Overwintering Sites - Western Monarch Count

What is an Overwintering Site?

Overwintering sites are the protective habitats in which monarch butterflies spend the winter; they’re usually a grove of trees and/or shrubs. There are more than 400 documented overwintering sites in California alone–these primarily serve the western monarch butterflies. Monarchs migrate to these special habitats each fall, seeking shelter from harsh climates. Here’s a migration map showing this phenomenon.

Overwintering Site Selection

Each overwintering season, Western Monarch Count (WMC) participants are asked to monitor a minimum of one monarch overwintering site. Regional coordinators assign participants their overwintering site(s) to monitor, though participants are welcome to suggest the sites they’re most interested in. It’s important that participants follow through on their site assignment whether the site is “active” (recent history of monarch clusters) or not. Even sites without recent monarch activity are important to survey, and there have been several cases of monarchs returning to sites that were unutilized for more than a decade. If you’re assigned one of these “historic” sites, think of it like a treasure hunt (maybe one day you’ll make an incredible discovery!) and know that you’re still submitting valuable data, even if your monarch count is zero.

Confirm your site(s) with a regional coordinator prior to conducting surveys!

Work with your regional coordinator to select an overwintering site. Regional coordinators may have special instructions for a site, so it’s always a good idea to run your monitoring plans by them first, even if you’re a returning volunteer. Working with regional coordinators helps ensure (1) good survey coverage across all sites in a particular region, (2) key information about sites are communicated to the survey team, (3) the WMC project maintains positive relationships with land managers. Additionally, some sites may be designated as sensitive sites, meaning that information about these site is not publicly available. Participants assigned to sensitive sites are required to keep the location and site information private.

Site Monitoring Frequency

Visit your assigned site at least once during each official monitoring period (e.g., early, mid and late season counts). Visiting multiple times per monitoring period is highly recommended! The more monitoring the better.

Photo: Several monarch cluster trees within the San Leandro Marina Golf Course overwintering site, California, 2021-22. By Christina Garcia.

Consider These 3 Things:

Discuss site access and more with your regional coordinator. Some sites have caveats. Communicate with your regional coordinator about important details (site access, terrain, count history, etc.) related to the overwintering site assigned to you. Additionally, the Xerces Society will sometimes determine that certain sites meet a higher priority threshold and require additional monitoring attention during the overwintering season. If you have capacity to do extra monitoring or aren’t attached to any particular site, please let your regional coordinator know so that they may ensure proper coverage of all priority sites.
Think about timing and capacity. Monarch counts should occur early in the morning so that temperatures are cool and clusters remain intact from the evening before. Consider choosing a site near your home or a place you frequent often so that you can guarantee an early arrival. Or, if you prefer adopting sites in more rural locations, consider camping nearby and making an annual adventure out of it!
Determine Site Access. Some overwintering sites require site access coordination and permissions before monitoring activities can occur. It is vital that WMC volunteers attain written permission from the relevant landowner or managing agency prior to entering a private site or site with limited access. The Xerces Society and WMC regional coordinators will do their best to coordinate and communicate site access details with volunteers, but ownership and access data may be limited. At times, volunteers may arrive at a site where access is denied, limited or unclear. In these situations, it is best to refer to the Code of Conduct and err on the side of caution; trespassing is never condoned.

Perks of Regular Monitoring

Although not a requirement, monitoring the same overwintering site each year can prove both fulfilling to you, the volunteer, and beneficial for the monarch butterflies. For example, developing deeper familiarity with an overwintering site may allow you or others to quickly identify site disturbances and/or potential threats to the overwintering monarchs and their habitat. You are encouraged to note any site disturbances or threats in your Monarch Count or Habitat Assessment datasheets and the Xerces Society may reach out to you for additional information or clarification. In this way, you are not only engaging in a fun community science project, but also serving as an advocate and conservationist for monarchs and their habitat. Additionally, you might notice that you’re monitoring more efficiently and effectively as you gain a better understanding of the nuances and trends within a site (e.g., when and where monarchs tend to cluster, what environmental resources they rely on, etc.). If you can commit to several years of monitoring, please let your regional coordinator know. Still curious about monarch overwintering sites? Learn more about overwintering site management and protection here.