Hunting Brown Hairstreak Eggs In Winter

BH Egg

Where do butterflies go in winter? Well some overwinter as adults, some as pupae, some as caterpillars and some as tiny eggs. One species that overwinters as an egg is the rare Brown Hairstreak which lays its eggs on young blackthorn growth.

Although it seems unlikely, it is easier to find the eggs than the adult butterfly as the latter spend most of their time out of sight in the treetops or along hedgerows. So the best way to monitor them is to count the eggs in winter, a painstaking task but one that gets good results and helps us track the fortunes of this beautiful butterfly.The eggs themselves are nearly always laid in the fork of a branch on young growth not covered in lichen.

They are usually laid from 20 – 200cm above ground, and on the sunny side of hedges, so in easy sight of the diligent observer (see photo).

Last week-end I helped a group of 10 volunteers from Dorset BC Branch to count eggs at our Alners Gorse reserve in North Dorset. We fortunately had a dry day, though our hands were very cold by the end. In order to quantify our search effort, we simply record the time spent searching and the number of people counting, to get an average of numbers seen per hour.

The bigger the population, the higher the density of eggs. If the density keeps dropping, we know things are going badly! If numbers increase, we know things are going well.

Our results showed that the number of eggs per hour of search fell in 2012 as follow:

2011: 19 eggs in 360 minutes searching = 3.2/hr

2012: 27 eggs in 600 minutes searching = 2.7/hr

We were pleased by this result as last summer was the wettest on record and many species did badly. But it looks like the Brown Hairstreak did OK.

The Brown Hairstreak is unusual in that most colonies breed over large areas of habitat, taking in several square kilometres of hedges and woods where young blackthorn is found. So the eggs at Alners are just a small part of the population. We also did a count on some neighbouring land which showed a far bigger drop from 7.5 to 1.9 eggs/hr.

Despite the poor numbers, the beautiful adults were regularly seen at the reserve during late July and August, so why not come along next year and see if you can spot one. Or you can join us in next years count, its not difficult, but you do need patience!

Hunting this elusive butterfly is well worth the effort.

Martin Warren

Chief Executive

Twitter - @martinswarren