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Background
About Weather and Spring Migration Fall Weather Conditions & Migration In the fall, it's easy to see why monarchs come south on cold fronts (or 'northers' they are called in Texas). It's like catching a bus going your way - in this case the ride is even free! Monarchs probably ride the layer of uplifted air associated with the advancing edge of these cold fronts. But that is not all they ride. They generally move with any wind that has a northerly component, and may still be seen traveling days after the front has past. Spring Weather Conditions & Migration In the spring, the situation is not as clear. However, from my own observations, I'd put the behavior I see in one of three categories. (And you'll notice that most are in the third, "catch-all" category!) #1) Clear, Directional Migration It does seem that I see the most butterflies clearly migrating northward at a rapid clip when there's a strong south wind. And here in Texas, the prevailing winds are out of the south this time of year. (When there are strong north winds I hardly ever seen them in the spring. Spring north winds are usually associated with cold rainy weather, and often thunderstorms.) #2) Flitting About At other times that I see them, they're milling around in classical butterfly fashion, and it's hard to find any directionality in their movement. (At these times, the winds are usually not strong.) Females are earnestly seeking ovipositional plants (milkweed). They're flying within a meter of the ground, flittering about, and lighting momentarily on possible milkweeds. (They apparently taste the candidate plant with receptors on their forelegs. If the plant passes the taste test they stop to lay an egg.) Males are seen anywhere between 1m of the ground to the height of the local trees. They might be flying low, patrolling the milkweed patches for females. And they often sit on the trees and presumably (now this is an assumption!) are watching for females. Both males and females might be seen mating, or flittering around and apparently looking either for nectar or milkweed. 3) Combination of Above The 3rd category--and probably a lot of butterflies are here--are combinations of #1 and #2 above. Observations You Can Make/How Do Monarchs Migrate in Spring? It would be fascinating to know what factors are involved in the timing and direction of the spring migration! Are monarchs simply blown along? Could super strong south winds blow them up all the way up to Iowa, for example? Or do they follow in the wake of emergent milkweed, so as not to outpace the appearance of the plant needed for reproduction? We just don't know what factors are involved, but you can help collect information during spring migration that might help us understand. When you see a monarch this spring, take note of the following. (Our goal is to separate migrating monarchs from those that are "resting"/not moving.) Directionality of Flight Is the flight purposeful, in a clear direction? Which way is the butterfly moving? Pay careful attention to the direction the butterfly is traveling vs. its orientation. That is, watch which way the butterfly is FACING and which way is it actually GOING. (In strong winds, every now and I see them out of control, faced in a different direction than they're traveling, just being blown along.) Height of Flight Are they low to the ground--within a meter or so? If so, are they every so often landing on a plant momentarily to taste it? Are they landing on flowers? Are they chasing another butterfly? If your answers to these questions are yes, it is likely that they are A) a female seeking a site to lay eggs or B) nectaring (this behavior is obvious) or C) a male looking for a female with which to mate. Wind Direction and Speed Also pay attention to wind direction and speed. It would seem that days when winds show a strong, southerly component would be the days the most monarchs are migrating--are moving in obvious directional flight. Record the speed and direction of the wind at the time of your observation.
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