Monarchs at
Sea: The source of monarch butterflies observed flying inland
from the Gulf of Mexico. (adapted from tvectors)
Gibo, David. March 29, 1998.
There is an interesting report by an observer, Francis Harvey, in the March 24 update by Journey North
on the northward migration of monarch butterflies. About 50 butterflies were reported
coming in off the Ocean on the 15 of March and flying against the wind. Journey North
considers the sightings unusual and has made it the basis of their weekly Challenge
Question feature, in this case, a question about verifying submitted data. I don't find
the sightings unusual because I have made similar sightings on the Outer Banks of North
Carolina in the latter weeks of August while on vacation with my family.
A check of the weather maps on Journey North, and the Weather Map for March
15shows that a high pressure area was to the west of the observation site during the
time of the observations. The clockwise wind around the center of the high would have
produced an offshore (North) wind, as reported. At some time around midday, inland areas
became warm enough relative to the surface temperature of adjacent Gulf waters to trigger
the sea breeze, a onshore breeze that can penetrate up to 50 miles inland. At this point,
the surface wind would swing around nearly 180 degrees and become southerly, as reported.
The probable source of the monarchs becomes clear when we consider that: (1) migrating
(have a preferred direction) and dispersing (no preferred direction) monarch butterflies
both engage in soaring flight, (2) high pressure systems are often ideal air masses for
generating thermals, (3) thermals always drift downwind, and (4) when a thermal that has
formed over warm land drifts out over a cooler body of water, the supply of warm air
feeding into the thermal is cut off and the thermal dissipates.
Assumptions: (1) Plume (column shaped) thermals were being generated inland and drifting
towards the coast. (2) Migrating (or dispersing) monarch butterflies that encountered the
thermals would begin to soar upward in the rising air. (3) The temperature at inland
locations in the morning was at least 75F (24C), the same as the temperature reported near
the shore (it was probably warmer inland). (4) The butterflies stop climbing (by partially
folding their wings) when the air temperature drops to 14C. (5) A monarch butterfly of
average mass and wing loading descends at about 1 m/s.
Because Francis Harvey did not mention clouds we will assume a clear sky and use the dry
adiabatic lapse rate (rate of cooling with altitude) of about 3C per 1000 ft to calculate
the maximum flight altitude for the butterflies. Assuming that the butterflies cool by 10C
during the climb, then they can soar upward in thermals (10/3) x (1000) = (3.3) x (1000) =
3,300 ft above the ground before the air around them cooled from 24C to 14C. Because 3,300
ft is about 1000m, a butterfly that began a gliding descend of 1 m/s as soon as it
detected that it was being carried out over the sea would require 1000/60 = 16.7 minutes
to get to the surface.
How far out to sea would it drift before it reached the surface? It depends upon the wind.
The surface winds for March 15 are indicated to anywhere from force 2 (3 - 7 kts) to force
3 (8 - 12 kts). Because the observer said that the butterflies were bucking a strong
headwind, lets assume 10 kts, or about 11.5 mph (18.5 km/hr). During a 16.7 minute
descent, butterflies that either spiraled down, or simply flew parallel to the coast,
until they reached the surface would be carried (16.7/60) x (11.5 mph) = 3.2 miles (5.2
km) out to sea. However, if the butterflies began to glide towards land, as soon as it
crossed over the beach, it would reach the surface closer to shore. With a gliding speed
of about about 3 m/s, or 6.7 mph (10.8 km/hr), monarch butterflies would be carried out to
sea at a net speed of 11.5 -6.7 = 4.8 mph. During a 16.7 minute descent, the buterflies
would loose ground and end up 16.7/60 = 1.3 miles at sea. Still a considerable
improvement. It could then fight its way back to land by staying close to the surface and
avoiding the full force of the wind. Of course, if the butterflies take longer to react to
being drifted out to sea while riding thermals, perhaps not responding to the
deteriorating situation until long after the thermals have died, the butterflies may be
carried so far off shore that they have to fly an hour or more to make it back. If the
offshore wind picks up, they may never make it back. On the other hand, when the Sea Breeze
starts, butterflies that were struggling against a headwind may suddenly find themselves
in a tailwind. They could then climb to the higher velocity wind aloft and be swept back
to shore, once again zooming high over the beach, but in the opposite direction.
What did I see in North Carolina that is similar to the sighting reported by Francis
Harvey for the Florida Gulf Coast. On days with an offshore breeze, Ospreys and other
large birds could be seen soaring in the thermals offshore. At irregular intervals,
occasional monarch butterflies and numerous dragonflies would suddenly fly in off the
Atlantic ocean and make their way inland. The butterflies and dragonflies stayed close to
the water until they reached the shore, then rose up about a meter as they were able to
use the wind shadow of the dunes, sea oats, etc. On one memorable occasion, a single pulse
of thousands of large dragonflies and a much smaller number of monarch butterflies flew in
from the Ocean for about 20 minutes. I believe that they were all from one huge thermal
that had formed well inland and drifted towards the ocean, picking up soaring dragonflies
and monarch butterflies (probably dispersing individuals rather than migrants) all along
its path and lifting them thousands of feet above the ground. The thermal apparently
crossed both the sound and the outer banks in the vicinity of Nags Head and was headed out
into the Atlantic before its insect passengers bailed out, descended to the surface, and
began to fight their way ashore.
Is there an adaptive advantage of soaring in thermals near the coast, even at the risk of
being drifted out to sea? Perhaps. The sea breeze forms a reasonable dependable source of
lift called the sea breeze front. The sea breeze front is a band of lift paralleling the
coastline that is formed when the sea breeze encounters an opposing wind, like the
situation on March 15, of forms at the front of the advancing mass of cool air moving
inland. This system forms most days from spring through fall and can be used as a highway
by soaring animals to travel long distances in relatively straight lines. Lake breeze
fronts also form, particularly in the Great Lakes
region.
Monarch butterflies may be adapted to exploit this dependable source of lift, or they may
simply be exhibiting their normal soaring behavior and are swept up in the lift from the
front as soon as it forms. Butterflies that were soaring in thermals further inland would
have have been carried S by the N wind and neared the coast just as sea breeze began to
move inland. The butterflies would have been swept up by the front could have flown in
lift for the rest of the day. Butterflies that started further to the south may have been
drifted over the sea and had to fight their way back, but once the sea breeze developed,
they too, would have been swept into the front and been able to soar in lift for the rest
of the day. Is this an adaptation, or simply their normal soaring program being played out
in a favourable environment. I don't know. I do know that it is about as much
sophistication we can expect from an insect unless it has senses that we don't know about.
So what is the most likely source of the monarch butterflies seen coming ashore in
Florida? The region of Florida immediately inland of where the sightings occurred. Why
didn't Francis Harvey see the butterflies being blown out to sea. They were probably 1000
- 3000 feet overhead. Were the butterflies some of the migrants from Mexico? Perhaps. But
they may also have been locals. Were the butterflies exhibiting a complex and
sophisticated flight tactic? Unlikely. Were the butterflies simply unlucky individuals
that happen to have started too close to the coast when they picked up their first thermal
of the day? Probably. Were these sighting unusual? Not at all.
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