Yesterday was the last of the really warm days in the current weather forecast, so I took the chance to do the two insect surveys that were due – a butterfly survey and a bumblebee survey, both at Lark Rise in Barton. It was also the day for the mid-April Westfield bird survey in Comberton, so it was a pretty full day out and about, and there was lots to report on. 

On Tuesday, I made a short video all about the benefits of hedgelaying for some of our moth caterpillars and, in turn, the potential that they are food for cuckoos. Listen, I ended by saying that we were hoping to soon hear our first Cuckoo.

Lo and behold, stop press, Mystic Vince Predicts: on Wednesday there was a cuckoo calling from the direction of Telegraph field!

The caterpillars I spoke about in a previous blog post belong to the lackey moth, this species lays clumps of eggs on the twigs of young growth bushes such as blackthorn and hawthorn. The caterpillars hatch at the time of leaf break, and feed on the soft, nutritious young leaves. They build a protective web to hide in and when the local leaves have been depleted they move camp and set up a new web home to feed on more leaves.

They are also protected by irritant hairs which most birds can’t cope with, but cuckoos have evolved to wipe these hairs off on twigs before consuming the caterpillars. Because these hairy caterpillars are quite conspicuous, they are good easy pickings for this specialist feeding behaviour, but there is a problem in the general countryside with a big decline in many of the hairy caterpillar species.

In the case of the Lackey and Small Eggar moths which were seen all over the laid hedges this week, they are susceptible to annual mechanical hedge cutting; the eggs are on the winter twigs which get trimmed off before the eggs hatch, so when spring comes around the eggs are disconnected from the food supply and a generation is lost. With hedge laying, the cutting is drastic but only happens once every decade or so, giving the hedge many years of regrowth when the eggs are safe from trimming off. The vigorous regrowth of a laid hedge is also much more nutritious for the caterpillars.

Leaf break varies from tree to tree, both individually and between species, with hawthorn being one of the earliest and beech one of the latest. My garden Blackbird nest is in a beech hedge and is still hidden behind the dead leaves of last year, with no sign of fresh leaves yet (this morning, the chicks looked about ready to leave the nest).

The two most widespread and important trees in the British countryside are Oak and Ash, and the old saying is "Oak before Ash, summer’s a splash; Ash before Oak, summer’s a soak". This saying is not backed up with statistical evidence, but it does have some bearing on the weather leading up to spring.

Ash leaf break Ash leaf break is essentially governed by the changing daylength, so varies very little from year to year, although cloudy springs may delay leaf break a bit. Ash leaf break is governed by temperature, so in warmer springs the leaves come out earlier than in cold springs. However, the cloudiness or temperature of the spring does not tell us much about the summer to come!

Oak leaf break Today, I came across both trees in leaf break and both in winter dormancy, all on the same area of land at Westfield… so you can take your pick as to predicting the summer. All I do know is that after the wettest February on record, we are in a prolonged dry spell, with the first drop of rain I’ve seen this month falling on the garden overnight being something to celebrate.

All these fresh new leaves will be food for thousands of moth and butterfly caterpillars, and the woodland birds are timing their egg-laying to coincide with this major hatch. It’s a lot easier for resident birds like the blue tits to get this timing right – they have been inspecting the dormant buds for weeks to see when to start nesting. Those that come here just for the summer are in some cases finding a mismatch with their arrival time and the emergence of caterpillars, this has been shown in the case of the pied flycatcher for example.

Dr Vince Lea
Head of Wildlife Monitoring 

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