Red Admirals increasingly overwintering

[Jack Harrison]

https://www.bbc.co.uk/news/uk-66395590

We have known that for a long time.

I think I am correct in saying that Red Admirals don't hibernate in the true sense as do say Peacocks or Small Tortoiseshells.  I have seen one basking in November sun when the temperature was just +2C.  I suspect that Red Admirals might be opportunist on fine winter days.  The ability to find nectar sources has become easier now that gardeners grow more winter-flowering shrubs.

Vince.  You're the Red Admiral expert.  Your thoughts please.

Jack

[Padfield]

Vince is the expert, but I'll add my ha'pence anyway!

The Great British Butterfly Count evidence doesn't obviously have anything to do with overwintering in Britain, since the count is in July. It is more probable, in my opinion, that increased numbers then reflect greater immigrant numbers, for whatever reason.

From my own observations, it is the length rather than the depth of winter that kills red admirals. In Switzerland, they can make it through to February, regardless how cold January was, but very few make it through to March. If there are successive warm, sunny days in January and February, they will come out of torpor, sometimes in good numbers, and mate and lay, and maybe these eggs produce adults later in spring. But the January and February adults do not survive. One year I photographed as many as I could in February, specifically to see if I could identify any individuals that went back into torpor and woke up again in March. None did. That February flight was their last flush.

Guy

[Vince Massimo]

Thanks Guy for adding your thoughts and observations. I hope that others will come in as well.

Jack, I have decided that some context is required before I come to my conclusion, so apologies for the length of my response.

The Red Admiral is primarily a migrant to our shores, although sightings of individuals and immature stages in the first few months of the year, especially in the south of England, mean that this butterfly is now considered resident. This resident population is considered to be only a small fraction of the population seen in the British Isles, which gets topped up every year with migrants mainly arriving in May and June that originate in central and southern Europe. Unless there has been a significantly favourable climactic event here in the British Isles in the preceding summer and winter, the number of adults seen in any one year is therefore dependent on the number of migrants arriving. Numbers will fluctuate as a result, so in some years this butterfly can be widespread and common, while in others rather local and scarce. The butterfly is not able to hibernate in the strictest sense of the word and the majority of adults leave the UK in late August to mid-October without mating and fly south to the winter breeding grounds in southern Europe. Those that remain are presumably trapped by the onset of cool conditions or bad weather, or are the result of late larvae producing adults in winter. These butterflies are usually unable to withstand our winter, especially in the cooler regions of the British Isles, however in the 10 years that I have been studying the lifecycle of this species and collating reports from UKB members, there have been increasing records of small numbers of adults overwintering during mild winters. Regular records also exist of late eggs and larvae which develop slowly in very sheltered locations, usually on or near the south coast, and produce spring adults. It has also now been established that eggs can be laid throughout the winter and larvae can hatch in any month of the year. Because it does not hibernate, the butterfly can become active on any sufficiently warm day in winter, so can be seen in any month of the year. When surveying for overwintering butterflies during the winter of 2014/2015 I recorded and photographed a specific individual butterfly on 6th December 2014 and again 64 days later on 8th February 2015. Another notable observation by Paul Harfield was of two Red Admirals roosting in his Hampshire garden over the winter of 2018/2019 - one for 33 days and the other for 92 days (27th October 2018 to 27th January 2019).

Some say that the hot, dry summer of 2022 resulted in a greater number of Red Admirals breeding in the British Isles, leading to more individuals overwintering and triggering the population boom we are now seeing. However, I saw few Red Admirals last summer and lots of dead and parched nettlebeds, so I personally do not think this theory works. Also, given the exceptionally cold winter of 2022/2023 and the wet/cold spring of 2023, I would not have thought that a large number of larvae would have survived the winter. However, data provided to me by my fellow Red Admiral enthusiast Dave Harris, who monitors larvae that regularly develop during the winter in his Newhaven garden, suggests that there was actually a slight increase in the number of larvae that successfully produced adults compared to recent years, but development was delayed compared to last year. There were 10 adults emerging between 26th April and 1st June this year compared to seven in 2022 (emerging between 14th March and 10th May). To put this into perspective, I have monitored over 500 eggs and scores of larvae in the wild over a number of winters and have never observed a resultant pupa or adult, unless I reared them on in the garden. Also, in my view, the jury is still out as to the contribution that overwintering adults make to the next generation beyond February. Even if they play a pivotal role, I did not observe anything unusual in early 2023 here in Crawley in terms of adult numbers, with the first one being seen on 21st January, five on 6th February, three on 19th February and nothing more until 5th April, so although this is only a very small data-set, the available evidence seems to suggest that only an average number of larvae and adults successfully overwintered. That being the case, then migration must be the dominant factor accounting for the numbers we are now seeing (as it is in most years). It was reported that on 4th May this year there was the start of a significant influx of Red Admirals and other migratory species into Sussex . Then, around 2nd July, there was the start of a another very large influx from mainland Europe into southern England, possibly coinciding with a home-grown emergence which accounted for the high numbers being recorded on the Butterfly Count.

The report on the BBC News website suggests that “Climate change sees migratory species stay in the UK”, but other than quoting the number of adults counted between 14th July and 2nd August, gives no further evidence. The count ends on 6th August. Whilst I am in no doubt that climate change is a factor, it could be that its effect on the winter breeding grounds in southern Europe could just as easily be the reason for the increase in numbers.


Reference material:
The Butterflies of Britain and Ireland (Thomas and Lewington 2010)
UK Butterflies (Red Admiral Species Page)
UK Butterflies (members reports)
Adastra Report of Sussex Butterflies 2023 (Martin Kalaher)
Observations by Paul Harfield
Observations by Dave Harris
Observations by Vince Massimo

[millerd]

I can add my own observations from my local patch near Heathrow (a bit of a "heat island", but well away from moderating coastal influences in the winter).

The last adult of 2022 was seen on 25th November (and was probably egg-laying).

Eggs and caterpillars were observed throughout December 2022, and into mid-January 2023. A final instar caterpillar was noted on 9th January, and the last larval tent on 13th.

Adult butterflies were then seen on 5th and 19th February 2023.

The poor March weather meant there were no further adult sightings until 5th April, after which they became regular:

5/4 1
7/4 1
9/4 3
10/4 1 (quite fresh)

On 11/4, there was a sighting of a Painted Lady, clearly a migrant.
13/4 2
15/4 2
16/4 1
17/4 1
19/4 2
20/4 1
3/5 2
4/5 2
5/5 7 (some fresh-looking butterflies)

7/5 6

Sightings then became more frequent, reaching double figures on 19th June, but it wasn't until the first week of July that they really started to climb. The peak number counted on my usual circuit was an astonishing 153 on 6th July.

I'm not sure what firm conclusions may be drawn from this. At the time, I thought the rather worn butterflies seen in early April had survived the winter, as I suspected those seen in February would have done. However, some of the worst weather of the winter was in March, and the sighting of a fairly fresh Red Admiral on 10th April and the immigrant Painted Lady the following day made me reconsider.

By the time of the explosion in numbers in July, there would have been a mix of new immigrants and locally-hatched individuals which were the offspring of earlier arrivals and (possibly) any over-winterers. Many of the July butterflies were sparkling new insects, and were joined by good numbers of new Commas and then exceptional numbers of new Peacocks. This is likely evidence of a really healthy and nutritious crop of nettles this spring, which would have benefitted all three species.

Dave

[Padfield]

Those who read French might be interested in the relevant parts of this article on butterfly migration, written by two friends of mine, Vincent and Michel Baudraz:

https://lepido.ch/observer-dans-la-nature/migrations/

The red admiral is le vulcain in French. If you scroll down, there is first a detailed account of seasonal movements in Europe, then lower down a description of the practical research done in 2016 capturing and marking red admirals as they passed en masse over the Col de la Croix. I was working that day ...

Guy

[Jack Harrison]

I can't read French but Chat GPT can. Here is the first paragraph in English.

The migration of butterflies is a fascinating phenomenon, but relatively little known. Everyone knows about the migration of birds, who can traverse the planet from north to south, crossing oceans or deserts. But insects are also capable of such feats, which is even more surprising given their very small size.

All species present in Switzerland are capable of moving daily over tens, or even hundreds of metres, navigating their habitat and its various structures. Some species additionally have a tendency to disperse, allowing them to colonize new favorable sites. These are particularly the species that require a specific and uncommon habitat, whose populations are fragmented, and for which dispersion also allows essential genetic mixing. Dispersion distances range from a few hundred meters to several kilometers, and probably do not exceed 10 kilometers. It is mainly the females that engage in dispersion, searching for their host plant to lay their eggs, while males are often more territorial.

However, in Switzerland, there are 22 species (or nearly 10% of species) that are considered migratory, meaning they can travel much longer distances in their lifetimes (in Europe, up to about 3,000 kilometers).

Jack