Every September an incredible migration phenomenon begins. Clouds of stripy orange monarch butterflies set off on a 2,500km journey, travelling from southern Canada to warmer climes in southern California and Mexico. Come spring they follow the milkweed blossom and travel back up north. No butterfly completes the entire trip: after flying many hundreds of kilometres the female butterflies lay eggs and pass the baton to the next generation. Now a new study, published in Biology Letters, reveals how these amazing insects make use of the weather to aid their journey.
Miniaturised radio transmitters were attached to the butterflies and their journey tracked using a series of automated telemetry towers. The results show how monarchs soar high to take advantage of strong tailwinds, powering along at up to 31kph. Those that have to travel furthest seem to travel fastest, but all butterflies took rest days every now and then. Warmer temperatures also help (though only up to a certain point) and on a good day they managed to travel over 100km. Light rain didn’t seem to have any adverse effect, but the researchers note they didn’t track any individuals during heavy rain events. Perhaps they shelter and make up lost time later?