Wednesday, May 18, 2016

The One Bird Theory


Prothonotary Warblers are great...I was more than happy to see this one in Goleta, in Santa Barbara County, last fall on the way to visit friends/family/birds in Ventura. In 2015, eBird has records for Prothonotary Warblers from 10 different locations in California. Could this just be the same bird wandering around? Of course not, but birding is rife with empty questions like this.

According to The Great Ornithologist Felonious Jive, there are more birders now than ever. I am inclined to agree with him. Birders are birding harder than ever before. Birds, in much of the country, are really getting birded.  Perhaps as a result of increasing coverage over the years, I am constantly hearing the same thing...the one bird theory.

The one bird theory is simple, though a bit cumbersome to explain. If a Ruff is seen in Oregon one day, and another Ruff is seen in Washington a few days later, birders will wonder if it is the same bird. If a Common Crane is seen in New Mexico, and a Common Crane is seen in Texas a few days later, birders will wonder if it is the same bird. If an Emperor Goose is seen in Humboldt County one day and an Emperor Goose is seen in Sonoma County the next day, birders will wonder if it is the same bird. So rather than assuming or deducing two different individuals are involved, a birder will wonder (and that is the key word here) if the same individual has been found in both places.

Now some of you might think they know where I am going, that I think the one bird theory has nothing to it. That is not true. The one bird theory turns out to be correct quite often. What does bother me is how often I hear or read the phrase, "I wonder if the [Species Blablabla] seen at [Location A] is the same bird that was found at [Location B]." It is usually just left at that, without any discussion. Well friends, you don't have to just put your wonderment out there for the world to behold, you can actually have an informed opinion...you could even decide yourself!

But how can one do this? Why think when one can wonder? Examining the one bird theory is not so difficult, it just comes down to considering a few factors and asking a few questions.


Elegant Terns are obvious birds...you know when they are around. It is important to know when birds are around, and when they are not.

Were they seen at the same time?

Overeager birders who quickly spout their one bird theory hypothesis often don't even check to see if the different individual sightings involved were actually happening simultaneously. A bird can only be in one place at a time, not two. Do your research people, don't make others do it just to answer your own question.


Migratory habits of many North American species are well understood. Black Terns go south in winter, north in spring. Not too complicated.

What direction is it going?

This question is especially important to consider during migration. Let's revisit our Ruff example from above. Let's say the sighting happens in October. In October, shorebirds migrate south. If a Ruff is seen in Oregon, it is incredibly unlikely it will turn around during the height of fall migration and show up in Washington. The Ruff will not suddenly figure out it is in Coos Bay and then try to hightail it back to Siberia in order to find the correct continent before it continued its southward migration. So, keep migration and dispersal patterns in mind when pondering the one bird theory.


Cassin's Finches, like several other high elevation species, periodically "invade" unusual places every few years in search of food, which can take them to unusual habitats and places where one would not expect them. When such patterns are evident, one need not spend much time contemplating the one bird theory.

Is there a pattern?

For various reasons (often unexplained), a region will occasionally experience an irruption of a vagrant species, such as with Black-bellied Whistling-Ducks, seabirds and northern finches. So when a lot of Common Ground-Doves sightings are popping up in the Midwest, which happened last year, there is obviously no reason to think it is just the same bird miraculously choosing to stop in heavily-birded vagrant traps and being found repeatedly.


An out of range Western Gull would be a worthy candidate of the one bird theory. They are not highly prone to vagrancy, and would stand out in a lot of places. A bird wandering around the east or the interior west could definitely be found in more than one place.

How rare is it?

There are rare birds, and then there are rare birds. If two Golden-cheeked Warblers were seen in two different places in California in one year, one should rightly consider if the same individual is involved...it is just incredibly unlikely one would get here in the first place (and we do have a record!), let alone two around the same time. However, if two Chestnut-sided Warblers show up in different places in California in the same season, the idea of them being the same individual would be a ridiculous notion unless both birds were melanistic or banded or something like that (we will get to that soon). Chestnut-sided Warblers are not at all unexpected in California, and the state gets many records every year, up and down the state. It's quite rare for the one bird theory to be in effect when a "expected" species is involved.



In the eastern United States, there is no shortage of habitat a wayward Western Tanager could use spring through fall. How many Western Tanagers are eastern birders missing? Probably hella.

Think about birder coverage.

In most, not all, parts of the country, birders are not covering all available habitat very well. Most is inaccessible, due to lack of birders, roads, trails, or because land is private or government property that the public can't get to. When this is the case, think about all the area where birds are being missed...what are the chances that the subject of your one bird theory will somehow be found in more than one place, and somehow not vanish into the abyss that is all this other unbirded habitat? Think about the odds...they usually are not very good.


Ah, the Common Nighthawk. An easy bird to see in some places, but how many of the Common Nighthawks out there are people really seeing? Nightjars are hard...they can be hard to identify, but just finding them is typically the problem. Finding them roosting is blind luck, and birders can't see them foraging in the dark. The unfortunate truth is that the vast majority of nightjars go completely undetected in their normal range, let alone as vagrants! 

Think about detectability.

Some birds are easier to see than others. An Ivory Gull is easy to see. A Common Poorwill is not. A Ferruginous Hawk is easy to see. A Gray-cheeked Thrush is not. A Long-billed Curlew is easy to see. A Wilson's Snipe is not. A Black Skimmer is easy to identify. A Common Sandpiper is not. A Laysan Albatross is easy to pick out. A Tristram's Storm-Petrel is not. When birds are hard to find due to their habits or simply hard to identify, you can bet that a lot of these birds are going undetected, even when they are right in front of us! Then there is the fact that most birds in our area go undetected because they are in places where we don't bird, or because they fly right over us during migration...when birds migrate, they don't stop in every single county on their way to their destination. Think about this: a Yellow Rail seen in one place is practically guaranteed to never be seen in another. So to put it all together, one must ask how many of a certain species we are missing in a certain area. This has everything to do with likelihood of the one bird theory holding up for a vagrant. Keep these things in mind when the one bird theory is wracking your brain...the more you think about the detectability of a species, the more the one bird theory has a tendency to lose water.


This Pink-footed Shearwater is a good example of what a bird can offer that would help identify it on an individual level. This bird has a lot of molt going on in the flight feathers, definitely something to key in on, and the underwing pattern is a great thing to examine as well.

What does it look like?

I saved the one of the most obvious things to consider for last. These days, many rarities are photographed well. So before you put your public wonderment out there, see if you can compare photos of the bird(s) in question. You can compare age, plumage, sex, molt, wear, patterning, bands, etc. This is what typically provides a definitive answer when questioning if the same bird is involved.

And there you have it...it boils down to knowing status and distribution, what the birds look like, and some focused questions about the odds of the same bird being found twice versus the odds of a different individual being involved. The one bird theory will always be out there, and for good reason, but now you are equipped to test it. You are ready. Wield the hammer of this knowledge in listservs, forums and Facebook groups, and you too can make birders better.

6 comments:

  1. I observed two Prothonotary Warblers together near Bishop last year. There are rare birds all over the place, they are like electrons around elements with long names. It is a heartening thought. Sometimes it is the same one who cares if Santa is real or not I just got a bunch of presents.

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  2. Bravo, Steve. I've always found this marked tendency of people (not just birders, but well displayed by them) to want to make what are surely separate animals into one to be perplexing. I think it comes from our preference for the narrative of the brave/crazy/pioneering/tragic individual soul off on a grand quest/adventure over almost anything else, especially over a much less romantic tale of a couple or more animals more or less randomly getting caught up in the same or similar set of circumstances.

    Not our greatest instinct as a species, but an important and defining one, and one that people who want to get others to value birds and conservation have to reckon with.

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    1. Thanks Jeff. You have a point, but I feel like a penchant for uncritical thinking is more to blame...another thing entirely to reckon with! I think there is room to both embrace the grand quest narrative and mix in some logical (and generally uncomplicated) analysis...migrants and vagrants certainly are on an adventure of sorts, whether seen by birders once, twice, or not at all.

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  3. Well said! But what about how far away are the sightings from each other? I have an easy time believing that the Northern Gannet on the Farallons and Alcatraz and Half Moon Bay is just one individual. But I have a harder time believing that the Kelp Gull on the Farallons, Half Moon Bay and Ano Nuevo is the same as the one at the San Gabriel Spreading Grounds (although that seems to be the consensus). And when it comes to a Crested Caracara first seen in Long Beach in 2002 being considered the same as one that took up residence in Fort Dick in 2008, I have sincere personal doubts.

    The philosophical underpinning is which scenario makes the fewest unsupported assumptions. All things being equal, which is the most conservative approach?

    I've thought about this a lot, and have yet to see a clear path forward.

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    1. Considering the number of caracara sightings in California over the last two decades, it seems highly unlikely to me that those caracaras would be the same birds. Detailed comparisons of the primaries of the Kelp Gull from photos in LA and on the Farallones pretty much seal the deal that it is the same bird though, at least as far as I'm concerned.

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