They fit in the palm of your hand.
Every year, they fly from Mexico to Canada and back.
They are also listed as an endangered species in Canada.

You guess it right, it's the monarch butterflies.






The data was collected over the .... iNaturalist

The raw dataset contained 703,394 records spanning from the 1800s to 2025. The first thing I noticed was that more than half the records (371,805) had no recoverable date, meaning neither the eventDate field nor the year, month, and day fields were populated.

As visible in the chart below, data before 1980 is extremely sparse, so I restricted the analysis to 1980 onwards. The spike around 1999-2001 also stood out. Digging into it, it turned out to be a bulk submission from Monarch Watch, a dedicated monarch monitoring program based out of the University of Kansas that has tracked the species since the early 1990s. These records were aggregated annually and lacked day and month information, making them unusable for phenological analysis.
After removing records with unrecoverable dates, missing coordinates, duplicates, and pre-1980 observations, the dataset was reduced to 316,273 records. Given sparse coverage in the early years, the analysis was further restricted to 2010 onwards, yielding a final dataset of 312,235 records, roughly 44% of the original.

The maps below show monarch butterfly sightings across North America colored by month. The full migration corridor is immediately visible, with purple and blue dots cluster in Mexico during winter, greens push north through spring, and reds and oranges spread across the US and Canada through summer and fall. Comparing across periods, the density of sightings increases noticeably over time, largely reflecting the growth of iNaturalist as a platform rather than the actual monarch population. To better understand the migration pattern, I focused on the Atlantic coast corridor and traced the path monarchs take as they move north through Georgia, the Carolinas, Virginia, and up through New England.

The table below shows the number of sightings per state per year. The iNaturalist effect is immediately visible: records in every state jump dramatically after 2017, reflecting platform growth rather than monarch population changes.

Despite the uneven coverage, a clear pattern emerges when we look at early arrival dates. For each state, I calculated the 5th percentile sighting date across years with at least 100 records, effectively when monarchs first consistently show up. Only years with at least 5 valid years of data were included.

The results tell a clean story. South Carolina sees its first consistent sightings around March 30, Georgia around April 17, and the signal moves steadily northward through Virginia in June, reaching Maine by early July. The map below visualizes this progression along the corridor. This is the spring northward migration captured in citizen science data. While not perfect, it is remarkably coherent given the limitations of the dataset. A trend analysis to investigate shifts in arrival timings was attempted but with at most 10 years of reliable data per state and significant growth in citizen science participation over the same period, it was not possible to draw statistically robust conclusions.

Unlike the eastern population which follows a north-south corridor to Mexico, western monarchs follow an inland-coastal pattern. Sightings in Salt Lake City peak in July, reflecting the summer inland breeding season. As fall approaches, monarchs begin their journey westward to the California coast. San Francisco sees its peak in October as the migration arrives, while San Luis Obispo, one of the most famous overwintering sites in North America, peaks in December as monarchs settle in for the winter. The data captures this difference clearly.

This population has suffered a catastrophic decline. The western overwintering population has dropped more than 95% since the 1980s, making it at greater risk of extinction than the eastern population. A dedicated analysis of the western corridor warrants further investigation.