Mourning Cloak Mysteries: The Butterfly that Hibernates

By Joe Eaton
Friday March 07, 2008

Posted Sun., March 9—We were out at Lafayette Reservoir a couple of weeks ago, looking for the bald eagle that wasn’t there. But there was a fair amount of butterfly action: a probable echo blue, some small hyperactive orange jobs, and three or four mourning cloaks, sparring or courting—it’s hard to tell with butterflies. 

There’s no ambiguity about a mourning cloak: it is, as Roger Tory Peterson said of the adult bald eagle, “all field mark,” its deep maroon wings bordered with a broad pale band. On close inspection of the reservoir butterflies, you could see that the band had faded from yellow to bone white and that the wings were a bit ragged. These guys weren’t fresh out of the chrysalis; they had been around all winter. 

Adult hibernation is an uncommon life strategy among butterflies, but the mourning cloak, along with its close relatives the California tortoiseshell and Milbert’s tortoiseshell, does just that. Adults that emerge in midsummer or fall spend the cold wet months holed up in some sheltered place. Some have been known to winter under the eaves of houses or in cellars. Arthur Shapiro, UC Davis butterfly maven and co-author of Field Guide to Butterflies of the San Francisco Bay and Sacramento Valley Regions, says that whatever the weather is like, they rarely stir before January 25.  

They wake up hungry. Shapiro says local hibernators seek out willow catkins for nectar. In Wisconsin, according to a 1980 study by Allen M. Young, they rely on tree sap to fuel themselves for courtship and egg-laying, frequenting sap wells drilled by the yellow-bellied sapsucker. I don’t know how important this food source would be for California populations, although our red-breasted sapsuckers winter in the coast ranges until March or April, overlapping with the overwintering mourning cloaks. And what about mourning cloaks in Europe, where there are no sapsuckers? 

British lepidopterists, who have their own nomenclature, know this species as the Camberwell beauty. It was first collected in Cool Arbor Lane near Camberwell (now a densely built-up part of London) in 1748, and has turned up periodically ever since. However, it has never bred in the British Isles. Permanent range includes temperate Eurasia east to Japan, and the mountains of Central and northern South America. Apparently temperature-limited, mourning cloaks avoid the lowland tropics and subtropics.  

California has two behaviorally distinct mourning cloak populations. In the coast ranges, they’re resident year-round, producing at least two, sometimes three broods. Elsewhere, they’re altitudinal migrants like their tortoiseshell relatives. Shapiro, who has been monitoring a series of transect points from Suisun Marsh to Castle Peak in the Sierra for over 30 years, has observed mourning cloaks flying upslope along Interstate 80 in June. Their larvae feed on mountain willows. Some of their progeny hibernate in the mountains as adults; others return to the Valley for the winter.  

Tracking migrant butterflies has its technological limitations: you can’t rig a radio transmitter on a mourning cloak. But Shapiro wonders whether some of the stable isotope techniques used with migratory birds could be applied to these fragile travelers. The ratio of hydrogen isotopes in a warbler’s feathers in winter can indicate how far north it was when it grew those feathers before migrating. A butterfly’s tissues should contain a similar latitudinal signal. 

Something happened seven years ago to disrupt the mourning cloak’s migration cycle: after a breeding failure in the Sierra, the butterflies have remained rare in the mountains and the Sacramento Valley. Shapiro found none at Donner Summit last fall, for the first time in 36 years. “The cause of all this remains a mystery,” he says, “compounded by the simultaneous regional decline of all our other willow-feeding species in the Valley,” the willow hairstreak, Lorquin’s admiral, and sheep moth. There are still plenty of willows, and the admiral and the moth are holding their own elsewhere.  

Mourning cloak females lay large batches of eggs, and the caterpillars—spiny black creatures with red spots—stick together. 

Sometimes a brood will defoliate its host tree. They also pupate in clusters. A couple of sources say the pupae twitch in unison when disturbed, which is something I would pay to see. (Shapiro’s field guide describes mass pupal twitching in the California tortoiseshell.) I’m not clear about what kind of sensory apparatus a pupa has while it’s being reorganized from a caterpillar into a butterfly, or how you would alarm one, let alone a whole clutch. 

More mysteries. 

When an adult mourning cloak emerges from its pupa, it voids—how can we put this delicately?—a drop of blood-red liquid. “In medieval Europe,” Shapiro writes, “such ‘red rain’ was taken as an omen and often stimulated civic disturbances and demonstrations of religious fanaticism.” Those were nervous times, with all the wars and plagues and crusades and massacres, and it’s understandable that people would get all wrought up about butterfly poop. Good thing we’re not that credulous anymore.