The Rare Bird Theory

The Rare Bird Theory (RBT) is a simple concept from observations we’ve had reviewing bird sightings. But before we delve into what the RBT is, we must first define what a rare bird is. A rare bird can be any bird out of their geographical range or a bird out of season. There are different levels of rarities, from overwintering birds to first ABA-area records, but the concept is the same.

The RBT has two main points:

  1. The more birders in an area, the more rare birds will be found
  2. Rare birds aren’t as rare as they seem but a factor of the number of birders in an area

Two birders can find a rare bird, but wouldn’t ten birders have an easier chance?
The end of the line

The first part of the RBT is similar to the “Patagonia Rest Area Effect” – a rare bird triggers an increase in birders to an area which means more rare birds are discovered, triggering even more birders to visit. This cycle doesn’t continue forever but has been proven in more than just Arizona (where the Patagonia Rest Area is). A few years ago when a Vermillion Flycatcher was discovered in DeKalb county (Illinois) another birder found a Say’s Phoebe while looking for the Vermillion.

The Chicago area gets a lot of rare birds. While the location plays a big role, it is the number of birders that determines how many rare birds are found. On any given day there may be 10 to 20 times the number of birders out at a famous place like Montrose, while the forest preserves in our county are lucky to get one birder a week.

Crimson-Collared Grosbeaks are found in southern Texas where there are a lot of birders – does this mean they are not at the under-birded areas?
Crimson-collared Grosbeak

So, is Chicago really a magnet for rare birds or are these birds just found easier because there are more eyes looking for them? Think of your own favorite birding areas. Are they rare bird magnets or are the rare birds just more likely to be found?

It is our belief that there are hundreds and hundreds of rare birds lurking in patches of habitat all over the place that just go undiscovered. These birds collectively reduce their own rareness, meaning they are just naturally occurring vagrants or wanderers. Of course, sometimes a mega-rarity shows up that is a great find and something truly out of the ordinary.

But the next time a rare bird shows up think about the Rare Bird Theory. Is it really likely that this one bird at one location is the only such bird in the area? How do the odds of a single birder finding this rarity compare to the odds of a birder missing many other rare birds? And how much more likely would finding a rare bird be if there were 20, 50, or 100 times as many birders out birding?

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