Migration is widespread in animals, particularly in vertebrates, insects, and marine invertebrates. Migrants range from tiny insects to blue whales, and comprise a remarkable proportion of animals worldwide, and the distances they travel vary greatly, with some species making just local movements, and others traveling thousands of kilometres between continents and even hemispheres.
Like other animal groups, many butterflies show migratory behaviour. However, whenever the term ‘migration’ comes to our head, we are likely thinking of the same individuals completing the full journey, showing two-way movement, and using the same route, potentially several times throughout their lifetime – but the phenomenon is slightly different for butterflies, mostly due to their short lifespan. For example, in the case of the iconic monarch butterfly migration, it takes 3-5 generations for them to complete the migration cycle! In the cases of other species, we observe one or two butterflies, flying in the same direction and it continues for several days. While this might not attract our attention, because of the low number of individuals involved, they however are migrating.
So, how widespread is butterfly migration? In our recent research, published in Biological Reviews, we discovered that several hundred butterfly species show evidence of migratory movements. Indeed, the rate of ‘discovery’ of migratory movements in butterflies suggests that many more species might in fact be migratory (Figure 1). Butterfly migration occurs across all families, in tropical as well as temperate taxa; Nymphalidae has more migratory species than any other family (275 species), and Pieridae has the highest proportion of migrants (13%; 133 species).
Migration in butterflies was first recorded in the early-19th century by Charles Darwin during his famous journey on the Beagle, followed in the early 20th century by other notable contributions. Since then, observing and recording migrating butterflies has become popular among professional and amateur lepidopterists worldwide, and now hundreds of species are known to be migratory.
Discoveries of migratory butterflies have increased steadily in recent decades, yet much of this research effort is focused on a very small number of species, with little work on others. Nearly 55% of the studies we located were focused on just two species, the monarch Danaus plexippus (215 studies) and the painted lady Vanessa cardui (124 studies). For 76% of migratory butterflies, we found only one or two research studies, with more than 10 publications for only 5% of migratory butterflies. For less-studied species, most authors used relatively weak lines of evidence to diagnose migration, such as records of mass movement, or nomadic behaviour. Very few studies have reported physiological (5%) and experimental (11%) lines of evidence.
Our results also reveal that there is a clear disparity between documentation of migration and the global distributions of species in which migration has been documented. This presumably indicates either a spatial bias in where migration is studied, or that these species do not migrate in all parts of their geographic distribution.
Migratory butterflies occur worldwide, although the geographic distribution of migration in butterflies is poorly resolved, with most data so far coming from Europe, USA, and Australia. Migration is much more widespread in butterflies than previously realised – extending far beyond the well-known examples of the monarch and the painted lady – and actions to conserve butterflies and insects in general must account for the spatial dependencies introduced by migratory movements.
Migratory species can be acutely vulnerable to threats since they depend on a series of habitats throughout their migration route. Yet, reports on the conservation status of migratory butterflies are sparse except for monarchs. While European populations of some well-known migratory butterflies are considered stable [e.g., painted lady Vanessa cardui, red admiral Vanessa atalanta, and clouded yellow Colias croceus], there has been a ca. 80% decline in the North American migratory monarch butterfly over the last decade (Semmens et al., 2016 Scientific reports, 6(1), 1-7). If the acute declines recorded for the North American monarch are replicated in other migratory butterflies around the world, there might be hundreds of such declines that are going undetected.
Several factors including loss of larval food plants and nectar resources, degraded overwintering habitats, climate change, influence of invasive species and natural enemies, global wind patterns, artificial light, habitat fragmentation, pathogens, agriculture and genetically modified crops, and declining fecundity, etc., have all been implicated, demonstrating the complexity of diagnosing and reversing declines in populations of migratory species. Successful conservation of migratory butterflies will depend on achieving a much better understanding of which species are migratory, and whether and why populations are declining.
If you are interested in reading the full paper, here is the link.
This article was first published in the IUCN Butterfly Specialist Group newsletter.
