Monday, March 12, 2007

Missing Birds 2: The Phantoms

Continuing my thoughts from an earlier post, I am naturally lead to the matter of North America's avian phantoms. There are, of course, three of these: Eskimo Curlew, Ivory-billed Woodpecker, and Bachman's Warbler. For some reason, one of these birds has ignited high passions and fierce debates for many, many decades; the other two have generally been approached in a more cool-headed, though still controversial, fashion. I'm going to pretend for the moment that all three of these species can just be dealt with as birds, not religious icons.

The prime question about all of these phantoms is always this:

Is it possible that they could have escaped detection for decades at a time yet still exist?

My back-of-the-envelope numbers suggest that in fact it is possible. Indeed, if they have managed to survive with populations numbered in the 10s or less, it might not only be possible, but likely and expected that many, many years would elapse between sightings. But, what is the difference between these birds and species like the Snail Kite, Whooping Crane (population formerly numbered in the teens), Kirtland's Warbler, and other birds with very small populations that are seen all the time? Well, it seems to me the difference is quite simple: We don't know where to look. We lost track of them. Without knowing a specific, small area to search in, the difference between very few birds versus no birds at all becomes extremely difficult to determine. Statistically, the amount of effort needed to either rule out or have a good chance of unequivocally detecting the presence of a small relict population lurking somewhere within a large area can be enormous.

But, don't we know where to look? Well, what we know are general patterns of historical occurence, and locations of recent unconfirmed sightings. However, this doesn't narrow our search down to small places and specific times where we'd stand a good chance of finding the birds if they did exist. Just because there have been multiple reports of Eskimo Curlews from Galveston Island doesn't mean that this is an especially favored spot for the species; it just means that this is where most of the birders on the Texas coast are. The nesting and wintering grounds for all three phantoms are in places that have not been especially swarming with birders and ornithologists during the last 50 or 100 years. Even the bottomland habitats of the Bachman's Warbler and Ivory-billed Woodpecker, though located in the heavily-populated eastern US, are probably the most inaccessible habitat in the region. It is not so much a matter of being remote or impenetrable. It is more about roadlessness and "No Trespassing" signs.

There's a fundamental problem with searching for low-density birds of poorly-known distribution by using targeted searches. If you focus intense effort in a small area, you are gambling that you picked the right spot. Suppose (oh no, here he goes again), that you've got ten areas, each of them one square km in size, and there is a single bird somewhere in the whole 10 square kms. This bird is one of those "typical" birds I talked about before, that you will on average find once every 10 hrs if there is one of them per square km. You have 100 hours of effort to put into searching for that bird. If you focus all 100 of those hours on a single square km, you have about a 10% chance of finding the bird. But if you spread that effort out, 10 hrs in each square km, your odds of finding the bird are closer to 70%. In the targeted search, if you guessed right you will almost surely find the bird multiple times, but 9 out of 10 times you will have guessed wrong. In the dispersed search, you stand a better chance of finding the bird at least once, though you are unlikely to find it more than just once or twice.

If you don't like these sorts of hand-waving calculations, just do an experiment that many of us do every year. On some June morning, go out and spend 4.5 hours walking around your favorite park, focusing your effort in a small area. The next morning, go do a Breeding Bird Survey, making 50 very short visits to 50 different, randomly selected spots. Then compare your species tallies for the two mornings. Odds are, you detected quite a few more species with the dispersed effort than the concentrated effort. This same phenomenon is why Breeding Bird Atlas projects have turned up so many surprises. Getting observers spread out and dispersing their effort revealed occurences and patterns that had been missed by many decades of clumpy, clustered effort.

So what is the answer about the phantoms? Even if one or two intensive searches (or a lucky chance encounter with a tourist and a camera) were to prove the continued exsitence of one of these species, we still would hardly know anything about the totality of the situation. Could some form of "Birding Blitz" help sort this out? If it were possible to get large numbers of experienced birders to each contribute a modest amount of (personal, volunteer) effort, spread throughout the areas of possible phantom occurence, would we emerge with more confidence in our assessment of whether or not these species still exist, and if so where and how many? In the past, it has been possible to pursuade large subsets of the hard-core birders to break their usual routines and go places they never would have gone otherwise, in the name of science and discovery. Is the notion of a "Curlew Blitz" in the prairies and western gulf coast in the spring, and paired "Woodpecker and Warbler Blitzes" in the bottomlands in winter and spring even worth thinking about? It might be more demanding on the observers than the Atlas projects were, especially the "Swamp Blitzes." Those might involve boats and private property far more than did the average Atlas block. But just for the sake of argument, if we could get say 200 people to each contribute 10 hours a year for each of five years, we'd have 10,000 hrs of observer effort. Might that be enough? Would we need 1000 people and 20 hrs a year to give 100,000 hrs of effort to be confident that we'd find the phantoms if they are out there? Could such a thing even be carried out? The "Curlew Blitz" would at least turn up a whole bunch of other shorebirds, including some nice rarities. The "Swamp Blitzes" might be harder to tempt people to volunteer for, especially since you'd need to have people with good ID and documentation skills for any of these efforts, similar to what is requres for the Breeding Bird Survey, and those folks are often already over-committed as volunteers.


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