Comparing eBird Data to Colloquial Perceptions


This map shows the April pattern of distribution of Black-throated Gray Warbler eBird reports for all years 1900-2011. (Image sourced online at

On 9 March 2012 a Black-throated Gray Warbler was reported from Bandon State Natural Area on Oregon's south coast. There was modest debate on the local listserv over whether this bird was overwintering or, perhaps, an early northbound migrant. I was in the overwintering camp.

Just a week earlier, I was melding together the Oregon and Washington sections of the Fall 2011 report for North American Birds when I noted that my co-editor, Brad Waggoner, had characterized a November 1st Black-throated Gray at Vancouver, Washington as being "a month tardy."

It's important to note that Brad lives on Puget Sound about 180 miles north of where this bird was seen. Vancouver is just across the Columbia River from Portland, where I live. Since low numbers of Black-throated Grays are still passing through Oregon during the first two weeks of October, I would have described this bird as being about two weeks late.

These discussions, which involved highly experienced birders, demonstrate that each of us has unique notions about the migratory timing of Black-throated Gray Warblers. These perceptions are shaped by where we live and our own set of experiences with this species. 

My experiences with migrant Black-throated Gray Warblers have come primarily from the Willamette Valley in western Oregon. In the southern end of the valley–where I lived from 1984-86 and from 1998-2010–the first northbound Black-throated Grays would appear in during the first few days of April. Skinner Butte in downtown Eugene would almost always yield the earliest sightings for the valley and often for the entire state, in part because it gets daily coverage from late March through May. The first reports Portland (110 miles to the north) would come several days later, usually not until about the 10th of April or later. The fall migration of warblers through Oregon is diffuse and there are rarely weather systems that create noticeable fallouts. Along the outer, coast the southbound "passage" of warblers borders on undetectable. In the interior lowlands, focused efforts along riparian corridors, especially those bordering north-south running waterways will produced small mixed flocks of migrants that usually include a few Black-throated Grays until about 15 October.

Since my ideas about the migratory timing of this species were not the same as other equally experienced observers, I decided to compare my perceptions about Black-throated Grays with the data that one can easily mine from the eBird database. Here's what I found.


January eBird reports of Black-throated Gray Warbler 1900-2012. (Image sourced online at


February eBird reports of Black-throated Gray Warbler 1900-2012. (Image sourced online at


March eBird reports of Black-throated Gray Warbler 1900-2012. (Image sourced online at

At a glance, the three maps above, which show the distribution Black-throated Gray Warbler reports for January, February, and March are remarkably similar. Compare these to the April map at the top of the article, which shows a major influx of Black-throated Grays all along the Pacific Coast during that month.

We can also look at the year-long bar graphs, which offer better resolution of what happens within each month. The bar and line graphs below provide a closer look at the migration timing in Northern California, Oregon, and Washington.


This graphic shows the annual bar and line graphs for just those counties in the northern one-third of California, basically drawing line from Placer on the eastern border (at the elbow of California) west to Mendocino. It illustrates that Black-throated Gray Warblers are reported almost year round, with the spike of spring migrants starting just before 1 April and peaking during the last two weeks of April. (Image sourced online at


The annual bar and line graphs for Oregon shows that Black-throated Gray Warblers are essentially absent from the state November-March. The spike of spring migrants starts right about 1 April and peaks about the first week of May. Note that the number of fall reports remains pretty high into October, with a sharp drop-off after the second week of October. (Image sourced online at


The line graph for Washington is nearly identical in shape to that of Oregon, but it is more compressed. The spring spike in Washington doesn't really kick in until about 15 April, but still peaks right at beginning of May. Note that in Fall the sharp drop-off comes about 1-2 weeks earlier than it does in Oregon. (Image sourced online at

One of things that is exposed in looking at these graphic representations is that the spring migration of Black-throated Gray Warbler is incremental in nature. When we think about migration, we are captivated by those species that make long trans-oceanic crossings and non-stop flights that extend for multiple days. By contrast, the migrations of many passerines involve a more protracted series of short hops. In looking at the bar graphs above, it is apparent that the first influx of northbound Black-throated Gray Warblers Northern California about two weeks or more before they reach Washington. The southern end of the area represented by this set of graphs is approximately 500 miles to the south of the southern border of Washington. If we break down this distance as it relates to the number of days (about 15) between arrival dates at each end of this 500-mile divide, it translates to about 30-40 miles of northbound flight per day. Surely, this is an over-simplification, but the data does suggest that Black-throated Grays are either coming north in a series of short flights or making longer hops and then stopping to rest and feed for multiple days along the way.

In the end, this exploration confirmed my thoughts about what constitutes "early" and "late" in Oregon. More importantly, it demonstrated that the perceptions of my colleagues are equally valid given the experiences one would have living where they live.


Based on the many years that I collected first arrival Rufous Hummingbird. I was able to calculate the approximate rate for the arrival front for that species. It worked out to about 16mi/day. Rufous Hummingbirds (on average) arrive much earlier than Neotropical warblers and this may (in part) explain the more leisurely rate.

As for the spiky bits that represent peak movement, especially in the spring, species that migrate fairly late in the season seem to show more compressed spikes than do those that arrive eariler. Compare Orange-crowned Warblers (for example) with Yellow Warblers. The pattern shows up quite well in bird banding data from the Neawanna Banding Station for most Neotropical species. Fall migrations tend to be more strung out in most species and movement spikes, at least on the Oregon North Coast, are mushier.


and here’s the eBird graph from BC:


Thanks Mike for the great comment. The combination of more data contributors and ever-improving capacity to display the data graphically is allowing us to truly “see” how migration looks upon the landscape.

Thanks Dick for sharing the eBird graphs of Black-throated Gray Warbler for B.C. I found it very interesting that the shape of the line graph for B.C. is the reverse of the California, Oregon, and Washington graphs, with the greatest frequency of detections coming in Fall rather than Spring. I have to wonder if this reflects the pattern of observer effort in B.C. Since the bulk of British Columbia’s population lives on or near the coast, presumably most of the birding effort occurs there. Northbound Black-throated Gray Warblers are very low density migrants along Oregon’s outer coast, at least compared to the number that one encounters inland. Perhaps the same is true in B.C. and few are found in Spring along the coast. It seems that southbound Black-throated Grays are bit more conspicuous along the outer coast, but it would take some data mining to make that determination. Mike Patterson may be able to offer some insights on this as well.


A quick question:
Aren’t the quick drop-offs on those WA and OR graphs in fall at exactly he same point? Sep 15, with the decline steep until Oct 15? I don’t see how that pair of graphs shows a difference in fall timing for WA and OR. And at least to the extent I can read the graph correctly, I don’t see how if can be claimed that the sharp drop off in OR occurs after Oct 15.

The ‘tails’ of the fall migration do seem to lag a little in the OR graph, but not in the way described , in my opinion.


In reference to the comments above by “anon,” I looked at the green histogram at the top of the page, which shows broader bars for Oregon in both the second and third weeks of October. Also, note the differences in percentages along the vertical access of the line graph. The Oregon line drops below the 2% of checklists line later in the season than it does for Washington. I think that the eBird data paints a pretty accurate picture of reality. This article is based on my layman’s interpretation of the data as provided and presented by eBird. Others may explore this data and arrive at differing conclusions.


A perspective from Victoria, B.C: in a typical year here, I tend to record at least 5 to 10 times as many Black-throated Grays in fall as I do in spring. In my experience, a decent-sized fall mixed warbler flock, often feeding in Garry oaks or riparian deciduous habitat, will typically contain a Black-throated Gray or several, while in spring passage I’ll generally have to sort through a lot more birds before I find a Black-throated Gray. Spring passage seems to peak around the end of April/beginning of May here, and in fall I’ll see migrants daily through all of August and September, typically with a drop-off around the last week in Sept. Here at the southern end of Vancouver Island, a high count of mine was 16 at one site on Aug. 9, 2010; in spring, even on excellent fallout days with rain and low clouds, seeing more than a couple Black-throated Grays is an exception. By contrast, Townsend’s Warblers seem about equally abundant as spring and fall migrants here, in my experience anyway.


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