Here is a link to a field of view calculator on Bob Atkins' website:
http://www.bobatkins.com/photography/techn...ld_of_view.htmlUsing your sensor size multiplier of 1.6, a 600mm lens at 100 yds would give you a vertical field of view of 7.5 feet, making your heron about 50% of the image height (for a horizontal shot).
You could of course get a 300mm and ad a 2x converter, but with questionable results. Not only because of the equipment quality but because 100 yds is a lot of air to shoot through. Air quality and heat waves will limit your results even with the best glass.
100 yds is a long way for bird photography, even big birds. Great bird shots take time and patience. Learning habits and characteristics of the subject, finding a setting that gets you close but not threatening to the bird and then sitting there for long periods waiting for the right moment.
I don't have any great photos of birds because I haven't committed the time...