While there are known populations of winter-breeding monarchs (i.e., monarchs that do not migrate), they are often located in areas separate from the overwintering sites. For example, winter-breeding monarch populations are present in Florida, and overwintering, non-breeding monarchs are typically located in Mexico. However, winter-breeding and overwintering monarchs were reported roosting at the same site in the San Francisco Bay Area during the 2024/2025 winter (James et al., 2025). While this circumstance has occurred in Australia, this was the first recorded instance of breeding and overwintering monarchs occupying the same roosting area in North America.
The co-occurrence event took place in a selection of conifer trees, where the overwintering populations were found roosting, surrounded by a mixture of both native and non-native milkweed species that were supporting the breeding monarchs. While the presence of breeding monarchs and active milkweed has not been shown to affect the reproductive dormancy of the neighboring overwintering monarchs in Australia, some people have voiced concern that this may not hold true for the overwintering monarchs in California. As this is a rare and previously undocumented circumstance in North America, scientists will be monitoring co-occurrences in the future to determine if the nearby presence of breeding monarchs impacts the behavior of overwintering individuals.
As the overwintering numbers for the western monarch population in California continue to be drastically low, some people are hopefully that these winter breeding populations will help support the larger western population and provide a possible adaptive solution to rising temperatures in the area. Additionally, one challenge that has been identified to the preservation of the western monarchs’ migration cycle is the difficulty overwintering female monarchs face in finding milkweed in the spring. If native milkweed growth adjacent to overwintering sites doesn’t impact the reproductive dormancy of overwintering monarchs, this nearness could be beneficial to the overwintering females come spring. As scientists continue to explore this new development in the winter behaviors of the western monarch population, it will be interesting to see what they discover.
Mykayla Hagaman
References:
James, D. G., Schaefer, M. C., & Agarwal, R. (2025). Concurrence of Overwintering and Winter-Breeding Monarch Butterflies (Danaus plexippus (L.) (Lepidoptera: Nymphalidae) at a Milkweed-Abundant Location in the San Francisco Bay Area. The Journal of the Lepidopterists’ Society, 79(3). https://doi.org/10.18473/lepi.79i3.a8