Painted Lady Butterfly Migration at Gibraltar: Why They Do Not Funnel Like the Birds

14 September 2026 Last checked 2026-09-14

Gibraltar counted 873 painted ladies across three springs and 94 per cent of them landed in one year. The Rock is a refuelling stop on a relay of at least 12,000 km, not a raptor style bottleneck.

Eight hundred and eighteen of the 873 painted lady butterflies recorded in three springs of transect counting at Gibraltar turned up in a single year. That is 94 per cent of a three year total landing in 2006 alone. Before you plan anything around this migration, that one ratio tells you most of what you need to know.

The painted lady does not arrive on a timetable. It arrives in pulses. In a quiet year you can stand in the right place, in the right week, on a perfect morning, and see almost nothing.

What was actually counted on the Rock

The only butterfly count from Gibraltar dense enough to argue with is a three year transect study at Windmill Hill Flats, published in the European Journal of Entomology in 2014 by Keith Bensusan and Charles Perez of the Gibraltar Ornithological and Natural History Society with British and Polish co-authors.

The method was plain. Line transects of 130 metres, two of them in 2006 and 2007 and one in 2008, walked every day from 25 April to 5 June, three times a day at roughly 10am, 1pm and 4pm, recording every butterfly within 3 metres of either side. That came to 414 separate transect surveys.

The result was 2,508 butterflies of 19 species. The painted lady was the most numerous single species at 873 individuals, 34.8 per cent of everything counted. Together with the clouded yellow and the red admiral, the three migratory species accounted for around 70 per cent of every butterfly seen. All the resident species combined made up the remaining 30 per cent.

Read the last column, because it is the one that changes how you plan. The painted lady was the most abundant butterfly on the site and yet it showed up in only 34.5 per cent of surveys, while the clouded yellow, with 181 fewer individuals, appeared in 50.7 per cent of them. The painted lady averaged 2.11 butterflies per survey with a standard deviation of 7.78. A number scattered that badly is not a trickle. It is mostly nothing, interrupted by a great deal at once.

The counts also hand you a date. Painted lady, clouded yellow and red admiral all peaked between Julian days 130 and 140, which is 10 to 20 May. The median date for the painted lady was day 135, which is 15 May. If you get one week in Gibraltar for this, that is the week.

For scale, GONHS lists 609 species of Lepidoptera recorded in Gibraltar up to 30 October 2020, the great majority of them moths. Gibraltar's butterfly fauna at the time of the study stood at 32 species, and 22 of those had been recorded regularly at this one patch of 12 hectares.

The journey those 873 butterflies were part of

No individual painted lady completes the circuit. The species runs it as a relay, and the butterfly returning to a place is several generations removed from the one that left it.

The distances have kept growing as the fieldwork has pushed further south. Hydrogen isotope work published in 2018 showed sub Saharan origins for migrants collected around the Mediterranean in February, evidence of a northward trans Saharan spring leg, and put the annual circuit at 12,000 km with two Sahara crossings. A 2021 analysis called it the longest annual insect migration circuit known at 12,000 to 14,000 km. Then came seven field expeditions across six African countries in the Decembers and Januarys between 2017 and 2020, whose authors write that their results expand the Palearctic African migratory annual cycle up to 15,000 km.

We are quoting that last one rather than paraphrasing it, because the wording is ambiguous about whether 15,000 km is the new total or the size of the extension, and we would rather hand you the sentence than guess at it. What is not in doubt is the direction of travel. You will still see 12,000 km quoted almost everywhere, including in headlines written this year. That was the 2018 figure, and every study since has made the circuit longer.

A 2024 isotope study of 118 specimens collected either side of the Sahara describes the annual cycle as 8 to 10 overlapping generations of continuous movement and breeding. Autumn legs exceeded 3,000 km, some of them 3,500 km from northern Europe to West Africa. A generation takes roughly a month in the warmth of the Afrotropics and an adult lives a few weeks, so the butterfly leaving the Sahel in February is four or five generations removed from the one that left Europe in September.

Then there is the flight that makes the Strait look trivial. In October 2013 painted ladies were found on Atlantic beaches in French Guiana, where the species has no business being. Genomics, pollen DNA and multi isotope geolocation published in Nature Communications in 2024 put them at a minimum of 4,200 km over open ocean from West Africa, crossed in 5 to 8 days on trade winds, and probably more than 7,000 km from a European start. Same species. Same equipment as the ones on the transect at Windmill Hill.

Why 2006 and not 2007

The year to year swing is not noise, and it is not local. An analysis of butterfly monitoring data from northeastern Spain, the Netherlands and England and Wales across 1994 to 2015 found the size of the spring immigration into western Europe varies by more than 100 fold between successive years. Three things predicted it best: winter vegetation greenness in the savanna and Sahel south of the Sahara, spring greenness in the Maghreb, and how often the wind blew the right way during the migration windows.

That study names its influx years: 1996, 2003, 2006, 2009 and 2015.

2006 is on the list. It is also the year Gibraltar's transects produced 818 of their 873 painted ladies. Two datasets, gathered by different people for different reasons, agreeing on the same year.

The effort argues the same way. 2007 had more transect surveys than 2006, 166 against 152, and produced 165 butterflies of all species against 1,771. More looking, fewer butterflies.

It is worth being clear about what that does and does not prove. Three springs cannot test a rainfall signal, and the survey effort was not identical across them, with two transects in 2006 and 2007 and one in 2008. What the agreement supports is the direction of the effect, not its size. The scale of a Gibraltar spring is set in the Sahel the winter before, thousands of kilometres from anybody watching the Rock.

The thing that is not established

Gibraltar's name as a migration crossroads was built on birds, and the mechanism behind it is specific. Soaring raptors climb on thermals, thermals form over land and not over water, so birds heading for Africa converge on the narrowest gap they can find. That is about 14 km at the Tarifa pinch, and it produces the autumn raptor crossing over the Rock. From Gibraltar itself the African coast is 21 km away.

Painted ladies are not bound by any of that. Radar in southern England recorded their southbound autumn flights averaging over 500 metres in altitude at around 30 mph, close to 48 km/h, selecting a wind rather than hugging the ground. The 2024 Sahara work describes the crossing as broad front and parallel, running north to south rather than squeezing east or west. An insect that crosses 4,200 km of ocean has no use for a 14 km shortcut.

There is also a direct measurement from Gibraltar, and it is the most awkward fact here. A 2009 study in Animal Behaviour caught spring generation painted ladies in southern Gibraltar, presumed to be arriving from North African winter breeding grounds, and used flight simulators to record which way they tried to go. They showed no evidence of northward headings. The scatter was wide and the mean direction was west.

The same paper found laboratory reared autumn butterflies in the United Kingdom orienting south southwest as expected, and butterflies with no view of the sky flying at random, which is the evidence for a sun compass. But a 6 hour clock shift produced no change in orientation, which is not what a textbook sun compass predicts. The picture is genuinely unfinished.

The 2014 transect authors are careful in the same way. They note that migrating butterflies are sometimes seen arriving from the sea at Gibraltar, and then state plainly that this remains to be demonstrated using standardised methods.

The people with the best butterfly data on the Rock are not claiming to have measured the crossing. That should tell you how to read everyone who does.

So our position. Gibraltar is a good place to see painted ladies and a documented refuelling stop: the surveys found adults in numbers and no larvae to go with them, which is what a stopover looks like rather than a breeding site. It is not a funnel. Any page telling you the Strait concentrates butterflies the way it concentrates honey buzzards is reasoning across from birds and has not checked. The same 14 km of water shapes the pilot whales that never leave the Strait and the raptors overhead in completely different ways, and the butterflies are a third case again.

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Where you can actually stand

Here is the awkward practical consequence. The best butterfly data in Gibraltar comes from ground you should not assume you can walk onto. Windmill Hill Flats is roughly 12 hectares of garigue at the southern end of the territory, managed by the United Kingdom Ministry of Defence as a military training area and run as an MOD Conservation Area inside a wider Special Area of Conservation. We are not going to send you there, because we cannot point you at a published public access rule for it. Ask GONHS or the Botanic Gardens first if you want to try.

What is open, free and genuinely good is the Gibraltar Botanic Gardens, the Alameda, every day from 8am until sunset with no admission charge. GONHS has written about seeing painted lady there alongside monarch, two tailed pasha, cleopatra, red admiral and small copper. It sits at the foot of the Rock a few minutes from the town centre, the planting is nectar rich, and the paths are steep in places, with disabled access along Red Sands Road.

The Upper Rock Nature Reserve charges 30 GBP for adults, 22 GBP for children aged 5 to 11, and nothing for under fives. The cable car is closed for refurbishment and is not expected back until 2027, so reaching the top means a licensed taxi tour from the frontier, which leave roughly every 15 minutes, or walking up. A free shuttle runs from the border to the cable car station between 10am and 4pm, though not reliably when cruise ships are in port.

Go in the middle of May. The published peak sits between 10 and 20 May, with a median of 15 May, and nothing about the wider research contradicts that window. For the autumn return we could not find a published Gibraltar count to set beside the spring one, so we are not going to invent a date. The southbound passage is real and well documented elsewhere in Europe. What it looks like from the Rock has not been measured the same way.

What we tell guests who ask

Most people book the Rock for the macaques, then ask what else is moving. In May the honest answer is to look down as well as up. A butterfly morning costs nothing at the Alameda and needs no scope, which makes it the cheapest good wildlife hour in the territory.

Set your expectations from the numbers, not from the photographs. In a strong year you will see clouds of them. In a weak year you will see a handful, and that is the same migration behaving normally. Anyone promising otherwise has not read the counts.

The frontier is the other thing guests get wrong. Since 15 July 2026 there have been no routine passport checks at the Spain border, so the open country of the Campo, which dwarfs Gibraltar's 6.5 square kilometres, is a short walk and a bus ride away rather than a queueing gamble. More thistles, more ground, more butterflies.

Victory Suites runs 100+ apartments in Gibraltar, on Devils Tower Road, about ten minutes on foot from Grand Casemates Square and a short walk from the frontier for anyone heading into the Campo for the day. Every apartment has a full kitchen, which matters more than it sounds for a mid May trip: you can be out for the 10am peak and back for lunch without paying restaurant prices twice a day. Rates are quoted in GBP, and the on site team can tell you what is flowering in the gardens before you walk down.