Insect Migration - It's a Family Affair

Monarch Butterflies in a fir tree (pendens proditor)

I'm in the process of studying for my comprehensive exams. Without going into detail 'comps' is the biggest test of a PhD students life and I've been tasked with learning 'ALL THE THINGS' about migration, among other topics.

I'm trying to read as broadly as possible about migration and not just focus on my favorite taxa, birds. Lately I've been reading many many pages about insects and noticed that there are some remarkable parts to insect migration we just don't see in other species.

One BIG difference between bird migration and insect migration is often insect migration is multi-generation. Which means that it takes several generations to make the complete circle of migration.

A prime example of multi-generation migration is the monarch butterfly. Monarchs winter, primarily in Mexico, in huge groups where they cover the fir trees. In the spring they start to move north. Each generation moves a bit farther, lays eggs and then dies, then their young hatch, grow to adulthood, migrate a bit farther and the process repeats until they reach the northern extent of their breeding range (see map). In late summer a special generation of butterflies is born, these will migrate hundreds of miles back to Mexico, all in one big movement. These individuals overwinter in Mexico and then start the process back north the next spring.

North American Monarch Butterfly Range. (Journey North)

Migrating through several generations is remarkable for several reasons, the biggest one is that each individual insect doesn't get a chance to learn from the previous generation or have any room for trial and error. One of the big things we look at in bird migration is how the bird navigate, how do they know where to go year after year? Some species are well documented to teach their young where to go, others head out on their own seemingly programmed with the knowledge they need. Often young birds end up in odd places, but the idea is they learn and the next year they are  better prepared for migration.

Insects don't get to take advantage of trial and error during migration. Monarch Butterflies live for only a few weeks and they have never been where they are going. This is their only shot.

Another remarkable thing about migratory insects is there are often differences between generations of a species if one generation is migratory and the next is not. Migratory individuals will have longer more developed wings. Both grasshoppers and locusts have different phenotypes for migratory and non migratory individuals [1]. Change in body shape between generations is remarkable because there is something happening at the genetic level deciding whether or not an individual will be part of the migratory generation. These patterns are well suited to predictable weather environments. For example, if we had an early cold snap in the fall, a non-migratory individual would be ill prepared to south.

Insect migration has been found to be highly influenced by weather patterns. Large scale wind systems, such as prevailing wind systems and fronts move large numbers of insects. Studies show insects are very selected for the wind direction, only leaving on nights when winds are very precisely in the right direction. Following weather patterns makes sense because insects can be easily pushed by the wind. We have a less clear understanding of how smalls scale weather patterns impact insect migration [2].

Tagging a Monarch Butterfly (Anna Barnett)

Like birds there are groups of insects who migrate during the day, and others who migrate at night. Diurnal (daytime) migrants seem to use the sun, or if the sun is obscured, polarized light to navigate [3]. Nocturnal (night) migrants navigation is not well studied, but it could be magnetic [4].

There are many methods used to study the migration of insects, from specialized analysis of radar data [5] to applying tiny numbered stickers to butterflies wings on the breeding grounds (during the late summer) and recapturing them on the wintering grounds. By having monarchs tagged all across the country we are able to track Monarchs across North America and understand how populations from different parts of the U.S. move [6].

The most impressive feat of insect migration I've found is the migration of  dragonflies across the Indian Ocean. This round trip flight across the Indian Ocean spans over 14-18 thousand kilometers (8500-11000 miles) [7,8] While assisted by wind, it is still quite impressive. They, like the monarchs take several generations to make their huge journey.

Monarch Butterflies, like many other organisms will start migrating soon. While birds will migrate fairly quickly, just taking a few weeks to make the journey north, butterflies will slowly make their multi-generational way north, moving a few hundred miles and laying the eggs of the next generation. They won't be arriving in the mid latitude states until mid May and the upper Midwest by mid June. If you see Monarchs around your neighborhood this spring, report them here and help us better understand how these remarkable critters are moving across the continent. Spring will get here eventually, so keep your eyes peeled!

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On a non-migration related note, today I read about hornets and how they defend their hives from other insect invaders by surrounding the invader and vibrating their thoraxes to create so much heat that the intruder DIES! Nature is so cool sometimes.

- Auriel

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Citations

[1] Denno, R.F. (1994) The evolution of dispersal polymorphism in insects: the influence of habitats, host plants and mates. Researches on Population Ecology 36, 127–135.

[2] http://www.annualreviews.org/doi/pdf/10.1146/annurev.en.33.010188.001151

[3] Hyatt, M. (1993) The use of sky polarization for migratory orientation by monarch butterflies. PhD thesis, University of Pittsburgh, Pittsburgh, Pennsylvania.

[4] Baker, R.R. (1987) Integrated use of moon and magnetic compasses by the heart-and-dart moth, Agrotis exclamationis. Animal Behaviour 35, 94–101.

[5] http://bioscience.oxfordjournals.org/content/53/5/503.short

[6] http://monarchwatch.org/tagmig/fallmap.htm

[7] Anderson, RC (2009). "Do dragonflies migrate across the western Indian Ocean?".Journal of Tropical Ecology 25: 347–348.

[8] http://news.bbc.co.uk/earth/hi/earth_news/newsid_8149000/8149714.stm

Migration - in all its flavors

Sandhill Cranes Migrating (Serge Melki)

Migration is one of the wonders of the natural world. Millions of animals move thousands of miles every year in an attempt to produce as many offspring as possible. When most people think of migration they think of Canada geese flying south for the winter and their return in the spring. You might not realize, that while these birds demonstrate impressive feats of migration, there are others kinds of migration in the bird world.

What makes all these things migration is they involve going back and forth between two places. It doesn't matter if its 2000 miles south or 3 miles up a mountain.  No matter the difference, birds are always migrating in some way during the year.

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'Loop' Migration
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Loop migration is what you'd normally call migration, where birds arrive in the spring and leave in the fall. Loop migration can be broken down further into a few categories, long distance, medium and short distance migration. All of these are annual movements for the same purpose. Birds migrate to take advantage of seasonally abundant food. In northern latitudes insect populations, fruit and seeds BOOM in a huge way in the summer. Taking advantage of these resources allows birds to produce more young than they would have in the tropics. Northern latitudes also provide an advantage because the tropics have constant competition for resources and higher predation.

Migration is not without it's risks of course but if you reproduce enough it offsets the high risk of dying during migration. In this way, you and the rest of your species can continue into the future, so you win. 

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Altitudinal Migration
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Unless you are lucky enough to live in the mountains, birds migrating up and down in elevation is the kind of migration you are probably least familiar with. Many mountain species take advantage of the perks of loop migration without traveling as far. Altitudinal migration presents unique challenges like extreme weather and changes in snow pack.

Altitudinal migration is quite common in tropical areas, where moving up and down in elevation can help mitigate the impacts of the rainy or dry season on food supplies. By moving around the landscape a bird can take advantage of the habitats with the most food and increase their chance of survival. Birds who live on small remote islands also demonstrate altitudinal migration. Instead of migrating several thousand miles to the next land mass they move up and down the mountain throughout the year. They are likely migrating up and down in response to the annual wet/dry cycle on the island.

Altitudinal migration is unique since its not always done just for the purposes of breeding. Some species migrate up in elevation to take advantage of seasonal abundances of insects so they have plenty of energy to molt their new feathers quickly [1]. It is extremely expensive energetically, which means it takes lots of food and energy to accomplish. Birds feathers go through extensive wear in a years time; replacing them is important for keeping the bird the correct temperature and flying efficiently.

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Irruptive Migration
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Irruptive migration is an odd kind of migration because it doesn't fit the way most people think of bird movements. Irruptive migration doesn't  happen every year, instead they are normally triggered by an abundance or shortage of  food. When food is scarce in the north birds come south looking for other resources; this is often why crossbills irrupt. When food is abundant, such as a boom of lemmings, birds are able to produce large numbers of offspring. A boom in the lemming population is what we think is going with Snowy Owls this winter. Boone did a post awhile back that covers their irruption in more detail. 

American Redstart (Derek Bakken)

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Differential Migration
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Differential migration can happen in every kind of migration. It means a subset of bird populations migrate differently. When males and females migrate separately and winter in different places, or when juvenile birds go south their first year and don't return north for several seasons, they are both demonstrating differential migration.

For example, female Cooper's hawks migrate significantly earlier then males. Arriving earlier may allow them to take advantage of extensive resources and put on weight in preparation for egg laying or to find the best nesting locations [2]. American Redstarts males and females migrate to different areas for the winter [3]. This segregation is caused by behavioral dominance by the older males who set up territories and force other individuals (usually female) into less desirable habitats. The exact reason for most differential migration is still unknown, but understanding the different ways a population migrates help us to better conserve their habitats. 
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It bears mentioning birds are not the only group of animals who migrate. We'll try and round up a post on those as well here sometime soon. In the meantime, keep on the look out for signs of spring migration near you! SPRING IS COMING!

- Auriel

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Citations

[1] Rohwer, V. G., S. Rohwer, and J. H. Barry. 2008. Molt scheduling of western Neotropical migrants and up-slope movement of Cassin’s Vireo. Condor 110: 365-370.

[2] Hull, J.M. Pitzer, S., Fish, A.M., Ernest, H.B., and Hull A.C. 2012 Differential Migration in Five Species of Raptors in Central Coastal California. Journal of Raptor Research 46(1):50-56. http://www.bioone.org/doi/abs/10.3356/JRR-10-116.1

[3] Marra, P.P, Homles, R.T. 2001. Consequencces of Dominance-Mediated Habitat Segregation in American Redstarts During the Non-Breeding Season. The Auk. 118(1):92-104. http://www.bioone.org/doi/abs/10.1642/0004-8038(2001)118%5B0092:CODMHS%5D2.0.CO%3B2