Lepidopteran lodgers: the moths that live in bird nests

This blog post is based on the introduction of an article I penned in September 2018 for Atropos, an excellent magazine for British Lepidoptera and Odonata enthusiasts. It was published in issue 62 (2018).

Very hungry caterpillars

A caterpillar munching on a leaf is probably what comes to mind when most people picture the early life stages of butterflies and moths. But if Eric Carle’s classic is to be believed, caterpillars have a much more varied palette. It is certainly true that many species shun the conventional diet of leafy greens, even if lepidopterans that enjoy a diet of chocolate cake and Swiss cheese are confined to children’s storybooks.

Not all caterpillars eat leaves…. Image: Peter Vahlersvik / iStock (RF)

Take Cryptoses choloepi, a pyralid moth found in the forests of South America. The caterpillars of this species feed exclusively on sloth droppings. Or Hyposmocoma molluscivora, which spins silk to ensnare passing snails. Some species have to be quick. In Hawaii, a number of pug moths Eupithecia spp. eat flies. These caterpillars patiently adopt a twig-like posture, ready to strike when one lands nearby (check them out in action in this Youtube clip).

There’s no need to venture to the tropics to find weird and wonderful feeding strategies. Look no further than the remarkable china-mark moths Acentropinae, whose aquatic larvae feed on pondweed in waterbodies throughout the UK. A sizeable number of British species manage to obtain their sustenance from dead wood, while a couple have specialised on beeswax. As testament to how difficult it is to extract nutrients from honeycomb, the larvae of Greater Wax Moth Galleria mellonella can also break down plastics.

Given this wide array of feeding strategies, it may come as no surprise that some moths have taken to living in bird nests.

An unlikely home?

Moths are not the only lodgers in bird nests; 17 other invertebrate orders have been recorded living in nests globally and a single study in England documented over 120 different arthropod species. A nest full of hungry beaks would appear to be a rather perilous living arrangement for most insects. What can make living in the home of a predator worthwhile?

At first glance, bird nests might not seem that special. But to invertebrates, they represent a veritable trove of organic detritus: from nesting materials, such as dried leaves and moss, to feathers and nutrient-rich guano. Even the birds themselves may be food, both dead and alive (depending on whether you’re a decomposer or a parasite).

Tinea semifulvella
Tinea semifulvella: a common inhabitant of bird nests which seems to prefer nests made in vegetation to those in nest boxes.

It’s thanks to this varied assortment of resources—concentrated in one spot—that nests are able to support such diverse communities. On top of this, bird nests tend to be sheltered and can be relatively warm, especially while chicks are being incubated—all the makings of an invertebrate’s paradise.

The buffet inadvertently laid out by birds clearly benefits the invertebrates. But is this relationship ever mutually beneficial? Perhaps. Some lodgers may pay their way by recycling waste materials, helping to keep the nest clean. It’s also possible that birds enjoy a reduced level of parasitism if their nest contains a diverse community of invertebrates, as these will tend to include predatory species that help keep the number of fleas and mites low. Tree hollows, the natural analogues to artificial boxes, may be especially rich in these beneficial invertebrates, as these have faster rates of detritus decomposition and fewer ectoparasites.

Learning more about bird nest moths

In Britain, around 20 species of detritus-eating moths are supposedly found in bird nests. These can be easily studied by putting old bird nests in a sealed container—the only difficult thing is remembering to check for adults in the spring! This is a great activity to do over the winter when there’s not much else going on. If this sounds like your thing, check out the second half of my Atropos article (pg 45 onwards) for a ‘how to’ guide.

I spent much of my final undergraduate year doing just this (but on an almost industrial scale, with over 200 bird nests!). The quantitative findings were published in Ecological Entomology (pdf available here), and some of my natural history observations on the ten species of moth I found are in The Entomologist’s Record (pdf link). For the more visually-minded, see the poster I made for Butterfly Conservation’s Symposium last year.

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