Date: 5/10/26 3:49 pm From: maliadances via groups.io <maliadances...> Subject: [pen-bird] EBIRD FILTERS in San Mateo County
Hi All,
I’ve been meaning to write this for a while as I often get questions/comments about our eBIrd filters. I often get asked “Why is this filter set this way, the bird is expected” and conversely when questioning a bird that is reported outside of its’ known range/distribution within the county I get asked “If this bird is rare, why isn’t there a filter?”
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First I want to say that eBird filters or the way they are set should not be feared or disliked. They have a beneficial purpose and they play a very important role in assuring data quality for San Mateo County. And know that, the filters are in a constant state of review, analysis, editing and refining.
eBird filters are extremely complicated and I could write more words of explanation than anyone will ever want to read. Some filters can be set a certain way, others cannot. So I’m going to try to keep this post as uncomplicated as possible.
*San Mateo County eBird Filters are set to flag and suggest to observers that caution should be taken, and documentation should be gathered before making a report.*
*They are set to:*
· -Reflect the overall rarity of a rare species countywide i.e. Slaty-backed Gull
· ---- Address timing and seasonality i.e. early or late arrival and departure dates
· ---- Flag the record where there are identification issues for some “common”or “expected” species that can have a very restricted range during a particular time of year.
· ---- Where there are ID issues for some “common”or “expected” species that have confusion species challenges.
*AAnd know that there are some filters that cannot be set to help ensure data quality in eBird*
· San Mateo County has ONE FILTER SET for each species for the entire county which is basically pretty inadequate. San Mateo needs more geographically refined filters. But eBird is
n not prepared to give them to us. So there are likely many birds in our database that have been reported erroneously out of their known range/distribution that the filters did not catch at the time of the observation. The review team has to search for those potential erroneous records manually, on a weekly basis, and then contact observers for more details after the fact.
So I want to start by talking about each of those categories above:
*Reflecting the overall rarity of a species countywide* i.e. rarity status birds like Slaty-backed Gull, Broad-billed Hummingbird, Emperor Goose, Bar-tailed Godwit etc.
I would think this category is probably a no-brainer and it requests details for some of the rarest birds ever reported in San Mateo County.
*Address timing and seasonality i.e. arrival and departure dates*
Addresses reports which are reported outside of expected arrival and departure dates. I believe this concept should also a no-brainer. And we do know that each year we can have birds arriving slightly earlier or later than the set filter date. Over time, however, we may see changes and different trends in migration timing so we try our best to evaluate these sort of filter settings. Observers who upload media (identifiable audio and photos) have helped us immensely in being able to re-evaluate and refine this sort of filter setting scenario.
*Where on-going identification issues for some “common” or “expected” species have become apparent based on what the review team has experienced and continue to experience.*
Here are just a few examples:
*Rufous Hummingbird –* are pass through migrants. Only adult males can confidently be ID’d and only with a solid look at the complete orange back or with a look at the open tail feathers. We are routinely presented with photos of female/immature types claiming to be RUHU with no explanation why. Also we often get reports indicating simply “ID’d by Merlin”. Merlin cannot reliably differentiate between Allen’s and Rufous sounds. So by setting the filter to zero year-round, this suggests that the observer should take caution before submitting the report.
*Hermit Thrush (summertime) -* Hermit Thrush in San Mateo County is complicated. It is true that the bar chart shows Hermit Thrush year-round. If you filter the results of the bar chart to show only map markers Dec-Feb, you will see observations of Hermit Thrush is pretty widespread through-out the county. However, if you set the date filter to Jun-Jul, you will see that the range shrinks significantly to restricted areas along the Santa Cruz Mts.The problem lies in the fact that the wintering sub-species of Hermit Thrush are present throughout the entire county during the winter. Those wintering thrushes leave in the Spring and are NOT the same sub-species that arrive in Spring/Summer to breed here. *Our breeding sub-species of Hermit Thrush occupy a very small range along the Santa Cruz Mts.* The Hermit Thrush filter is set to “zero” because folks are used to seeing them all winter and some ebirders think they are here year-round AND are also confusing Swainson’s Thrush, thinking they are Hermit Thrushes. eBird users have proven with both photos and audio as well as descriptions that separating our summer Hermit Thrushes and Swainson’s Thrushes is quite challenging.
*Cassin’s Vireo -* Similarly, CAVI arrive to breed in *a very small range along the Santa Cruz Mts.* eBird users have proven with photos, audio and descriptions that Hutton’s Vireo and Warbling Vireo are a confusion species. As well as reporting CAVI way outside their known summer distribution during breeding season.
*Bank Swallow –* Once again, migrates here to breed. Has a very restricted breeding range in San Mateo County in the Año Nuevo area. ID issues have been identified with countywide reports of Bank Swallow where they are not expected. Also confusing Northern Rough-winged Swallow. Also eBird mobile app users type BASW trying to report Barn Swallow then click on Bank Swallow and don’t double check their checklists before submitting. Filter set to zero to suggest observer take caution.
*Northern Pygmy and Saw-whet Owl –* year round species – but heard only issues, where they are reported in unlikely parts of the county, where there are reported with time of day issues, where “ID’d by Merlin” has not proven to be the reported species.
Those are just some examples.
*If you’ve gotten this far,* there are many examples of species the review team cannot set filters for *as they are common in part of the county, but rare in another part of the county.* And *Many newer eBird users have no idea about regional distribution.*
· ---- There are species that only rarely occur on the bayside. Like Brant or Brandt’s Cormorant and get reported all the time.
· ---- There are species that only rarely occur on the coastside. Like Ring-billed Gull especially in large numbers, Oak Titmouse, White-breasted Nuthatch, American White Pelican and get reported all the time.
· ---- There are species that may never occur in the Santa Cruz Mountains like Northern Mockingbird, Horned and Eared Grebe, Swamp Sparrow, Red-breasted Merganser, Greater Scaup and are reported often.
There are many more species, that often get reported out of range. Sightings such as these, where we cannot set a filter for, require weekly manual search by the review team. There are about 45 species, depending on time of year where we need to search for species reported out of their known range. We will periodically try to write to observers who have reported out of range birds, but we often get the comment, “If this was flagged as rare I would have paid more attention and gotten photos” and the observers often don’t remember the observation.
Hope this all makes sense and please feel free to share with anyone who may find this useful.
If you have a question about a filter, please do not make a comment on your ebird record. Instead, please write to me personally <maliadances...>
