Why are more female monarchs recovered at the over wintering sites than males you ask. It is the difference between having a thicker thorax or thicker wings.

Crazy Huh? Let me shake it out for you.

Flight muscle size is the most important factor for the butterfly’s ability to fly. The flight muscles are contained inside the butterfly’s thorax and in general males have larger thoraxes than females to accommodate their heavier wing muscles. Having heavier wing muscles does not necessarily translate in the ability to fly long distances because a heavier thorax will produce more drag during flight. Drag will burn up precious lipids during migration and may offer insight as to why more males are captured and tagged along migration routes than females. Several multiyear tagging projects showed that female monarchs were far less likely to stop and nectar at tagging sites as were the males. Some data even showed a 70% frequency of males over females at many of these refueling areas. So it just may be drag that causes the males the need to refuel more often than females there by making them more likely to be captured than the longer flying females. Although having the big old thorax maybe be a drag, pun included,  those heavier muscles do help the male during mating because he needs the ability to not only lift the female but still be able to fly while coupled with her for many hours.

The key to the females flight success is the thickness of their wings. Females tend to have relatively thicker wings than the males. A thicker wing would have higher tensile strength there by making it more durable and less inclined to breakage. This could explain why females are usually in better shape than males when recovered and more likely to complete the trip to Mexico.

Regardless of thorax or wings size the monarch is still quite the agile aeronaut. Although a monarch butterfly can make a 90 degree turn in just one body length to avoid a problem, in general  noxious butterflies fly in a straighter path then more palatable ones. The more edible butterflies need to constantly bob and weave in an erratic pattern to avoid predators while the those with warning color patterns can be more straight forward while caring less about who’s for dinner. Since every turn and dodge burns fuel, it better suits the monarch to fly in a straighter pattern.