I must admit to having very limited experience of cameras, never having owned a DSLR ! but having checked back a year or two I find that the highest ISO shown on EXIF data has been 800, this in a gloomy woodland and at 40mtrs through undergrowth. Any darker and I would not have bothered to take a shot.
Used Nikon 8800, Nikon 900 and now the Nikon P1000 and almost every shot shows an exif of less than 400, taken in all normal lights ranging from overcast to brilliant sun, so why the need for high ISO ?
Den
Here's some camera background. A camera sensor needs a certain number of photos to hit it to make an image. A camera has 3 things that affect that amount of light: shutter speed, aperture, and ISO setting. They are all related in the concept of a "stop" (from old days when turning a knob clicked between detent stops). Each stop is a doubling or halving of light, so you can trade them as you want -- make one larger or another smaller.
Shutter speed: the faster the less light. It is a linear relationship. 1/30th of a second lets in twice the light (i.e. "1 stop") as 1/60th. When using a long lens (where small shakes make big blur) or shooting faster moving things, you need faster shutter speeds.
Aperture: This relates to how big the opening is inside the lens. A lens has a minimum aperture (wide open) and can then "stop down" to close the opening. You stop down to get more in focus (bigger depth of field). Note that lower apertures are bigger openings because it is measured in f-stops, not mm of opening. Aperture is a sqrt(2) scale: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, etc. That is because it is a 2-dimensional opening so making the opening sqrt(2) bigger lets in twice the light. So f/2 to f/2.8 is 1 stop.
ISO: This is how sensitive the sensor is. You could think of it as how much amplification the sensor applies to the signal from the sensor. ISO 100 is usually the base ISO, some cameras will go down to ISO 64. The lower the ISO, the better quality image -- i.e. the less amplification and noise. The larger the ISO, the more effective light you have. So, ISO 200 is twice as sensitive as ISO 200. It is linear, so doubling is 1 stop.
Now you can trade off stops. You can double the ISO or half the shutter speed or reduce the aperture by sqrt(2) to increase the amount of light the camera "sees."
In your example, the Nikons used a low ISO because they had plenty of light. In the gloomy light, they had to bump up the ISO because there was not enough light. Cameras -- depending on the mode -- will use different equations to figure out the minimum desired shutter speed (due to lens size or subject motion) and aperture side (depending on subject profile), then cranks the ISO up or down to make the exposure workout. Cameras have their own little brains and algorithms to try and pick the right settings.
For birds, many people will use Shutter Priority so they can pick the speed they want, then the camera will auto adjust aperture and ISO. Many also use Manual mode so they set both the speed and aperture and only let the camera pick the ISO.
Marc