Monarchs face a wider array of threats than scientists previously realized, according to a new survey of the butterfly's annual southerly migration.

Most investigations of declining monarch butterfly numbers have blamed habitat losses among the insect's wintering grounds in Mexico, as well as the decline of milkweed plants throughout the Midwest. But the new study, published in the journal Proceedings of the National Academy of Sciences, suggests scientists have underestimated the importance of the fall migration.

Part of the problem, according to the study's authors, is the fall migration is difficult to quantify.

"Getting accurate monarch counts in the summer is tough," lead author Sarah Saunders, former Michigan State University integrative biologist, said in a news release. "Finding them in the fall, though, is nearly impossible as they're moving hundreds of miles daily."

Over the last several years, conservation efforts have helped reverse habitat loss in Mexico and increase milkweed availability throughout the Midwest. Despite the improvements, the monarch's two-decade-long decline has continued.

Researchers suspected there were other unexamined threats. To identify them, scientists built a model to identify correlations between fall-specific ecological variables and wintertime population numbers.

A summer population index helped the model account for year-to-year population variations. The model considered the effects of temperature and landscape greenness on monarch numbers, as well as the negative impact of Ophryocystis elektroscirrha, a parasite that causes monarchs to grow smaller or deformed wings.

"The survival of migratory animals has been an important area of investigation as climate changes spark ecosystem and migratory pattern changes," said Elizabeth Blood, a program director at the National Science Foundation, which helped fund the study. "This research boosts our understanding of larger processes -- from regional to even continental scales -- and the impact they have on migratory animals."

Instead of using a single wintertime population total, the new model is designed to analyze 19 known monarch colonies individually.

"We're the first to examine the winter colonies this way," said Elise Zipkin, Michigan State integrative biologist. "If you aggregate the winter colony data, you can get the wrong result because there are important differences in habitat quantity and quality at the individual colony sites."

"Unsurprisingly, our model shows that all seasons are important; summer, fall and winter factors are all connected," said Saunders, who now works as a quantitative ecologist at the National Audubon Society. "In particular, we found that landscape greenness during the fall migration, in addition to the peak summer population size and the amount of habitat at local winter colonies, were the key factors influencing the winter population size."

Saunders and her research partners say the new model was only possible thanks to several decades worth of data collected by ecologists, climatologists and citizen scientists in both the United States and Mexico.

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