Vanessa at night
ngd at ceh.ac.uk
Wed Jan 23 15:00:41 EST 2002
[This is long - so be prepared! - and there's more to follow in a separate post!]
I have followed with interest the recent thread discussing whether butterflies fly at night. I decided to put the question to uk-leps, a UK-based email group, to see if anyone had any evidence of butterflies flying at night.
One of those who responded was from Eddie John who has studied the migration of the Painted Lady in Cyprus. I have copied (with permission) into this email two of his posts.
"The Entomologist's Record, last month (Vol 113 Part 6), published my account of a Painted Lady (Vanessa cardui) migration across Cyprus in March last year. No night sightings, as such, were observed but there were very early morning (08.00 hrs) coastal records of 'streams' of butterflies already heading north or north-west, inland away from the sea at Episkopi and Larnaca, for example. Given the geography of the area, their early morning arrival (they may have arrived much earlier, i.e. before dawn) and the continuous stream of migrants observed until mid-late afternoon on three successive days, it is inconceivable that they did not fly at night. Waves of migrants crossed the island over a three-day period (I received over 50, separate, one-minute counts from several observers) with all activity ceasing by late afternoon.
The most likely departure point was Israel where the migrants had been observed at a time consistent with their later arrival in Cyprus. They were later observed arriving on the south coast of Turkey shortly after being seen leaving northern Cyprus, on a northerly course. The extent of the front was estimated at 200 km and many millions of butterflies were involved, so the idea that these could launch themselves from an overnight pause on board ship seems far fetched, as there would have to be a large number of conveniently spaced vessels in order to maintain the 200 km front of the migration!"
In response to which, and wanting some clarification and to be sure I had properly understood all this, I asked the following questions?
"Were the Painted Ladies actually observed coming in off the sea at that early hour in the morning? How long does it take them to do the crossing? Are you sure they weren't sitting on the beach and coast, having arrived late the previous day, waiting for an early morning start?"
and Eddie's reply:
"No, there were no night-time sightings, but I consider the following to be compelling 'evidence' of a night or partial night, crossing. On 18 March at 09.30 hrs a very keen observer, Lyndon Thomas, noticed migrant V. cardui passing through his garden, close to the shore at Episkopi. The following morning, at 08.00 hrs he positioned himself, his wife and two house-guests over a 500 m area close to the coast at Episkopi (but, unfortunately, not close enough to witness them coming in from the sea). By this time the migration was seen in full stream (between 22 and 25 specimens per minute counted in separate streams over 16 x one-minute counts). The migration was watched continuously for an hour before the group moved 18 km inland where similar activity was observed an hour later (25 - 30 per minute counted over no less than a three-hour peak period;the butterflies travelling through a 25 m wide valley). It seems unlikely that such a large number would have left the coast at convenient intervals to allow continuous observation over five hours (the count adds up to 7,500 - 9,000 over the three-hour period at this inland site, with numbers falling by 15.00 hrs). Had they arrived at the Cyprus coast the previous evening isn't it more likely that they would have left en mass after nectaring the following morning, and have quickly cleared the island? Migrating cardui are purposeful and direct in flight (Torben Larsen, Ent Rec 1988 Vol 100:131, has 'clocked' migrating V. cardui at 30 km per hour) so within three hours all would have travelled through. As I see it, the only reasonable explanation for a 200 km, continuous island-wide crossing observed from early morning to mid-afternoon, is a night crossing. In this case, the evidence points to Israel (about 340 km away) as the source. No nectaring was observed until late afternoon when those still passing over AS DUSK FELL (sorry, I can't use italics to emphasise this point!) paused for the night. Migrant cardui were easily recognised from the resident populatio
with their foraging as though nothing was going on', as streams of migrants flew, non-stop, overhead.
Another piece of 'evidence' which negates overnight congregation at the coast is that on three successive days, numbers of migrants were seen to DWINDLE in the late afternoon at Episkopi (on the coast), rather than there being a huge increase in numbers, as would be expected if congregation were to occur."
Almost finally a short section from Eddie's paper relating to the above:
"Migrants were recorded arriving at the south coast of Cyprus from 08.00 hrs (but may have begun arriving much earlier) indicating that at least part of the passage took place in the hours of darkness. T.B. Larsen has clocked migrating V cardui flying at a speed of about 30 km per hour so the journey of approximarely 340 km fromthe coast of Israel couod have been accomplished in 12 hours or less. Yet observations of V. cardui behaviour in Cyprus, and probably Israel too, show that migratory activity ceases in mid-to-late afternoon, when the principle emphasis turns to nectaring. Nevertheless, the timing points either to an unlikely evening departure from Israel or a much slower journey commencing, perhap, 18-20 hours before the arrival time in Cyprus."
Eddie also writes in an article to be published in a local entomologist's group annual report:
"Torben Larsen accurately predicted the arrival of V. cardui on the south coast of Ireland, after a sea crossing of around 40 hours, having observed a migration northwards through Portugal (Larsen 1988 and per comm.)." (Not got access to Larsen's paper at the moment as our library is beng refurbished!)"
Sorry this is so long, those of you who have reached this far - but I trust it has been informative!
Mr J Nick Greatorex-Davies
(Butterfly Monitoring Scheme co-ordinator)
NERC Centre for Ecology and Hydrology
(Formerly the Institute of Terrestrial Ecology (ITE))
Cambridgeshire PE28 2LS UK
Tel: (+44) (0) 1487 773 381
Fax: (+44) (0) 1487 773 467
E-mail: ngd at ceh.ac.uk
Web site: http://bms.ceh.ac.uk
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