Yesterday morning I did the bird census down our road. It was a lovely morning, cool, but not unpleasantly so, clear and sunny. As the sun rose, it warmed up, so by the time I got down to the meadows at the end of the census route, it was feeling pretty comfortable. Perfect weather for migration. It wasn’t just the birds who were on the move, though – so were the monarchs. I haven’t seen very many monarchs since arriving here, just the odd one here and there. Yesterday morning I probably saw about a dozen during the census – enough for me to note their increased abundance.
In northern climates, where the environment cools down or freezes during the winter, animals have evolved various ways of coping. For insects, most of which are unable to be active when it’s very cold, there are two strategies, stay or leave – overwinter (essentially a hibernation of sorts) or migrate. Monarch butterflies fall into the latter category. Monarchs are long-distance migrants, and the individuals I saw in the meadow yesterday will, in a few months, be spending a nice balmy winter in central Mexico.
There are three distinct populations of monarch butterflies – one east of the Rocky Mountains, one west, and one in Central America, all of which have unique migration patterns and overwintering destinations. About 90% of Canada’s monarch population live east of the Rockies, and all of them will head down to one of perhaps a dozen spots in central Mexico to spend the winter. These sites are all high-elevation oyamel fir forests located within about 800 square kilometers (309 square miles). This area has been designated as the Mariposa Monarca Biosphere Reserve.
The population from west of the Rockies winters further north, in central and southern California. There are about 200 known overwintering sites in the state, with anywhere from dozens to tens of thousands of individuals present at each site. The Central American population doesn’t undertake a latitudinal migration, but mostly moves short distances of 10 to 100 km (6 to 60 miles) from highland to lowland areas.
Not surprisingly, with populations overwintering in such concentrations, both habitat loss and natural disaster pose serious potential threats. I would be extremely surprised if the population hadn’t gone through such events before, and obviously had survived and rebounded, but it does pose some concern, especially in the face of climate change that has the potential to produce more numerous and more severe storms and conditions than the butterflies are used to weathering.
A monarch butterfly hatched at the beginning of the summer may only live for two months, but those hatched at the end, the ones that undertake migration, will enter a non-reproductive state that allows them to live for up to 9 months, enough to get them through the winter and started back north to breed again. Of course, it’s not enough to get our Canadian butterflies back home to us, so those monarchs that I saw in the meadow will not return. Rather, monarchs have developed a strategy to circumvent this short lifespan and still allow them to migrate. The butterflies that overwintered in Mexico will move north to breeding sites in the southern US in March and April. There they lay eggs and go through two or three generations. The latter generations are the ones who continue the push north, as the original adults by that time have died. This step-by-step generational approach to migration allows the monarchs not only to take advantage of milkweed as it starts growing in each region, but also helps to build up the population, which suffered losses over the winter. As well, by going further north, the species can produce up to three additional broods beyond when milkweed begins to die off in the southern US in June.
Our Canadian monarchs finally make it home in late May or early June. Those west of the Rockies may not return to British Columbia in all years. The best years are warm, dry summers, or summers with extended periods of sunny weather. The mechanisms by which the later generations find their way back north again is still unclear, but there is obviously a genetic component to it.
During southbound migration, monarchs prefer to stick to dry land and are reluctant to cross large bodies of open water (such as the Great Lakes). Part of this is because of their migration method – they use air thermals, rising columns of warm air heated by the sun striking the earth, to gain altitude, where wind currents higher in the sky will help give them a tailwind, or where they can glide down to the base of the next thermal (much like hawks). These thermals can sometimes take the butterfly to incredible heights – sometimes up to a kilometer (0.6 mile) high.
Air thermals don’t form over open water, so it’s much more work for the butterfly to cross a large lake than it is for them to follow a shoreline. However, sometimes following a shoreline leads them into what are often called migrant traps – places where the geography of the land naturally causes migrants to accumulate (this applies to both birds and butterflies). Because going in reverse counters their natural instincts, the migrants remain at the trap until such time as favourable weather occurs for crossing over the lake. Sometimes this is simply the next day for butterflies, if they’ve arrived mid-day, or sometimes they may hang around for a few days, if a string of cold or wet days occur.
One such migrant trap is the Leslie Street Spit (Tommy Thompson Park) on the Toronto waterfront. Although I was away most years, I was fortunate to be able to experience the migration last fall. Often monarchs will make a significant push on just one or a few days, leading to huge concentrations (for here, anyway; they’re paltry compared to those on the wintering grounds). We lucked out in catching the one big one last fall – we’d noted increased numbers of monarchs passing through earlier in the morning, and later in the day a trip to the tip of the spit revealed clusters of butterflies clinging to the tree branches, waiting for the next day to cross the lake.
We estimated there were perhaps 15,000 to 17,000 butterflies out there, split up over three or four locations, but with at least two thirds of them in a single large woodlot at the end. It was an amazing sight, but interestingly, a surprisingly cryptic one. When we first approached the woodlot we weren’t sure there were many butterflies there. It wasn’t until we actually walked inside amongst the trees, and disturbed a cluster, that we began to notice them. Then, once we started really looking, they were everywhere. Walking through from one end of the woodlot to the other was magical; as you passed the butterflies would rise from the branches where they were hanging and filled the air in golden clouds, before settling back down again once you’d passed. So light on their wings, it gave the space a more airy feeling than if it had simply been empty.
I probably won’t see numbers like that again without a special trip down to Lake Ontario; although monarchs will still roost together in smaller groups, such migrant traps just don’t exist away from the lakeshore. But even still, watching the butterflies dance among the meadow flowers is also very captivating and peaceful.
You can join in and help track monarch migration with the citizen science program Journey North, which invites you to submit your monarch observations to be included in a map compilation. The site also has lots of other great info about monarchs and monarch migration, including maps of this year’s migration.