How do you think people took nature photographs 100 years ago? They carried view cameras in wilderness. Ansel Adams made most of his best known photographs with very heavy gear carried on his back: eg, Monolith, the Face of Half Dome was made with a "61/2 x 81/2 Korona view camera, with two lenses, two filters, a rather heavy wooden tripod, and twelve [...] glass plates" which Adams carried over very rough ground including a 1200m altitude gain (Examples: The Making of 40 Photographs, p3). The camera weighs about 6kg, and his glass plates weighed about 1kg each, so his total camera gear must have weighed 25kg. Edward Weston, on the other hand, said "Anything more than 500 yards from the car just isn’t photogenic". Which is fine - but it is not a fact, it is a choice.
It is not that everyone - Adams included - would not rather carry less and spend less but that where an individual draws the line between "possible" and "impossible" is a unique, personal choice. The second highest step is the same distance from the top of every ladder, but not the same distance from the bottom.
There are distinct genres of Nature photography. Ansell Adams was photographing landscapes that have that useful tendency to hang around, albeit the uniqueness of the scene is all about the weather and time of day etc - and not least the person making the image. the focus of this thread is on the most cost-effective and ergonomic solution to secure the best possible images of organisms - small bodied and mobile - exemplified in smaller birds especially. These are elusive subjects for which a camera & telephoto on a tripod too often slows one up capturing the moment.... Handholding works best as does the ability to be free to move relatively quickly.
It's true that a great many excellent wildlife photos are captured from set hides and mobile hides (vehicles) where lugging in 2+ DSLRs and 2 or more big lenses AND more gear is so feasible over those well trodden steps between hide and home.
For the past 3+ decades l have often had to carry heavy packs all day in tropical climates and over rough country. On occasion over 30kg of gear for camping out - mainly food and litres of water. It is no fun. Grueling after a while. On a couple of these trips my companions gave in in exhaustion, and they were regular runners....the heat and the loads and the gradients take no prisoners. A conspicuous cohort in the generations I was privileged to grow up in Zimbabwe (then Rhodesia) through the 1960s into the 1980s took such conditions as the means to the ends... But quite a few refused to go to such lengths of backpacking so much gear to enjoy the gains "Out There". (In fact, the strategy of very heavy backpacks was tried and proven in the counter insurgency bushwar.)
The lesson in the preceding paragraph is only a minority of us can carry such heavy loads and even fewer can put the core equipement to speedy use when the event of the moment happens..... Moreover as one ages this ability to perform at the margins declines...
My Nikon gear was too often part of the payload. I have learnt by first hand experience that keeping camera + lenses under 10kg pays dividends in mobility and the ability to capture the proverbial moment. Under 5kg is even more optimal for mobility etc = DSLR + 3kg telephoto with TCs is barely optimal IMHE.
It is hard to imagine an extreme photographer fitter and better capable to capture the moment in mountains than Galen Rowell, where it was not unknown for him to run up slope etc to get the shots. Quite a few of these shots were taken at altitudes several kms above sea level eg his Tibetan Rainbow capture. This context of light gear for outdoor photography underwrites Rowell's adage - "when you can't take it with you" - link above. (Even though he had a full house of Nikon bodies and glass - he got to know firsthand what optimal system worked for his more extreme conditions.... In my book his little bundle of gear sets the tight bracket on optimal photo gear for Nature photography and his was mainly landscapes. Too often we need the exotic telephotos for wildlife! As the options stand today, this is 3-4kg.
There's also the irritating facts of fiscal stricture as to how many telephotos can be purchased, or even loaned
Costs and Logistics plus ergonomics squeeze the choice too often to a single telephoto. This choice hangs over many travelers, pertinently bird photographers who fly to the tropics and poles and elsewhere.
The 6th post in the series on my blog frames the more technical aspects of my argument in the market forces I see are driving the changes to more lighter ergonomic telephotos - in the tradition of the 300 f4 PF Nikkor. And Teleconverters are all the more central to this solution -as reiterated by John Koerner above....Especially on hikes where 2 telephotos is 1 too many! Yes, it's the trade off for ergonomics to get the images but we still strive for the best IQ that's possible.
And all the better if the telephoto one invests in the future will be (hopefully) enabled with integral bespoke TC(s). What a Game changer! sales of such optics will accrue provided not overpriced. As with what's been achieved in space science, IT and genomics etc, the more audacious suggestions and demands for innovative solutions are first treated with voluble pessimism. But history tells us technology so often triumphs, where after the doom-and-gloom brigade pretends they were believers in any case..... Just a decade back, who would have believed if a forum post had postulated the mass production of the Dinky phase-fresnel telephoto prime (that overcame the glitches of the Canon)?
While my long term interests in choosing the optimum camera gear are to meet my own peculiar needs, I've tried to understand what factors and demands are changing the overall market, not least the potential that is possible today thanks to new technologies in materials and innovations. One has to consider where markets are improving, and for telephotos the needs of nature photographers on the move are swelling in numbers.
This is excellent for several reasons, and more will benefit where technology delivers innovative solutions. IMHO, the telephoto design space has only begun to acknowledge the keystone role of the Humble Teleconverter :-)