I totally get it...on your long haul flight - the plane isn't emitting say, 92Kg per passenger per hours flight...your plane is (based on an average occupancy of 330 passengers) emitting 92.5kg/hour created by nasty "tourists" and 0kg/hour created by 2 "eco-tourists"
Obviously (given long haul flights average occupancy of 333 passengers) tens of thousands of eco-tourists around the world each year are just "absorbed magically" into existing flights...they don't create any extra demand...
Lets be honest, most of our birding creates a significant carbon footprint, from manufacture of our stuff...through driving right up to long haul flights...so none of us are without sin...(though the bikers and walkers amongst us are doing pretty well!) by all means enjoy your world birding...but at least have the good grace to feel a little guilty about it and not try and pretend it brings "environmental benefits"
The carbon cost incident to my being on the plane is negligible compared to what I save regularly via other choices, so it is not logical to feel guilty about it. No, the 331st person added to a flight of 330 is not "magically absorbed" - but the increased CO2 impact is quite small - certainly not another 92kg/hour.
The simplistic 92 kg/person-hour value that gets thrown around is (at least) an order of magnitude too high an estimate of the rise in CO2 emissions due to the addition of a single passenger on a typical jet. (Using the "logic" by which that number is customarily derived, one would conclude that a half-full flight emits half the CO2 that a full one does.) The amount is small enough to be easily offset in a month or less of simple lifestyle decisions. In my case, I don't own a car or drive one, my wife has a hybrid, she works from home, we rarely drive anywhere anyway, I do not own or use any gas devices such as mowers or snowblowers (and I live in Minnesota, for Pete's sake!) and I don't even use public transportation; I walk everywhere around town and average 10 miles a day, so I make up for a gallon of gas from a 40MPG car in less than a week. I will confidently claim to be "greener" than most self-professed greens... and no - no guilt about flying, thanks.)
In a scenario in which my NOT flying would cause the airline to CANCEL the flight, then YES, I think I might feel guilty about getting on it (assuming the cancellation actually results in a net reduction of flights and not a rescheduled one, thereby keeping the total number the same.) But that isn't how it works. Birders make up a small part of any group on most planes and unless we all were on the same aircraft, our deciding to fly or not isn't making that much of a difference. If every birder, assuming we are 1% of the general population, decided to stop flying, it would almost certainly result in no change in the total amount of major flights flown, and far less savings than N*92kg/person-hour, where N is the number of now non-flying birders. The only exception I can see would be those rare flights to "speciality" remote places where few others would go: perhaps the short puddle jumps to some of the Aleutian Islands, for example. But routes to Miami? To Mazatlan? To Rio? Sorry, not seeing it. Just not enough of us could stay home to dent the demand. I don't see airlines cancel flights because 1 or 2% of the seats didn't sell. They seem to have found plenty of other ways to recoup those costs.
All that being said, I despise being on planes and detest airports with a passion. I look forward to my last visit to any of them. And I would be happy to take alternate transportation if it existed. Any ideas on how else I can get from Minnesota to Uganda?