A University of Minnesota study on Monarch butterflies suggests a connection between the long migration and parasites.
Volunteers counted Monarchs and tallied levels of a common parasite. They found higher levels of parasites in the north, and fewer in the wintering grounds in Mexico. That suggests heavily-infected butterflies didn't survive the fall journey.
Researcher Karen Oberhauser said they also found Monarchs in areas packed with lots of butterflies suffered more from parasites.
"So it suggests it would be a lot better to keep larger areas for organisms to prevent the kind of crowding that might lead to parasite build-up," Oberhauser said.
The study is published in the current issue of the journal Ecology.
Oberhauser said the volunteers who collected data were essential to the study.
"It's pretty exciting that people are out collecting data, and sending in these data to scientists, and then that the data are being used to support really important research on how organisms interact with their environment," she said.
Oberhauser said the Monarchs have just left Mexico and are on their way north.