Two overcast days on the trot here - so here is my last instalment of the Broxhead Blue study to date.
Roosting preferences
At the Broxhead lek around 8.30 am one day in late June 2017, I counted 11 females of which 9 where roosting together on heather in the space of a 5 metre square; four of them on a single heather bush no more than one metre diameter. When the sun shone, they all opened their wings, and when a cloud came over they all closed them back and sat once again in the roosting position. The males roosted more or less at random spacings across the lek while this was going on - only the females were roosting communally but some of the males were already active and flying about. I wondered about this all-female roost because the books say nothing about single-gender roosting, so two days later I went back at 6 am just after sunrise with the idea that none would have moved from their night roosting positions at that time. The place where I saw 9 females all together, with 4 on one bush was precisely replicated. They were all there again, even the four on the same bush, but they were accompanied by one male this time. This suggests that roosts are “rooted” so to speak. The same individuals appear to go back to the same locations to slumber overnight it seems. I found another communal roost at the other end of the lek which had four males and two females and another just outside the lek which had four males and one female but there were a number of males dotted around not in communal roosts which must have been still in their night-time positions because I prodded one with my finger and he did not fly off. This shows that all the communal roosts were bi-gender after all but additionally it proved that not all individuals roost communally. I did not observe a female roosting singly though.
They roost pointing downwards to minimise the effect of predation of course. This explains why they often have bits missing from the edge of their wings. This is where it has had a close call with a bird. If they roosted face-up, a bird, usually approaching from above would get the head end first, more likely killing the butterfly. But roosting face down means the bird is more likely to peck at the rear and tips of the wings. A butterfly in this position can just drop from its perch and fly off and away from the bird's beak more effectively with only a tatty wing-edge to show for it. This was proven to me during the roost around 6.15 am when a predatory insect in the shape of a dragonfly approached one roosting male from above and he merely dropped from his perch down to a lower perching point, effectively evading the dragonfly entirely in this one simple, energy-conserving move. Incidentally it seems that the more insidious predators take their opportunities at night or around dawn. One morning I saw a poor male Silver-studded Blue that had succumbed to the ambush of a Misumena vatia crab spider which was sucking the juices from its victim where he rested, still in his roosting position; and I kept seeing harvestmen and hunting spiders on the move at this time.
- 4 males and 1 female roosting
Wing-rolling
A behaviour where the hind-wings are “rolled” over part of the forewings. This was the first species I saw do this. I don’t intend to go into this here as there is a discussion topic on UK Butterflies under the “General” theme, entitled:
“Wing-roll behaviour of some of the members of the Lycaenidae family”
viewtopic.php?f=16&t=9738
Please feel free to add your own experiences and comments there. Thanks.
The only thing to re-iterate here is that SSB’s are capable of wing-rolling with their wings fully open and ajar as well as fully closed. When they do it with their wings open it reminds me of the Samba dance move!
Emergence and Decline
My book tells me that after the eggs have been layed, the larvae emerge in the following spring during March or early April and grow through four instars before they are ready to form a chrysalis. The ants either take or goad the caterpillar to their nest and the chrysalis forms within the nest. Three weeks later the butterfly emerges on a warm morning.
As a brood the Broxhead bunch begin their adult emergence in early June just around the time that the new blooms start to appear on the “pioneer” heather which the butterflies need for nectaring. The butterflies as a brood are then on the wing until mid August although individuals only last a couple of weeks within this time frame. There is usually only one brood in this country because we are at the northernmost limit of its temperature endurance. In warmer climes in Europe and elsewhere in the world there can be more than one brood and on the rare occasion even southern England may get a small second brood but I have not yet seen this happen at Broxhead.
One day I noticed a melee of males getting very excited in one spot on the Broxhead lek so I went over to investigate. Their centre of attention was what at first appeared to me to be a mutant male – I thought they were mobbing the poor thing for some reason. Then I spotted this “mutant” was climbing away from a black ant nest that had “erupted” with ants scurrying everywhere and then the true nature of the event came to me. This was a newly emerged butterfly just appearing for the first time from the ant nest where its chrysalis had been. Its wings were contorted into a crumpled mass as it had not yet had time to inflate them. Thus it could not yet fly but only walk. The crowd of male butterflies had congregated around it because they hadn't yet worked out whether it was a male or female. Each one wanted to be first in line should it turn out to be a female. After a while the group dispersed, I guess once it was realised amongst them that it was a male after all.
Video stills of the general melee...
- Silver-studded Blues crowding
...and the newly emerged male with its still-crumpled wings.
This incident made me cast my mind back to a previous year when I saw what I thought was a mutant male mating with a female. I now realise it was more likely a newly emerged male with its wings partially folded up. At the time I just noted it and walked on without taking a picture. I am now regretting that!
Even when a newly emerged male has inflated its wings, it may still not be ready to fly. I have seen a mature male bothering such a newly emerged male not realising it was not a female. The newly emerged male did not seem to be able to open its wings fully yet, just setting them very slightly ajar at best, and so could only walk away through the heather. What this seems to suggest is that males tell other males from females by seeing the blue and black markings of the other males' upper-wings or brown of the females', not by the bluish tinge of the underneath of the male and not by scent. If gender recognition is only achieved by this butterfly looking at the upper-sides of the wings one has to ask why they have evolved the silver studs on the undersides of their wings that give this species its name. I have as yet no definitive answer to this but it seems it has nothing to do with mate finding; perhaps it has more to do with predator evasion. After all the studs only appear along the edge of the hind wings, those wings that remain visible while roosting. There are no studs along the edges of the forewings. Perhaps the reflectivity of the studs against the non-reflective background of the rest of the wing is an adaptation that confuses some types of predator, for example dragonflies.
Because the Broxhead brood normally starts quite early in the season and there is only one brood, it can also finish quite early. The decline tails off gradually if the weather is fine all that time. By 10th July 2017 I counted only 3 males and 3 females in the Broxhead lek, all looking pretty tatty. There were none at all in the immediate surroundings and only one elsewhere. My book suggests the main flying period is from the beginning of July to mid August, but in 2017 at Broxhead, the brood was one month ahead of this timing and I am told Bramshott is even earlier. It was also true when I quartered the reserve in 2015 to count how many there were in total. In that year the decline was very evident by 8th July.
And that is it for my report on the study of Silver-studded Blues at the Broxhead reserve to date, although it is by no-means complete and I still have lots to find out about them yet.
I'm hoping to catch up with them next week when we're on holiday further south. Interesting fungus too - I especially like the first one.
