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.

To investigate whether arrival dates are shifting, I fitted a linear regression for each state using year as the predictor while accounting for growth in citizen science participation. After controlling for this, no state showed a statistically significant trend. Virginia showed the steepest slope at -51 days per decade, but with a p-value of 0.15 this is not reliable. The most likely explanation is that 10 years of data is simply not enough to detect a real biological signal above the noise as a longer time series would be needed to draw confident conclusions.

State	Slope (days/decade)	P-Value
Virginia	-51.41	0.1477
Maryland	-32.24	0.2200
Georgia	-28.97	0.5203
North Carolina	-26.74	0.3758
New York	-22.52	0.2861
New Jersey	-20.95	0.2122
Connecticut	-4.17	0.8957
Delaware	-3.73	0.9614
Massachusetts	0.43	0.9847
Maine	14.01	0.5101
South Carolina	72.98	0.2187

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. This deserves 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.

• Extend the analysis as iNaturalist data matures: by 2030 there will be nearly 20 years of consistent coverage which should make trend detection more reliable
• Incorporate climate covariates such as spring temperature anomalies to separate biological signal from sampling noise

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.