Cuckoos can stomach toxic caterpillars, but it appears they are also impervious to being stung externally, as bird expert Colin Miskelly discovers.
A previous blog on this topic referred to shining cuckoos seeking out and eating prey that are toxic to other birds – including caterpillars with urticating hairs.
‘Urticating’ refers to hollow spines that have a venom gland at their base. The spines function like a hypodermic needle, injecting poison into any foolhardy animal that ventures too close.
The example referred to in the blog was a shining cuckoo photographed eating furry larvae of the gum leaf skeletoniser moth.
These caterpillars come with a health warning. If they contact human skin, their hairs can cause local pain (sometimes severe) and swollen itchy welts.
Cuckoos are able to eat prey with poisonous spines, as their stomach has a thick mucous membrane that the spines become embedded in.
The membrane is regurgitated, shedding the spines in the process.
The term ‘urticating’ is derived from Urtica, the genus name for stinging nettle plants (and derived from the Latin urere – to burn). There are nine species of Urtica nettles in New Zealand, with the most infamous being the tree nettle Urtica ferox.
As its name suggests, this is a shrub that it is advisable to learn how to recognise and avoid.
Nettles are covered with tiny poisonous spines that provide the plant protection from browsing animals. There are a few animals that have developed ways to overcome these defences, including the caterpillars of red admiral butterflies.
These caterpillars feed only on nettle leaves, and gain protection from their own predators by hiding among the nettle’s spikes.
While red admiral caterpillars are themselves spiky, they are not considered to have urticating hairs. However, other members of the same genus in the northern hemisphere do have urticating hairs.
As a result of posting the previous blog about shining cuckoos eating toxic prey (including monarch caterpillars), I was sent the following remarkable images of a shining cuckoo catching and eating red admiral caterpillars behind a South Westland beach.
The images were taken by Gerry McSweeney of Wilderness Lodge Lake Moeraki on 5 November 2016. They show a cuckoo repeatedly entering a dense patch of tree nettle on the coastal fringe, and flailing about while searching for caterpillars.
The bird was watched for about 45 minutes, and allowed observers to approach within four metres.
Apparently immune to the nettle’s stings, the cuckoo would then land on the nearby beach sand to consume its spiky prey, before returning to the tree nettle for another caterpillar.
The series of images suggest that cuckoos have both internal and external tolerance for urticating hairs.
With many thanks to Gerry McSweeney, Norm Twigge, Nathan Hill, Rob Lynch, Gonzalo Avila and Jeremy Rolfe for their fantastic images providing insights into the ecological relationships between New Zealand’s birds, plants and insects.
This all looks like a variation on ‘anting’, apparently a common behaviour in our introduced birds such as blackbirds, and like dust-bathing in sparrows.
Anting birds become totally engrossed in their behaviour, adopting body positions just as in Gerry’s photos. I think the cuckoo is using the nettle urticating hairs against feather lice and mites, and at the same time recognising the caterpillars as food.
Gerry’s subject bird probably took a long time to realise it had an onlooker.
And nettle is host to a leaf-mining fly, and at least 3 moth species that feed only on onga-onga.
Thanks very much for this intriguing suggestion John. We usually think of nettles as having a subdermal effect, with the toxin being delivered after the spines have punctured the skin. I suspect that ‘anting’ of tiny ectoparasites would require the toxin to be spread through the feathers, and for small invertebrates to be affected by contact with the toxin (rather than being pierced by a spine). Whether these can occur could be determined experimentally.
This was very interesting, thanks very much. I’m amazed that Red Admiral butterflies feed only on nettle leaves – there are a lot of red admirals about but I have no awareness at all of nettle trees. I’m going to have look for them!
I remember stinging nettles from when I was a child but I hardly see them at all these days.
Thanks very much for your comments Olwen.
Tree nettle is the primary host for red admiral butterfly caterpillars, but they can eat other nettle species. There is no tree nettle on the Chatham Islands, and the endemic subspecies of red admiral butterfly there is dependent on the smaller Urtica australis.
I live in part of Wellington where I’m fairly sure there are no (or very little) Urtica nettles. But there are a lot of admiral butterflies, feeding from the koromiko flowers etc. Either the butterflies fly in (at least a km or so, which I guess isn’t much in a good Wellington wind), or maybe the caterpillars can feed on the weedy plant pellitory-of-the-wall, which is abundant where I live and in the same family as Urtica.
Pellitory-of-the-wall photos: http://naturewatch.org.nz/observations?place_id=6803&subview=grid&taxon_id=57281
I too see red admiral butterflies regularly near central Wellington, and assumed that there must be nettles lurking off the beaten path in the town belt. It would be an interesting citizen science challenge to find out where and what are the host plants for Wellington’s red admirals.