Butterfly surveys ... in winter?

Confused? So was I when asked to help with December butterfly surveys. But, these weren't normal butterfly surveys. No, we were searching for butterfly eggs!

Although for most butterflies the most obvious way to search for them is in the spring and summer months as adults, for Brown Hairstreaks (Thecla betulae), egg searches are the most effective way to record this species and even to uncover a colony. The sites I surveyed have risen from 36 eggs to 186 eggs across only a year or two!

Winter hedge habitat for egg searches (Jen Jones)

Butterfly lifecycles

There are a few habitat specialists which are surveyed in a different way to standard butterfly transects (example recording forms and methods are on the UKBMS webpage). These include the Brown and White-letter Hairstreak egg counts, but also Duke of Burgundy, and Large Blue egg counts and Marsh and Glanville Fritillary larval web counts. Interestingly, the Green Hairstreak is the only UK Hairstreak species to not overwinter as eggs.

WLH egg (Peter Eeles)

WLH adult (Tim Melling)

White-letter Hairstreak (WLH) declined as Dutch Elm disease killed the Elms used for breeding. WLH have started using Elm suckers and Wych Elm (better resistance) in the Upper Thames. You should focus on Elms above 2m high (late June – early August for adults or late October – February for eggs).

BH adult (Iain Leach)

BH egg (Gillian Thompson)

Brown Hairstreaks (BH) adults are active late July to mid-September. Changes to hedge management (e.g., flailing) and their limited flying capability have caused them to decline in abundance and be listed as a BAP species. They prefer sites with master (Ash) trees during the breeding period.

Methods - Brown Hairstreak eggs

Survey timing

Butterfly egg surveys take place in the winter as these species overwinter as eggs until April. Ideally eggs can be surveyed from October to mid-February. The earlier, the better as less eggs will be predated or damaged; try to aim for after Blackthorn leaves have been dropped.

Where to survey

As the BH is relatively rare and declining in populations, survey areas can be decided based on previous presence records and suitable habitat (such as hedges and scrubby Blackthorn suckers). OS maps and satellite images can be handy to look for where open access hedgerows, scrubby patches or woodland edge habitats (along roads, lanes, and adjacent to public footpaths). If needed, this letter can be used if you want to contact a landowner to agree access.

Habitat suitability can be easily assessed by a drive-by/walkover to look at the Blackthorn on site. Typically, a sunny aspect sheltered from the wind has a high chance of egg success. Management of hedgerows can provide more young growth and introducing scalloped edges to increase sheltered micro-climates for breeding, feeding and moving through typically exposed agricultural landscapes (Merckx and Berwaerts, 2010 and Hedgelink, 2013).

As long as the hedge has one of the six criteria present, it can be classed as potentially suitable. If a hedge fits all 6 criteria of ‘unsuitable’ habitat, then do not survey it. The list below are the suitable criteria for BH:

Hedge and habitat suitability for BH egg searches

Zones for eggs from Devon Wildlife Trust

For suitable hedges, you should spend 30 minutes per 100m of hedge. More details of what to record can be found on the guidance document from UKBMS and Nf4 recording form. For each egg, information such as the height off the ground, aspect, shelter and which ‘growth zone’ of the hedge/scrub is present (see opposite). Record a location point for each egg you find.

Suitable Blackthorn hedges for BH eggs (Jen Jones)

Eggs

Eggs are very small, about the size of a pinhead and are usually laid on Blackthorn suckers (20-200cm high, about knee to head height is a good rule-of-thumb) in the junctions of stems and branches. BH eggs look like mini white sea urchins, they are well-textured and they stand out easily against the darker stem colouration. A hand lens is very useful to bring along, as other species do lay their eggs on Blackthorn (see below).

Hairstreak eggs on Blackthorn (Upper Thames BC)

Other eggs on Blackthorn (Upper Thames BC)

Egg size (Jen Jones)

5 eggs (Bekah Beaumont)

Eggs are mostly laid singly or in pairs as seen below, but if you are lucky, as my colleague was, she found 5 on one branch very close together! Because of this a relatively low 'strike rate' is expected, don't be surprised if you only see one egg every 10 minutes. Also consider how easy the site is locate as this transect should be walked yearly.

BH egg detail (Jen Jones)

Winter tree ID

BH favour Blackthorn so some winter tree identification skill is needed. Hawthorn is one confusion species, it has lighter stems and longer and sharper thorns than Blackthorn, with red rather than black berries. Other Prunus species may lack thorns and flower at an earlier date than Blackthorn so this is another characteristic to look for.

Health and Safety

Bring warm and waterproof clothing and make sure you have somewhere for regular breaks out of the cold (don't survey during snow, frosts, heavy rain, strong winds or fog as you will be very cold and not find eggs easily)
Be careful when surveying along lane and road hedges, and leave them out if it is unsafe to use them (check for cars regularly).
Hold branches firmly to avoid them springing back and whipping you in the face, and be careful of the thorns