Discussion with John of A DC Birding Blog

The White-Crested Elaenia that recently turned up in Texas was the first of its kind to ever be found in the U.S. and inspired a whole lot of excitement among birders who could afford a plane ticket. The situation got me thinking about extralimital birds. How many extralimital birds are actually found by birders? How many birds are missed?

I engaged John of the excellent A DC Birding Blog in a discussion of this topic, for your reading pleasure:

Say 100 White-crested Elaenias are released onto the east coast of the US. After a month, how many of them do you think have been found by birders?

ADCBB: I think it depends on where on the east coast you release them. If you release them near New York City, I would guess 80-90 would be found. In the less developed areas, I could see only 10-20 being found.

I agree that if the birds were released in a heavily birded area they are more likely to be seen than if they were released somewhere more remote, but that still leaves a chicken-and-egg question: Are certain areas heavily birded because they're productive or are those areas productive because they're heavily birded?

ADCBB: For the most part, I think it's the former. Places like Cape May were popular with market gunners who needed to find as many birds as possible long before there was a birding culture around them. I think that urban hotspots develop reputations because there are so many birders to track down every rarity that passes through.

I think I agree. Certain areas are popular because they are places were birds tend to congregate in large numbers, and therefore extralimital birds are more likely. On the other hand, I think a lot more extralimital birds would be found it birders spread out and searched less common areas (gull-filled landfills in the winter, for example). Do you think the Elaenia would have been found if it had landed in a private yard next to the conserved area it was found at?

ADCBB: Probably not, unless the yard belonged to a really sharp birder. Then again, nothing would stop it from wandering into the conserved area where it might be more likely to be seen by a birder.

Sure. Length of stay and movement definitely increases the likelihood of an unusual bird being discovered. Of all the extralimital birds that show up in the U.S., how many do you think are discovered?

ADCBB: I think that most extralimital species are discovered eventually; I'm not sure that most individual birds are since it is easy for individuals to land in an underbirded area.

But aren't most extralimital birds single individuals? Are you saying that species makes a difference?

ADCBB: If you have one extralimital bird that sticks to a particular location for an extended period, then I think you can say with some confidence that it is a single bird. But you don't really know for sure unless you band it or have some other identifying characteristic to separate individuals from each other.

As an example, a few winters ago a Rufous Hummingbird was visiting one of the Smithsonian gardens in D.C. There were actually two birds, but that did not become clear until the one that had been banded was recovered outside a window in Pennsylvania. Without that recovery, it would have appeared that a single individual stayed all winter. Now Rufous Hummingbird is not an extralimital, but I think its example can be applied to some of the single-record extralimital birds. There is speculation that the various Western Reef Heron sightings along the East Coast in the last few summers are a single individual, but I am not sure whether that has been confirmed.

That's an interesting point that refutes my position that most extralimital birds are not seen. If the same individual Western Reef-Heron was seen in several different locations it would indicate a higher level of birder coverage than I expected. However, I would temper this my saying that coastal birds - especially herons - are more easily seen than most birds (ie rails, warblers, sparrows) that stay out of sight. Therefore, extralimital birds of this type are less commonly discovered than more visible species like waders and raptors. Do you agree?

ADCBB: I think that is correct. The smaller and more cryptic birds are going to be harder to find, especially outside of breeding season, so some individual birds are going to go unnoticed. At the same time, most extralimital species seem to follow some sort of misguided route - a spring migration that takes them too far or a winter migration that somehow ends up on the wrong continent. Except for birds that only touch the edges of the ABA area, their route is likely to pass through some areas with few birders and others with lots of birders. So the (hypothetical) Northern Lapwing that is missed in Nunavut could be spotted around Cape Cod, Cape May, or the Eastern Shore.

Passing through the same areas that local migrants favor (and, with them, birders) would certainly improve the chances that they are discovered. So a bird will continue it's general north-south or south-north migration even though it's on a different continent?

ADCBB: I think that the north-south instinct is still pretty strong, even if the east-west coordinates are off. If you look at distribution maps for rare species, the sightings tend to cluster around certain areas of the country. There are large clusters along the southern border, mainly in the Southwest, and there are other clusters along the Pacific or Atlantic seaboards, which suggests birds that entered through the Arctic followed North American flyways. I think that both of these represent a misguided migratory instinct. It would be interesting to know if studies of vagrancy patterns would confirm that.

What about species that don't migrate as far? What do you think about patterns of species like Arctic gulls (Ivory, Ross' etc) or Great Gray Owls, that aren't necessarily following flyways?

ADCBB: The flyways are really just broad fronts along which birds move, whether because of geography, weather patterns, or some other factor. Each species, of course, has more specific routes. I think the shorter distance migrants still follow flyways, but just not as far. When Ivory Gulls come south, their destinations seem fairly predictable (see map).

Awesome map. OK, let's wrap this up with a prediction. I think that of all the extralimital birds that arrive in the US, only about 1/3 are found by birders. How many do you think are found and why?

ADCBB: I think it's a little over half. Extralimital birds that enter the country seem likely to end up in a spot frequented by many birders, so they ought to be found eventually, even if it takes some time. A lot of the more cryptic species will go unnoticed, either from being missed or being misidentified as common birds, but a higher percentage of the rest will be found.