How does eBird know a species is rare, or a count is high? The short answer is filters. The Wisconsin eBird Team recently replaced, species by species several times over, one of our workhorse filters which we called the South Filter. Formerly the South Filter covered 11 counties in South Central Wisconsin. These 11 counties are now covered by 7 new filters–reflecting local differences in bird status and distribution on a finer scale.
Why do records flag? Typically, records flag for rare birds or high counts. Additionally, uncommon species that pose an identification challenge often flag as “rare”. Some of the species that fit this description in South Central Wisconsin include: Trumpeter Swan, Greater Scaup, Brewer’s Blackbird, Alder Flycatcher, and Long-billed Dowitcher. For these uncommon species, please provide comments specifically describing key field marks you observed that rule out look-alike species.
If you wish to read more about the ins-and-outs of filters and the ebird review/data quality process go here.
Also, back in 2012 we wrote about improvements to the South Filter here.
Wisconsin’s filters have evolved considerably over time. Initially one or two filters covered the entire state. Date range limits were monthly. How many eBirders recall when every neotropical migrant flagged in late April?
Since 2012 date range limits could be adjusted at the daily level. Currently Wisconsin has 25 filters, which increasingly account for local differences in bird status and distribution. That said, they are not and never will be “perfect”. More on that later.
With a few exceptions (Horicon Marsh, Wisconsin Point, Crex Meadows & Fish Lake, Wyalusing & Nelson Dewey and now the Southern Kettle Moraine and Lower Green Bay) all filters cover a county or several counties.
Along with hundreds of tweaks to date range limits and allowable high counts, the South Filter was broken into 7 smaller chunks:
As you will see in the filter descriptions below, compromises are necessary in filter design even at the county level. Some well-documented, but very local populations will continue to flag. Documentation for these populations can be brief (e.g. “known location” or “counted by 2’s and 3’s to arrive at a high count of 20”).
Below is an incomplete summary of filter adjustments…
Columbia County—very similar to Dane in many respects, but marsh birds are more common. High counts are also lower for most species–probably due to less coverage. Breeding Upland Sandpipers are extirpated or nearly extirpated. The well-documented and burgeoning Eurasian Collared-Dove population won’t flag unless you see more than 15. The well-documented, but isolated Red-necked Grebe population at Schoeneberg Marsh will continue to flag.
Dane County—this eBird Checklist juggernaut gets its own filter due to the sheer volume of data submitted by the county’s eBirders. Yellow-headed Blackbird and Common Gallinule are declining and very local breeders—as is Common Nighthawk. The isolated Louisiana Waterthrush population near the Iowa County Line in Blue Mounds State Park will flag during the breeding season. Rare breeders like Pine Warbler, Hooded Warbler, Chestnut-sided Warbler, Cerulean Warbler, Yellow-bellied Sapsucker, and Acadian Flycatcher will also flag.
Green, Lafayette, and Rock Counties—This region still supports a decent population of the declining Upland Sandpiper. Marsh bird numbers are lower in these counties versus “wet” counties like Columbia, Jefferson, Waukesha or Walworth. Unlike the other counties, Clay-colored Sparrow is a rare breeding bird, while Grasshopper Sparrow is relatively common. The isolated, but well-known population of Hooded Warbler in the Cook Arboretum will flag, as will Cerulean Warbler counts greater than 1. A visit to the rich, but relatively isolated wildlife habitat composed of Yellowstone Lake State Park and Yellowstone Lake SWA will likely produce a fair number of flagged species (e.g. a good-sized breeding population of Blue-winged Warblers exists here). During appropriate seasons, the lake may harbor large numbers of waterfowl and under certain conditions, shorebirds that are atypical elsewhere in the region.
Iowa and Richland Counties—Very little eBird data exists for Richland. Both counties are relatively dry—lacking widespread wetlands. As a result, marsh bird limits are low, but grassland bird limits are higher. The isolated breeding population of Louisiana Waterthrush in Governor Dodge State Park will flag. An exception to the grassland bird rule is Lark Sparrow. One might expect this species to be at least uncommon in these counties, but very few are actually encountered–typically at locations with sandy soil near the Wisconsin River. Counts of this species exceeding 2 birds will require documentation.
Jefferson, Walworth, and Waukesha— Similar to Dane and Columbia Counties. Common Gallinule and Yellow-headed Blackbird are more common breeders in comparison to Dane. Outside of the Southern Kettle Moraine State Forest (SKMSF) Acadian Flycatcher, Pine Warbler, Cerulean Warbler, Hooded Warbler and Red-breasted Nuthatch are rare summer residents. Summer records of Yellow-bellied Sapsucker are rare anywhere in these counties.
Sauk County—The Baraboo Hills routinely host breeding warblers that are rare to non-existent in the other 10 counties (e.g. Louisiana Waterthrush, Mourning Warbler, Black-and-white Warbler, and Chestnut-sided Warbler). Winter Wrens are expected here and breeding counts of 3 or lower will not flag. Formerly Hooded Warblers were routinely encountered in the Baraboo Hills, but this is no longer the case (Why?). For now this species will flag in Sauk County. Currently, the Sauk Prairie Recreation Area supports an unusually high concentration of Clay-colored Sparrows. Counts of greater than 10 will continue to flag.
Southern Kettle Moraine—The Southern Kettle Moraine lies in Jefferson, Walworth and Waukesha Counties. Within the confines of the Southern Kettle Moraine Eastern Whip-poor-will, Acadian Flycatcher, Pine Warbler, Red-breasted Nuthatch and Lark Sparrow are expected breeders. One can amass impressive double digit counts of Hooded Warbler and Cerulean Warbler on a lengthy hike through the forest. Small, isolated breeding populations of Louisiana Waterthrush, Black-throated Green Warbler, Blackburnian Warbler, Golden-crowned Kinglet, and Blue-headed Vireo exist within the Southern Kettle Moraine, but these will continue to flag due to their rarity and uneven distribution.
eBird filters are never “finished” or “perfect”. Bird status and distribution is ever-changing. A large-scale prairie burn at the Sauk Prairie Recreation Area may diminish the currently abundant breeding population of Clay-colored Sparrows. Irruptive and occasionally common species like the crossbills, Pine Siskin, Common Redpoll, and Red-breasted Nuthatch require periodic filter tinkering due to the idiosyncrasies of their movements. High rainfall, flooding, drought or a temporary drainage of a small lake or pond may create spectacular conditions for unprecedented numbers of shorebirds and waterfowl. Filters do not account well for locally abundant populations or isolated breeding populations (e.g. Clay-colored Sparrows at Sauk Prairie Recreation Area, Prothonotary Warblers in the Bark River Bottoms, Yellow-headed Blackbirds, Common Gallinules and Red-necked Grebe in a few locations). A birder is highly likely to encounter 12+ Prothonotary Warblers and a few Brown Creepers while paddling certain sections of the Bark River. However, most birding does not take place via canoe or kayak and the vast majority of habitat covered by the Jefferson, Waukesha, & Walworth Filter (which includes the Bark River) is not river bottoms. A summer eBird report of 12+ Prothonotary Warblers and a few Brown Creepers in a local upland woodlot or suburban backyard would be nearly impossible and should be thoroughly documented.
Due to lower filter limits many species are likely to flag on long traveling counts. Remember that “[t]raveling counts should be limited to homogeneous habitats (e.g., creosote desert, short-grass prairie), and ideally not be longer than 5 miles. Exceptionally long counts are not as useful for rigorous analysis, nor are traveling counts that span a broad diversity of habitats” (What Data are Appropriate).
Remember, if a record flags you found something rare. Be excited! Please provide appropriate documentation, so your local reviewer doesn’t need to nag you more details. This saves everyone time.
Need a refresher on the best way to document your find? Read this excellent article.
Thank you for contributing to eBird! We could not improve the filters without your data. Future iterations and improvements of the state’s filters can only be achieved with your contributions.
Article and filter edits by Aaron Stutz