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 guessed it, it's the monarch butterflies!

I've explored a dataset of over 700,000 monarch butterfly sightings collected by citizens across North America. What started as curiosity turned into a full analysis of migration corridors, phenology, and understanding the limits of what volunteer-submitted data can tell us. This is what I found.


The monarch butterfly was listed as endangered under Canada's Species at Risk Act in December 2023, a designation that reflects decades of population decline driven by habitat loss, pesticide use, and climate change. The eastern population, which migrates between Mexico and Canada, has lost an estimated 80% of its numbers since the 1990s. The western population, which overwinters along the California coast, has fared even worse, declining by more than 95% since the 1980s.

The data used in this analysis was downloaded from the Global Biodiversity Information Facility (GBIF), which aggregates occurrence records from institutions and citizen science platforms around the world. The majority of records come from iNaturalist, a platform where anyone can log a wildlife sighting using their phone. What makes this dataset remarkable is that it exists entirely because ordinary people (hikers, gardeners, teachers, kids) took a moment to photograph a butterfly and record where they saw it. This is both the strength and the limitation of the data. More eyes means more coverage, but it also means the data reflects the number of citizens recording instead of actual population measurements.

The raw dataset contained 703,394 records spanning from the 1800s to 2025. As visible in the chart below, data before 1980 is extremely sparse, so I restricted the analysis to 1980 onwards. The spike in records 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 clustering 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 increase in participants and volunteers 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 participation bias is immediately visible: records in every state jump dramatically after 2017, reflecting an increase in citizen submissions rather than monarch population changes.

Despite the uneven coverage, a clear pattern emerges when we look at 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. To investigate whether arrival dates are shifting, I fitted a linear regression for each state using year as the predictor, controlling for growth in citizen science participation. After controlling for participation, no state showed a statistically significant trend. Virginia shows the steepest slope at -51 days per decade, but with a p-value of 0.15 and only 10 years of reliable data, the signal-to-noise ratio is too low to confirm this as a biological pattern rather than sampling noise.

This analysis establishes a baseline and a framework, and rerunning it in 2030 with nearly two decades of consistent citizen-driven data will be a much stronger test of whether monarch arrival timing is responding to climate change.
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.

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, and a subject that warrants a dedicated analysis of its own.

This analysis set out to explore what citizen science data can tell us about monarch butterfly migration patterns across North America. The Atlantic corridor analysis confirmed what biologists expect: monarchs arrive progressively later as you move north, from late March in South Carolina to early July in Maine. The fact that this signal emerges clearly from noisy, volunteer-submitted data is itself encouraging.

The western population tells a different story, following an inland-coastal pattern rather than a north-south one, with inland breeding in summer and coastal overwintering in winter.

Where the data falls short is in detecting change over time. The trend analysis, even after accounting for growth in citizen science participation, was inconclusive. With at most 10 years of reliable data per state, the signal-to-noise ratio is simply too low to draw confident conclusions about whether monarch arrival timing is shifting.

• Incorporate climate covariates such as spring temperature anomalies to separate biological signal from sampling noise
• Extend the analysis as data matures: by 2030 there will be nearly 20 years of consistent coverage which should make trend detection more reliable
• Conduct a dedicated analysis of the western overwintering population, whose 95% decline since the 1980s warrants closer examination

The analyses in this post are only possible because thousands of volunteers take the time to record and submit their sightings. If you spot a monarch butterfly, consider logging it on iNaturalist. Every observation counts and directly contributes to the scientific record.

If you are based in Canada, Mission Monarch is a citizen science program that tracks monarch and milkweed distribution across the country. In the US, Journey North has been tracking monarch migration since the 1990s and welcomes new observers.

You can also help by planting native milkweed and nectar plants in your garden, avoiding pesticide use, and supporting local conservation organizations working to protect monarch habitat.