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Fall Migration - How do they do it?

Sep 25, 2014

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  • Migration

by Candy Sarikonda

It has been a promising summer, Monarch Larva Monitoring Project (www.mlmp.org) data show that monarchs are reproducing well and we should see a modest increase in the overwintering population this year. Get your gardens ready for migrating monarchs to fuel up on nectar sources. These monarchs will survive on stored fat all winter, so they need to build up fat reserves by nectaring along their migration route. Learn more here: http://www.learner.org/jnorth/tm/monarch/nectar_lipid_graph.html, and to find out when migrating monarchs will be moving through your area, enter your latitude here: http://monarchwatch.org/tagmig/peak.html. 

Many people have questions about the dynamics of fall migration.  Let’s explore some of those questions.  First, how do monarchs migrate?  Monarchs begin leaving the northern US and Canada in mid-August. They usually fly for 4-6 hours during the day, coming down from the skies to feed in the afternoon and then find roosting sites for the night.  Monarchs cannot fly unless their flight muscles reach 55ºF. On a sunny day, these muscles in their thorax can warm to above air temperature when they bask (the black scales on their bodies help absorb heat), so they can actually fly if it is 50ºF and sunny. But on a cloudy day, they generally don’t fly if it is below 60ºF.

On 50-60ºF days in the fall, you may notice monarchs flying very low to the ground.  If you drive on east-west roadways during migration time, you may see monarchs in directional flight flying at car-level. They may fly just over the top of your windshield as they cross the road in front of you! These monarchs are staying low to the ground, picking up radiant heat from the earth to help warm them.  Early in the morning, they will fly almost at ground level. As the arc of the sun rises, and the air mass above ground begins to heat up, monarchs will fly at 1-2m high.  By mid-morning, on a clear day, thermals will form and monarchs will ride these thermals.

Why do monarchs ride thermals? Thermals are columns of rising air, caused by uneven heating of the earth. They form wherever the air is just a few degrees warmer than the air next to it. Thermals often form wherever large patches of dark ground are adjacent to lighter-colored ground, such as over parking lots, above farm fields, highways, next to rivers and lakes, and above mountains. Monarchs use thermals in the same way that hawks do. Monarchs are so light that they can easily be lifted by the rising air. But they are not weightless. In order to stay in the air, they must move forward while also staying within the thermal. They do this by moving in a circle. The rising air in the thermal carries them upward, and their overall movement ends up being an upward spiral. Monarchs spiral upwards in the thermal until they reach the limit/top of the thermal (where the rising air has cooled to the same temperature as the air around it). At that point, the monarch glides forward in a S/SW direction with the aid of the wind. It glides until it finds another thermal, and rides that column of rising air upwards again. To see what a typical monarch’s flight pattern might look like, see www.learner.org/jnorth/tm/ThermalLesson.html  Migrating monarchs use a combination of powered flight and gliding flight, maximizing gliding flight to conserve energy and reduce wear and tear on flight muscles.  Monarchs can glide forward 3-4 feet for every foot they drop in altitude.  If they have favorable tail or quartering winds, monarchs can flap their wings once every 20-30 feet and maintain altitude.
 
Monarchs are often counted at hawk watches, since both monarchs and hawks rely on thermals and have similar responses to physiographic features. For example, both hawks and monarchs have a reluctance to cross large bodies of water where there are no updrafts, and will instead prefer to follow land routes and shorelines around large bodies of water. But while hawks are heavily dependent on thermals for a free ride, monarchs often move across open terrain. Monarchs throughout the eastern US maintain a heading that results in a convergence in Texas, with most monarchs crossing into Mexico between Eagle Pass and Del Rio.
 
What pathways do the monarchs follow?  Elizabeth Howard of Journey North and Andrew K. Davis published a paper entitled, “The Fall Migration Flyways of Monarch Butterflies in Eastern North America Revealed by Citizen Scientists.”  Since 2002, Journey North has collected sightings of fall roosts. Three years of data were analyzed to determine the monarchs fall migration pathways. Two distinct flyways emerged: an “eastern flyway” and a “central flyway.”  The central flyway appears to lead directly to Mexico. The eastern flyway lagged behind the central flyway roosts in timing, suggesting that monarchs traveling the eastern flyway have a reduced chance of making it to Mexico. See www.learner.org/jnorth/tm/monarch/roost_findings.html. This conclusion was further supported by tagging data, which showed monarchs traveling the eastern flyway are far less likely to reach Mexico than their counterparts traveling the central flyway (see www.learner.org/jnorth/tm/monarch/tag_recovery_rates.html. Clearly, conservation efforts in the central flyway are key to the monarchs’ survival.
 
How high can monarchs fly?  Wind speed and air temperature affect the height of monarch flight. Air cools as you move higher into the atmosphere. Since monarchs cannot fly when their flight muscles are below 55ºF, cold air temperatures will limit how high they can fly (See www.learner.org/jnorth/tm/monarchs/sl/20/7.html).  We can’t see monarchs with the naked eye when they fly more than 300 feet high. NEXRAD (Doppler) radar has shown that monarchs travel most of the time within the planetary boundary layer, at around 800-1200 feet high. That means, on a good migration day, we most likely WON’T see monarchs as they fly far overhead. What is the highest a monarch can fly? Monarchs have been seen by glider pilots at 11,000 feet!

Male monarch on goldenrod

Surface winds affect the monarch migration.  Monarchs prefer a good tailwind, meaning a northerly wind that pushes them S/SW. Strong headwinds, i.e. southerly winds, can impede the migration. When winds are from the south, monarchs fly very low, often choosing to duck and find cover or refuel on available nectar sources. They may wait for the winds to change direction, and as a result, can form large roosts as they accumulate in a protected location. Once the winds change to a more favorable direction (N/NW/NE), the monarchs will burst out of their roosts after they have warmed themselves. They will fly to another spot to bask in the sun, nectar, or search for a thermal to continue their journey south. In the Great Lakes region, monarchs may accumulate at the tip of Point Pelee, while they await favorable winds to carry them across Lake Erie. They can be found on the grounds of the South Bass island lighthouse, nectaring and roosting for the evening. In other areas, such as the desert, monarchs will search out cooler, moist riparian areas and refuel as they await favorable winds to carry them into Mexico. And many observers will note that large numbers of monarchs come in to an area ahead of a cold front, as they ride the layer of uplifted air associated with the advancing edge of the front.

How far can a monarch fly in one day?  The average pace of the migration is around 20-30 miles per day. But tag recoveries have shown that monarchs can fly 150 miles or more in a single day if conditions are favorable. Recently, a monarch tagged on October 9th in Cape May, New Jersey was recovered October 12th in Harlem, Georgia. It had travelled 558 miles in three days.

So why do monarchs form overnight roosts?  Scientists still have many questions about roosting behavior. Monarchs migrate during the day, coming down at night to gather together in clusters in a protected area, usually a tree with a suitable microclimate surrounding it. Monarchs migrate alone—they do not travel in flocks like birds do. So they often descend from the sky in the afternoon to feed, and then search for an appropriate roosting site. Most roosts last only 1 or 2 nights, but some may last a few weeks. It appears that monarchs select roost sites based on multiple factors. Monarchs seek sites with protection from the wind, nectar sources nearby, and trees with a structure that facilitates clustering. In the north they might use conifers, while in the south they might choose oak or pecan trees, especially if the trees are overhanging a stream channel. We don’t know if monarchs actually seek each other out when they are looking for a spot to roost, or if large numbers of them are seeking the same microclimate and just happen to end up in the same locations.

How do monarchs find their way to Mexico?  We really don’t know for certain.  We do know that monarchs have a sun compass in their brain, and a circadian clock in their antennae. The clock and compass are integrated in the brain to form a time-compensated sun compass. Using the sun and polarized light waves, monarchs can maintain a general S/SW heading throughout the day. The Reppert lab also just reported that monarchs have a UV-light dependent magnetic compass as well  (http://www.nature.com/ncomms/2014/140624/ncomms5164/pdf/ncomms5164.pdf), which aids in allowing them to maintain a S/SW heading on a cloudy day. But how monarchs NAVIGATE is still a mystery. The difference between maintaining a compass heading and navigating can be illustrated by asking yourself a simple question: If you only knew which way was south, could you start where you are and end up at the monarch wintering sites in Mexico, without a map? How do they know to head hundreds of miles south, and make a sudden right turn to head into the Monarch Butterfly Biosphere Reserve? How do butterflies starting in Minnesota, or Ohio, or Maine end up in the same place, when they’ve had to travel very different routes to get there? How do they find a reserve area that is only about 1 degree longitude wide? And, how do they do all of this with a brain about the size of a pinhead?  There is so much more to learn about the wonderful monarch butterfly!

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