Cuckoos are well-known for eating spiky and poisonous foods, but did you know they can eat toxic monarch caterpillars?

Shining cuckoo

The pure, whistling call of the shining cuckoo is New Zealand’s harbinger of spring.

This small cuckoo is one of our few migratory landbirds, spending the winter on tropical islands east of Papua New Guinea, and returning to breed here from about the end of August on.

Their characteristic calls are first heard in the north, but by early October they can be heard throughout the country.

Although readily identified by its song, it is much harder to get a good look at this most iridescent of New Zealand’s birds.

Even in bright sunlight, they can be superbly camouflaged, particularly among the bright green of fresh kowhai foliage (a favoured tree for foraging in – see below).

While few bird-watchers are lucky enough to get a good view of a shining cuckoo, a few enterprising photographers have managed to get images that give insights as to how the cuckoos live.

Citizen science

About a week ago, Rob Lynch provided a stunning image to New Zealand Birds Online that showed a shining cuckoo in a gum-tree, carrying a small furry larva.

I was interested to know what the cuckoo was eating, and so Rob sent me a couple of close-ups of the bird and its prey.

The images were forwarded to the secretary of the Entomological Society of New Zealand, who circulated them to members. Within 10 minutes my email inbox started to fill with replies, with 12 of the eventual 15 respondents suggesting that the cuckoo was holding larvae of the gum-leaf skeletoniser moth (Uraba lugens).

This Australian species is a pest of gum trees (eucalypts), and was first discovered in New Zealand at Mt Maunganui in 1992. The initial outbreak was eradicated, but the species reappeared at Auckland in 2001.

It is now well established in the northern North Island, Hawke’s Bay and Nelson (where Rob took his images).

The email replies were mainly from professional entomologists involved in insect pest management, including four from the Ministry for Primary Industries, two from Plant & Food Research, two from Landcare Research, two from universities, and one each from Scion and AgResearch.

Several respondents expressed their surprise at cuckoos eating Uraba larvae, which have urticating hairs that can cause painful rashes if they are handled by humans.

Cuckoos and their toxic tastes

Cuckoos in general are well-known for feeding on hairy caterpillars that other birds avoid.

In addition to Uraba, shining cuckoos often feed on the woolly-bear caterpillars of the magpie moth (Nyctemera annulata), and the spiky caterpillars of the kowhai moth (Uresiphita maorialis).

They are able to do so because they have a thick mucous membrane lining to the gizzard that the hairs become embedded in. Patches of the membrane are sloughed off and regurgitated, getting rid of the toxic hairs.

Cuckoos are also able to eat other toxic insects that are typically avoided by birds, including large numbers of ladybird beetles, which (when alarmed) discharge a fluid toxic to most vertebrates.

Even more surprising, two other contributors to New Zealand Birds Online have provided images of shining cuckoos eating the brightly coloured caterpillars of monarch butterflies.

Monarch butterflies and caterpillars are well-known for their aposematic colouring, warning of the cardiac glycosides they contain.

The glycosides, which are bitter, cause vomiting, and affect vertebrate heartbeats, are obtained from the milkweeds that the caterpillars feed on (typically two species of swan plants in New Zealand), and are retained in the tissues of the adult butterflies.

Shining cuckoos, and other cuckoos worldwide, often bash their caterpillar prey to remove the gut, which contains freshly ingested toxic plant material. But they must have other as-yet undiscovered physiological mechanisms that allow them to eat ladybirds and monarch caterpillars with apparent impunity.

Both of the shining cuckoos photographed feeding on monarch caterpillars were young birds, with indistinct barring on their faces and throats, and so I wondered if they had yet to learn that the caterpillars taste bad (and will make them sick). But Anna Barnett from Upper Moutere reported that the bird she observed returned to take several caterpillars.

And the cinerarias?

When Dick Sibson prepared the shining cuckoo text for the 1966 ‘A field guide to the birds of New Zealand and outlying islands’, he wrote that the cuckoos were ‘often killed by cats among the cinerarias’.

The passage appealed so much to Barrie Heather’s sense of humour that he included it in the 1996 The field guide to the birds of New Zealand, co-authored with Hugh Robertson.

As a school-boy bird enthusiast, I occasionally visited Dick Sibson (then retired) in his Remuera home. While I recall the bed of cinerarias outside his window, I did not notice cats or cuckoos lurking therein! The cuckoo connection with cinerarias is that they are a favoured food plant for magpie moth ‘woolly bear’ caterpillars, bringing the cuckoos within pouncing range of cats.

Inclusion of the cineraria image on the shining cuckoo page of New Zealand Birds Online is a whimsical tribute to two of the more colourful and beloved characters of New Zealand ornithology, both sadly no longer with us.

Related blogs

Cuckoos and their toxic prey – ‘urticated’ inside and out

Seasonality in birds and birdwatchers