Jezebel butterflies seem super abundant in Hong Kong during early 2024

I’ve seen various social media posts and even a couple of media articles about large numbers of Red-base Jezebels Delias pasithoe in Hong Kong recently. It seems people especially notice them during cold spells, when many of these butterflies are grounded by the chill, maybe dying – though with weather warming, at least some spring to life and feed on nectar.

It’s odd to me that the Jezebels have only been widely noticed in recent weeks, as I noticed what looked like a huge influx underway in autumn last year – and I believe this influx and hence later numbers was related to conditions in breeding areas in China, leading to a bumper season last year. And, in turn, related to climate change.

While Red-base Jezebels can be found year-round in Hong Kong, migrants also arrive in autumn, though have previously been less noted than tigers and crows. I haven’t found much information on their migrations, though did come across a paper noting their occurrence on islands off the coast of Fujian province, se China [two similar species are mentioned; perhaps both are occurring in Hong Kong though I haven’t checked]:

In recent years, a considerable number of pierid butterflies of the genus Delias have been found from Jinmen Islands, where no available hostplants grow, rendering a proof of cross sea water migration of these butterflies. It is suggested here that these butterflies come from nearby continent of eastern China, visiting the islands for nectar acquisition in seasons when nectar is in short supply because they mostly show up in autumn months. 

On mysterious occurrence of butterflies in the genus Delias at Jinmen Islands, with clarification on the nomenclature of D. pasithoe (Linnaeus, 1767) and D. acalis (Godart, 1819) in southern China

In autumn 2023, I spent several sessions watching for migrating birds of prey over Lantau and Cheung Chau. Mostly not many birds passing; but, especially in November, I did see jezebels passing south in what I figured must be considerable numbers: might see one, then another a few minutes later, and another not long after… I saw them pass over Ngong Ping – altitude over 500 metres – as well as the south Lantau coast, and over north Cheung Chau; along with taking ferry journeys and also seeing some migrating over the sea.

Some clusters of flowers also attracted multiple Jezebel butterflies to feed on nectar.

Given this influx, I figure the numbers of Jezebels seen in Hong Kong during this winter are not so much related to changes in the local situation, but result from event[s] in China. What might these be?

Major China Heatwave One Year, Rainstorms the Next

One highly unusual event [well, may become more usual…] was a major heatwave and drought that hit around central to east China in summer and autumn 2022. This even has its own Wikipedia entry:

To date, it is the country’s worst heat wave to have existed. According to weather historian Maximiliano Herrera, it is the most severe heat wave recorded anywhere in the world.

…

The drought and persistent heat had also caused more forest fires in China, particularly in Chongqing. Also because of the heat and lack of rain, the level of the Yangtze sharply decreased, and the Three Gorges Dam was opened in order to refill water into that river.

Poyang Lake, which is located in Jiujiang, Jiangxi, has been reduced to just 25% of its usual size due to extreme weather conditions, causing a major drought.

2022 China heat wave

I figured this had to impact ecosystems; and believe it was the key reason for larger than usual numbers of several robin, thrush, flycatcher and other bird species occurring in Hong Kong during autumn and winter 2022. Several of these birds also arrived earlier in autumn than usual; and not during cold surges as had been typical in years past. I believe this was because they had arrived – or bred – in areas where they might normally spend at least some time, such as central-east China forests, and found very little food available, hence moved south in search of better conditions.

Last year, by contrast, there were rainstorms across much of China, especially in autumn. Again on Wikipedia:

Several floods struck China starting in July 2023, most of them caused by heavy rainfalls in different areas.

2023 China floods

Again, these rainstorms and floods will have impacted ecosystems; though sadly without information reaching Hong Kong that I know of.

I believe they led to plentiful food in central – east China forests and other habitats; and this, along with warmer than formerly normal temperatures led to a dearth of songbirds arriving in Hong Kong for the winter.

Perhaps also there were ideal conditions for a bumper breeding season for Jezebels; aided by predator numbers reduced by the previous year’s heatwave? And many of the progeny then migrated south, resulting in the Hong Kong influx.

Why, then, no remarkable numbers of tigers and crows, or other insects or even birds in Hong Kong this winter? I have mentioned this quandary – by email – to insect expert Dr Tim Bonebrake, who noted:

This winter has been reasonably good for the tigers and crows based on our numbers. Last winter was absolutely dismal – Delias [jezebel] numbers last year were also low evidently.

Greenfly and Ladybird Blizzards in UK Summer of 76

While I’ve seen reports about numbers of locusts, say, prone to “explode” as conditions change, I can’t recall something quite similar other than during Britain’s hot summer of 1976. Then, I remember a day or several at my home town of Scarborough, on the Yorkshire coast, when greenfly [aphids] suddenly appeared in “blizzards”; and a short time later were followed by hordes of ladybirds that filled the air – perhaps after their numbers boomed with numbers of greenfly to feast on.

The 1976 event is covered in Ladybird Population Explosions.