They're rich in carotenoids, which are essential for developing strong immune systems and vibrant plumage. They also provide a high water content, keeping chicks hydrated without the need for parents to carry water back to the nest.

 Perfect timing 

The timing of hatching is tied directly to the availability of insects.

Blackbirds and robins are among the first to hatch. Because they have a varied diet that includes worms, they can start nesting as early as March. Their eggs take about 13-14 days to incubate, meaning you can see the first hatchlings by late March or early April.

Blue tits and great tits are caterpillar specialists. They time their hatching perfectly with the emergence of winter moth caterpillars, ensuring a massive food supply is available exactly when the chicks are most demanding.

Synchrony and survival

From late April to May is the ‘Caterpillar Glut,’ and it is remarkably brief. This is when you will see the highest density of caterpillars in UK gardens.

However, this delicate balance is vulnerable to temperature shifts. If the spring is unusually warm, caterpillars hatch earlier. If birds do not adjust their nesting time accordingly, they miss the peak food supply, a phenomenon known as ‘trophic mismatch’. This gap between peak food demand and peak food availability can significantly impact the survival rates of a brood.

Research suggests that blue tits can actually fine-tune their egg-laying date based on the chemical signals released by trees as they begin to bud, helping them avoid the risks of trophic mismatch.