Maine's Filters

Have you ever wondered why a bird shows up as likely or rare on a checklist? When submitting a checklist you are shown the list ‘likely’ species, created from filters that check the species, count, location, and date for every observation. Here in Maine, we now have nine filters (plus a 10th state wide filter) dividing the state into smaller regions for greater accuracy. Anything that exceeds expected totals for a given species at a location on a specific date, will be flagged for review. This review process takes place in order for us to maintain the integrity of the database, and for it to be used fully by the science and conservation communities. Through our combined effort to maintain high data quality, eBird has become one of the most valuable large-scale data sets on bird distribution and abundance in the world.

If you are not familiar with the eBird review process then you may benefit from reading the helpful article “Understanding the eBird review and data quality process” available through the eBird Help Center. This article can be found at:

Maine’s filters are largely based on county boundaries with most comprising one or two counties; Monhegan Island being the exception.


There are currently 9 regional filters in Maine and one state wide filter. These regions use county boundaries to divide the state into areas with relatively similar habitat or bird populations. Counties are grouped to reflect these habitats since coastal regions will differ greatly from inland locations and especially areas with boreal breeding species. There is also a ‘semi-latitudinal’ divide to reflect the northern or southern extent of some bird populations. See the map below for a visual of how the state it broken up:


Species Known to be at a Location

There are some species in Maine that may be common or expected at certain locations but are still flagged for review. These species are flagged because of the large area covered by the filter they are associate with. Simply put, a species might be expected at a specific spot but rare anywhere else in the county. Complete descriptions or embedded photos should still be included in the notes of your checklists as they can help track important information.

As an example, Tricolored Herons are known to occur in the Scarborough Marsh each year. We can explore the Scarborough Marsh Wildlife Management Area Important Bird Area data frequency chart (inserted below) and see that Tricolored Herons actually occur on as many as 30% of checklists from that area. The filter that drives the checklists from the Scarborough Marsh is controlling all of York and Cumberland Counties for any given time of the year. If we look at the same chart as below but for the entire range of that filter, frequency only peaks at 4%. The point here is that we know Tricolored Herons occur at the Scarborough Marsh, but are very rare elsewhere in southern Maine. Complete descriptions of birds away from the Scarborough Marsh would confirm other locations they are using and details (especially on age) of birds at the Scarborough Marsh could provide proof of nesting


What the Future Holds:

In 2013, Maine split off its first sub-regional filter by creating one unique for Monhegan Island and several of the Mid-coast islands. These areas are so different from the mainland filters they were previously associated with that either too many species were being flagged as rare or species were slipping through without necessary review. For example, Tufted Titmice are extremely reluctant to cross water and therefore won’t venture out to offshore islands; follow these links to compare Tufted Titmice on Monhegan Island versus the rest of Lincoln County. Before having this sub-regional filter any Tufted Titmice reported on Monhegan could have gone undetected.

While it may take awhile, in the future more sub-regional filters will be developed to further strengthen the integrity of the data being submitted. Following the Tricolored Heron example above, the Scarborough Marsh and many of Maine’s other IBAs provide a starting point for these new filters. Kennebunk Plains is certainly unique enough from the rest of York County. And Messalonskee Lake in Belgrade with its Purple Martins and Sandhill Cranes would make a perfect central Maine filter. How bout coastal vs inland filters?